OF ARTS AND SCIENCES. 
35 
[From, the Author.] 
III. 
THE VERMICULTTES : 
THEIR CRYSTALLOGRAPHIC ANI) CHEMICAL .RELATIONS TO 
THE MICAS; 
TOGETHER WITH A DISCUSSION OF ITIIE GJrtJSE OF THE VARIATION 
OF THE OPTICAL ANGLE! IN THESE MINERALS. 
By Josiah P. Cooke, Jr., 
Erving Professor of Chetnistry and Mineralogy in Harvard College. 
Read, Dec. 9, 1873. 
Introduction. — In the American Journal of Science (VII. 55,1824), 
T. II. Webb described a mineral from Millbury, near Worcester, Mass., 
which has since been a mineralogical curiosity on account of its singular 
reaction when heated. The mineral consists of “ small foliated scales dis- 
tributed through a steatitic base.” “ When heated, it exfoliates prodig- 
iously, the scales opening out into long worm-like threads made up of the 
separate folia;. Exfoliation commences at 500° to G00° Fahr., and takes 
place with so much force as often to break the test-tube in which the 
mineral may be confined. Before the blowpipe it fuses at 3.5 to a gray- 
ish-black glass.” It was named by Webb, as he says, “from the Latin 
vermiculor, ‘ I breed worms.’ ” The hardness of the mineral is 1-2, 
the specific gravity 2.756, the lustre talcose, and the color grayish, 
somewhat brownish. It was analyzed by Crossley, who “separated 
with great care from the base the scaly mineral, which is the true 
vermiculite,” and his results were as follows: — 
• Oxygen Ratio. 
Oxygen Ratio. 
l. 
True 
App. 
• 2. 
True App. 
Silica . . . 
35.74 
19.06 
19.06 
11 
35.74 
19.06 
19.06 2 
Alumina . . 
16.42 
7.65 
7.65 
4 
1.6.42 
7.65 
Ferric Oxide . 
Ferrous Oxido 
10.02 
2.23 
11.13 
3.34 
10.99 1 
Magnesia . . 
27.44 
10.98 
13.21 
7 
27.44 
10.98 
10.98 1 
Water . . . 
10.30 
9.16 
9.16 
5 
10.30 
9.16 
9.16 1 
99.92 
101.03 36 
PROCEEDINGS OF THE AMERICAN ACADEMY 
The results of analysis in column 1, and the portions of the descr» 
tion of the mineral in quotation-marks, above, have been taken frtym 
“ Dana’s System of Mineralogy,” fifth edition, page 493 ; and the atomic 
ratio which is there deduced is, 
IV VI II II 
Si : ft : H : H = 11 : 4 : 7 : 5. 
In this analysis, however, Crossley could not have determined the state 
of the iron, which, in the specimen I have examined, is almost wholly 
in the ferric condition. If now we assume that the whole of the iron 
belongs with the sesquioxide radicals, the analysis would appear as in 
column 2, and the atomic ratio is then seen to be 2 : 1 : 1 : 1, which is 
undoubtedly the correct result. 
In the year 1851, Mr. W. W. Jeflferis, of West Chester, Pa., discovered 
at the ripidolite locality near that town a peculiar micaceous mineral 
which exfoliates like the Millbury vermiculite, but which, instead of 
occurring in small foliae, is found in large hexagonal plates. This 
mineral was analyzed by Professor Brush; and although at first referred 
by him, “with a query,” to vermiculite (Am. Jour. Sci., II. xxxi. 369, 
1861), was subsequently described as a new species (Am. Jour. 
Sci., II. xli. 248, 1866), and named Jefferisite. 
Several years later, Mr. John Hall, now of Philadelphia, sent to me 
for examination some rough six-sided prisms of a micaceous mineral, 
which he had discovered at East Nottingham, in Chester County, Pa. 
This mineral also exfoliates when heated. It is a new species, and I 
have named it, after the discoverer, Hallite. 
A year since I received from Colonel C. W. Jenks, in connection 
with other minerals from his corundum mine on the Culsagee River, in 
Macon County, N. C., a specimen of still another micaceous mineral 
having the same remarkable pyrognostic properties. It proved to be 
the best defined of any of this class of minerals which I had examined, 
and I shall designate it as Culsageeite. 
Besides the above, there have been found several other micaceous 
minerals whose pyrognostic and crystallographic characters indicate 
that they belong to the same family, but which have not yet been 
investigated. 
The remarkable exfoliation and great apparent increase of volume 
which the class of mineral under consideration undergo when heated 
are analogous to the well-known phenomena presented in the dehydra- 
tion of alum, borax, and other crystalline salts, when heated in a similar 
way; and it will be one object of this paper to show that the effect is 
due to the same cause, — namely, to the escaping of what we call water OF ARTS AND SCIENCES. 
37 
of crystallization. I also expect to show that the several minerals 
referi’ed to above are members of a family of hydrous silicates closely 
allied aud parallel to the well-known family of anhydrous silicates 
called the micas, and that their molecules differ from those of the mag- 
nesian micas chiefly in containing a definite number of molecules of 
water; that is, water of crystallization. I shall cull this family of 
minerals “ the vermiculites,” using the original name, as “ mica” is now 
employed, to designate a class; aud I shall call the three species (or 
varieties?) of this family Jefferisite, Culsageeite, and Ilallite; which 
correspond, as I expect to show, to the two varieties of Biotite and to 
Phlogopite respectively. It will appear that the original vermiculite 
has the same composition as the variety from the Culsagee mine. 
Finally, attention will also be asked to some unexpected discoveries to 
which the optical examination of these minerals has led. 
Water of Crystallization and Water of Constitution. — In the salts 
of the stronger acids, the distinction between water of constitution 
(basic water, as it is sometimes called) and water of crystallization is, 
as a rule, well marked. Water of crystallization generally escapes at a 
comparatively low temperature, and the loss is attended with a com- 
plete disintegration of the crystals, which usually fall to powder. 
W uter of constitution is held more firmly, and the salt must be heated 
to a much higher temperature before it is expelled. Moreover, 
athough the loss of basic water may greatly alter the external aspect 
of the body, yet there is not the same complete breaking up of the 
structure as before. To illustrate these points, it is only necessary 
to refer to the well-known reactions of the common rhombic sodic 
phosphate when heated. 
It is generally believed that the water of crystallization exists as 
such in the salt, the molecules of water forming a part of the structure 
of the crystals; and the facts to be presented in this paper tend to sup- 
port this view. But our modern theories assume that the so-called 
water of constitution, which may be also driven off when the salt is 
sufficiently heated, is formed under the influence of the heat from the 
atoms of hydroxyl H-O-, which are united to the acid radical of the 
compound. But in every simple salt, whatever may be our theories 
in regard to the mode of combination, the weight of the water of 
constitution, as well as that of the water of crystallization, must bear to 
the weight of the salt a definite proportion, which can easily be calcu- 
lated from the symliol; and the ratio between the amount of hydrogen 
and the amounts of the other radicals must be that of the atomic weights 
of these radicals or of their multiples. In any given class of salts, 38 
PROCEEDINGS OF THE AMERICAN ACADEMY 
moreover, in which the ratio between the atomicity of the basic and acid 
radicals has a constant value, all hydrogen, which represents water of 
constitution, will supplement the other basic radicals, and added to 
them will complete the relative amount of basic radical which the given 
class of salts require; while the hydrogen, which represents water of 
crystallization, will be in excess of that amount. 
In mineral silicates these relations are complicated by the phenomena 
of isomorphous replacements, and, although there may be some question 
in regard to the replacing capacity of hydrogen, vve cannot expect the 
same constancy in the amount of water of constitution, which these 
minerals contain, as in-the case of simple salts. The amount of water 
of crystallization, however, must be as invariable in one case as in the 
other, and the hydrogen must bear the same relations to the atomic 
ratio of the compound. Thus, if the mineral is an orthosilicate in 
which the atomic ratio between the sum of the basic radicals and the 
silicon is 1:1, all hydrogen in excess of the amount which this ratio 
requires must represent water of crystallization, while all required to 
complete the ratio must represent water of constitution; and we thus 
have a means of distinguishing between these two states of combination 
where the class of compounds to which the mineral belongs is known. 
Now, it is true of each of the minerals we have distinguished among 
the vermiculites,— 
First. That the water is driven off at a temperature below a red 
heat. 
Secondly. That the loss of the water is attended with a complete 
disintegration of the mineral. 
Thirdly. That the amount of the basic radicals, exclusive of the 
hydrogen, is sufficient to saturate the silicon, and that the amount of 
hydrogen is wholly in excess of the amount which the atomic ratio 1:1 
requires. 
Could it be proved that the vermiculites are orthosilicates, the last of 
the three facts just stated would be alone sufficient to establish the cor- 
rectness of our conclusion. Unfortunately such absolute proof cannot 
be obtained; and we only claim that the crystallographic and chemical 
relations of the vermiculites to each other, and to the magnesian micas, 
gives a very high degree of probability to our theory that they are 
orthosilicates. 
From these facts we have concluded that the water which enters into 
the composition of the vermiculites is water of crystallization. For the 
evidence of the facts, we refer to the descriptions of the several species 
given below. But, further, in order to justify our conclusion, we pro- OF ARTS AND SCIENCES. 
39 
pose to bring into comparison with the vermiculites a class of hydrous 
micas from which the water obtained is clearly water of constitution, 
and this class of minerals we shall first describe. 
JJamourites. — Delesse originally gave this name to a hydrous mica 
which occurs in fine scales in Pontivy in Brittany. Since then, micas 
of similar composition have been observed in several countries, and 
shown to be not unusual constituents of granitic rocks.* Among these 
we may distinguish several varieties, (or species?) marked by slight 
differences of composition and optical characters. But we would pro- 
pose to give the name Damourite to the whole class, distinguishing the 
varieties by separate names only so far as may be thought necessary, 
Under the family of Damourites, then, we class all unisilicate micas, 
which are chiefly silicates of aluminum and potassium, but in which 
a portion of the alkaline radical is replaced by hydrogen. 
Sterlingite. — A remarkable mineral of the Damourite type is found at 
Sterling, Mass., associated with spodumene, in a vein of a large bowlder 
rock. This mineral, for the sake of distinction. I have called Sterlingite ; 
but it does not differ from the original Damourite of Delesse except in 
the value of the optical angle. We give in parallel columns the char- 
acters of the two minerals : — 
Mica-like in structure. An aggre- 
gate of flexible and unelastic laminae, 
frequently an inch in diameter. 
Lustre, pearly. 
Color, yellow or yellowish-white. 
H = 2 3. 
Sp. gr. at 26°. 
1st ex. gave 2.832. 
2d „ „ 2.828. 
Cleavage basal, highly perfect, as in 
mica. Jointed parallel to the sides of 
a rhomb, having an angle of 120°, and 
also parallel to the shorter diagonal of 
the same. The markings of these 
joints, or cleavages, visible on the sur- 
faces of the lamina?. 
Double refraction strongly negative. 
Biaxial with plane of axes parallel 
to shorter diagonal. Divergence, about 
70°. Dispersion of axes very small. 
Before the blowpipe fuses on the 
edges with potash flame. 
Sterlingite. 
Mica-like in structure. An aggre- 
gate of fine scales. 
Damourite. 
Lustre, pearly. 
Color, yellow or yellowish-white. 
11 = 2 —3. 
Sp. gr. = 2.792. 
Double refraction negative. Biax- 
ial divergence, 10 to 12 degrees. 
* See the papers of Professor Haughton, cited below. 40 
PROCEEDINGS OF THE AMERICAN ACADEMY 
ANALYSIS. 
Sterlingite. 
Damottrite. 
Oxygen. 
l. 
2. 
Oxygen. 
Si02 
43.87 
23.4Q 
45.22 
43.41 
23.15 
Al2Os 
36,45 
17.00 
37.85 
35.17 
16.39 
Fe, Os 
3.36 
1.00 
4.62 
1.39 
MgO 
1.40 
0.56 
k2o 
10.86 
1.84 
11.20 
10.90 
1.85 
h2Q 
5.19 
4.61 
5.25 
4.50 
4.00 
99.73 
99.52 
100.00 
Of the two analyses of Damourite, No. 1 is by Delesse, of the 
mineral found in the gangue of cyanite, at Pontivy in Brittany, and 
No. 2 is by Igelstrhm, of the similar mineral, found at ITorrsjbberg, 
Wermland. The analysis of Sterlingite was made by Mr. C. E. Munroe, 
Assistant in the Chemical Laboratory of Harvard College, and the 
above numbers have been abundantly confirmed in repeated analyses 
by various students in the same laboratory. The alkalies were treated 
by Smith’s process, and the potassium was weighed as PtK2 Clti. This 
value, compared with the total weight of the alkaline chlorides, and that 
of the chlorine also determined, showed that the alkali in the mineral 
was almost wholly potash, although the presence of lithium and sodium 
was plainly indicated by the spectroscope. The water was determined 
by ignitiug the mineral in coarse powder, previously dried at 100° C. 
Even after ignition, the finely pulverized mineral is only partially 
decomposed by hydrochloric acid, and in the above analysis it was 
decomposed by fusion with sodic carbonate. The usual tests failed to 
indicate the presence of fluorine. 
Regarding the water as basic, and as forming a part of the protox- 
ides, the atomic ratio in Sterlingite, between the silicon, the sesqui- 
oxide radicals, and the protoxide radicals, is 
Si : R : R = 23.40 : 18 : 6.45, or nearly 4:3:1. 
The deviation from the simple ratio will not appear so great, as seems 
at first sight, if it is noticed that a difference of one-lialf per cent in 
the amount of water would make the ratio almost exact. 
The corresponding ratio in the Damourite from Wermland is 
23.15 : 17.78 : 6.41, or, as before, nearly 4:3:1. 
These ratios point to the general formula: — 
II VI 
R, R .Os. Sir )F ARTS AND SCIENCES. 
41 
The Damourite and Sterliugite are types of a very large class of 
hydrous micas, which in many places are widely distributed through the 
granitic rocks. This class of minerals has been especially investigated 
by Professor Haugiiton, of Dublin, whose papers may lie found in Phil. 
Mag., IV. ix. 272, and Q. J. G. Soc., xviii. 414, also xx. 280. We cite 
here a few of his analyses, selected from those given by Professor 
Dana, — pages 310 and 311, of the fifth edition of his “System of 
Mineralogy,” — in further illustration of the subject we are discussing: 
Dublin Co. 
— IIaughton. Optical Angle, 53°. 
Si 
A1 Fe 
Mg 
Ca Na 
K 
H 
43.47 
31.42 4.79 
1.13 
1.38 1.44 
10.71 
5.43 = 99.77 
23.18 
14.64 1.44 
0.45 
0.39 0.37 
1.82 
4.83 
23.18 
16.09 
3.03 
4.83 
23.18 
10.09 
7.86 
3. 
2. 
1. 
Mt. Leinster. — IIauguton. Optical Angle, 72°. 
Si 
A1 Fe 
Mg 
K 
H 
44.G4 
30.18 6.35 
0.72 
12.40 
5.32 = 99.61 
23.81 
14.07 1.90 
0.29 
2.11 
4.73 
23.81 
15.97 
2.4 
4.73 
23.81 
15.97 
7.13 
3. 
2. 
1. 
Glendalouoii.—IIaughton. Optical Angle, 70°. 
Si 
A1 Fe 
Mg 
Ca Na 
E 
H 
44.71 
31.13 4.69 
0.90 
1.09 1.27 
9.91 
6.22 = 99.92 
23.84 
14.51 1.41 
0.36 
0.31 0.33 
1.68 
5.53 
23.84 
15.91 
2.68 
5.53 
23.84 
15.91 
• 
8.21 
3. 
2.. 
1. 
In these micas again, if we regard the water as basic, we obtain a 
nearly constant ratio, but differing from that of Damourite in the 
relation of the two basic radicals. The general symbol of the last 
would be: — 
II VI 
R4 • 0M . Sir 42 
PROCEEDINGS OF THE AMERICAN ACADEMY 
There appear to be, therefore, these two distinct types of hydrous 
micas related to the species Muscovite, all rich in alumina and alkali, 
destitute, or only containing very small amounts, of magnesia, and 
having a wide optical angle. 
The anhydrous Muscovites have not been investigated nearly as 
fully as the hydrous varieties; and I can find no analyses of any of the 
beautiful specimens from our American localities. I cite here, for the 
sake of comparison, an analysis of the Fuchsite, from the Zillerthal, 
by Schafhautl (No. 1), and one of a mica, from Fahlun, by II. Rose 
(No. 2): — 
Si 
Al. 
Fe -0r 
Mg 
Ca Na 
K 
F 
(1) 47.95 
34.45 
1.80 3.95 
0.71 
0.59 0.37 
10.75 
0.35 = 100.92 
25.57 
16.05 
.54 1.24 
.28 
0.17 0.09 
1.83 
25.57 
17.83 
2.37 
Si 
A1 
Fe 
Mg. 
K 
H 
F 
(2) 46.22 
34.52 
6.04 
2.11 
8.22 
0.98 
1.03 
24.65 
16.08 
1.81 
0.84 
1.39 
0.87 
24.65 
17.89 
3.10 
If now we should add to (1) 6.93 per cent water, and to (2) 3.42 per 
cent, we should obtain the ratios : — 
(1) 25.57 : 17.83 : 8.52, or approx. 3:2:1, 
as in the hydrous micas analyzed by Haughton. 
(2) 24.65 : 17.89 : 6.16, or approx. 4:3:1, 
as in Damourite and Sterlingite. 
There would seem, then, to be two definite varieties of hydrous micas 
of the Muscovite family, distinguished by the atomic ratios 3:2:1 and 
4:3:1. Corresponding to these, it is probable that there are two 
varieties of anhydrous mica, containing dh excess of SiOv which, by the 
addition of sufficient water to saturate the excess of the acid radical, 
are reduced to one or the other of the two normal types. In a pre- 
vious paper (Am. Jour. Sci., II. xii. 217, 1867) the author suggested the 
idea that the excess of silica in this class of micas might result from 
a mixture of two isomorphous species corresponding to the two 
hydrates: — 
Hf Of Si and H2=OfSiO, OF A UTS AND SCIENCES. 
43 
and he there described a mica whose atomic ratio was very closely 
that of the second type, and which he called Cryopliyllite. Now, the 
simplest theory of the relation of the hydrous to the anhydrous Mus- 
covites would seem to be that, while in the molecules of the anhydrous 
micas a portion of the silicon atoms (the number varying in the different 
varieties) are in the condition of the first anhydride (fffOfSiO), the 
hydrous micas contain sufficient basic hydrogen to bring all the silicon 
atoms into the condition of the normal hydrate ( H = Of Si). The two 
graphic formulae which follow indicate, more clearly than any other 
language can, the relations we have attempted to describe. In these 
formula} ii stands for the double atom in the radical of the sesqui- 
oxides, having the tpiantivalence of six: — 
II Si K 
o bi b 
ll OklO li Ch&i-O K 
b b 
SiO/il ROfSi 
b o 
K- 0-Si- O R 0-&- 0-H 
1 «M 1 
0 Of 0 
k si k 
Hydrous Muscovite. 
Si K 
<« I 
0j O 
OSi-Oli-O-di-O-K 
b o 
SiOfR Hi Of Si 
b o 
K-0-Si(>R-0Si=0 
b o< 
k k 
Anhydrous Muscovite. 
The new mineral Sterlingite, whose examination has been the occa- 
sion of this discussion, is remarkable as being a very well-defined 
example of a hydrous mica occurring in large crystals, and exhibiting 
very marked characters. It does not materially differ in composition 
from Dainourite, aud it also agrees with the specimens of this mineral 
so closely in other physical qualities that we cannot regard the small 44 
PROCEEDINGS OF THE AMERICAN ACADEMY 
optical angle, observed by Descloizeaux on the minute scales of the 
Pontivy mica, as sufficient ground for separating the new mineral from 
the old species. We should include under this species all hydrous 
micas which are rendered by the basic hydrogen orthosilicates, and to 
it the name Damourite belongs by priority. Sterlingite is simply a 
variety of Damourite, having the ratio 1:3:4, with more marked 
qualities and a wider optical angle than the Pontivy mineral; and, 
provisionally, the name I have given will be useful in designating it. 
The hydrous micas, of which Sterlingite is a variety, have a special 
interest in connection with the subject of this paper, because they 
illustrate the characteristics of basic water, which will be contrasted, 
with those of water of crystallization, in our description of the fol- 
lowing species. The evidences that the water in these micas is basic, 
— that is, forms a part of the basic radical, — may be summed up as 
follows: — 
1. The amount of water in the different varieties is very variable, 
and bears no constant ratio to that of the other basic radicals.* 
2. The hydrogen of the water supplements the other basic radicals 
and fills out the amount required for a unisilicate, the type to which 
most of the micas conform. 
3. The water is expelled only at a very high temperature. 
4. The loss of water is not attended with any marked change in the 
appearance of the mineral.f 
Jefferisite, of West Chester. — This well-known mineral, found 
in the serpentine at West Chester, Pa., was, as I have said, care- 
fully analyzed by Professor Brush, of New Haven, who named it after 
W. W. Jefferis, Esq., of West Chester; and to this gentleman I am 
indebted for the specimens of the mineral whose crystallographic rela- 
tions I have studied. As I am informed by this careful mineralogical 
observer, the locality is some three miles south of West Chester, where 
* This fact is not shown so forcibly by the analysis cited above as by the 
series given by Professor Dana, on pages 310 and 311 of the fifth edition of his 
“ System of Mineralogy,” to which we would refer in illustration of this point. 
+ Since the above was written, we have received from Dr. F. A. Genth his 
very valuable paper on corundum and its associated minerals. He regards 
Damourite as one of the most important products of the alteration of co- 
rundum, and gives a large number of analyses of specimens from different 
localities, to which we gladly refer, as they illustrate the point made in this 
paper, even more markedly than the analyses cited above. The Damourites 
are evidently widely distributed minerals and characteristic features of certain 
rocks. OF ARTS AND SCIENCES. 
45 
the mineral occurs in several veins or pockets of the serpentine, not 
more than fifty yards from the quarry where the ripidolite is found. 
Jt is associated with oligoclase; and corundum also occurs in the same 
serpentine formation, in Newton township, ten miles distant, on the 
north edge of the serpentine ridges. The Jefferisite is found in broad 
crystals, or crystalline plates, which frequently have a distinct hexagonal 
form. On two of these plates sent me by Mr. Jefferis, measuring respect- 
ively two inches and three-fourths of an inch iu diameter, the form is 
as sharp as on crystals of Muscovite, and the edges make an angle of 
1 *20°, as nearly as it can be measured. Mr. Jetferis writes, — lu 
regard to the Jefferisite crystals which you have examined, the large 
crystals or plates occurred in the vein immediately under the sod, 
exposed to all frost, &c.; the small crystals were taken out four or five 
feet below, in the decomposed Deweylite, and were associated with 
striated oligoclase, of which I sent you specimens.” 
The crystals of Jefferisite cleave like mica, affording thin but un- 
elastic folia;. The cleavage planes are marked triangularly by lines, 
crossing at angles of 60° and 120°. In some cases there is a jointing 
as in crystals of mica, parallel to the shorter diagonal of the rhomb. 
One crystal sent me by Mr. Jefferis is the half of a rough hexagonal 
prism, an inch and a quarter high by two inches in diameter. The 
plane of the optical axes, as in the larger number 
of micas, is parallel to one of these lines, as 
indicated in Fig. 1, coinciding with the shorter 
diagonal of the rhombic prism, which appears 
to be the fundamental form in all this class of 
minerals, and from which the hexagonal form is 
derived by the truncation of the two acute angles. 
The double refraction is strongly negative, but 
the angle between the optical axes varies in the most remarkable 
manner. I liave measured angles on different plates, of 27°, 24°, 
and 10°, and observed many intermediate conditions. Owing to 
the deep yellow color, the plates become opaque at a very moderate 
thickness, and for this reason it is impossible to measure the angle 
with great precision. Some of the plates are apparently uniaxial; 
but this may result from the blending of the twro hyperbolas, due to 
the thinness of the plate. The dispersion of the axes is but slight, 
and only perceptible in the thicker laminae when < v. It is obvious, 
therefore, that the crystallographic characters of the mineral are 
identical with those of mica. 
The plates are generally, if not invariably, twinned, and the twin- 
Fig. 1. 46 
PROCEEDINGS OF THE AMERICAN ACADEMY 
ning is the cause of this most remarkable variation in the optical angle, 
as will be explained at length in connection with our description of 
Culsageeite. On this last mineral the same phenomena are more 
marked, owing chiefly to the greater transparency of the plates. 
In order to illustrate the chemical relations of the mineral to the 
Biotite micas, we give below: 1st. The results of the analysis of 
Jefferisite by Professor Brush. 2d. The same results, calculated for 
the anhydrous mineral. 3d. The residts of an analysis of a Biotite 
mica, from Pargas, Finland, by Svanberg. In each case I have added 
the amounts of oxygen in the several oxides, to show the atomic 
ratios: — 
Si 
A1 Fe 
Fe 
Mg Ca E H 
(1) 
37.10 
17.57 10.54 
1.26 
19.65 0.56 0.43 13.76=100.87 
19.78 
8.18 3.16 
.28 
7.86 .16 .07 12.23 
19.78 
11.35 
8.37 12.23 
i[i( ''•* 
5. 
3. 
2. 3. 
Si 
A1 Fe 
Fe 
Mg Ca K 
(2) 
42.94 
20.33 12.20 
1.46 
22.75 .66 .50 = 100.84 
22.90 
9.47 3.6G 
.32 
9.10 .19 .08 
22.90 
13.13 
9.69 
5. 
3. 
2. 
Si 
A1 Fe 
Mn 
Mg Ca K H F 
(3) 
42.58 
21.68 10.39 
•75 : 
10.27 1.04 8.45 3.35 .51 = 99.01 
22.71 
10.10 3.12 
.17 
4.11 .30 1.43 2.98 
22.71 
13.22 
8.99 
5. 
3. 
2. 
The general 
symbol of Jefferisite deduced from (1) would be,— 
II VI 
li4, R,. O20. Si5. GII20. 
A comparison of the results given in (2.) and (3) will show that the 
anhydrous Jefferisite corresponds very closely in its chemical constitu- 
tion with the Biotite mica from Pargas. The chief difference is to be 
found in the fact that the mica contains potassium and basic hydrogen, 
in place of more than one-half of the magnesium of the Jefferisite. It 
should, however, be remembered in this connection that the Biotites 
present a very wide variation in the ratio between the amounts of the OP ARTS AND SCIENCES. 
47 
protoxide and sesquioxide radicals which the various varieties contain. 
The limits usually assigned to this variation correspond to the ratios — 
R : § : Si= 1:2:3, and R : S : Si = 1 : 1 : 2, 
and the Pargas mica with the ratio 2:3:5 falls between these limits; 
but the Culsagee variety of vermiculite corresponds to the more com- 
mon class of liiotites, which have the ratio 1:1:2. 
Hut this resemblance in chemical constitution only appears when we 
compare the Hiotite mica with the anhydrous Jefferisite; while it is the 
crystallized hydrous Jefferisite which so closely resembles the magnesian 
micas in its crystallographic relations; and the question now arises, 
What is the condition of the large amount of water— 12j per cent — 
which the crystallized mineral contains? 
To aid us in forming a conclusion on this point, we have the follow- 
ing evidence:— 
First. As the above analysis shows the water is united in definite 
and atomic proportions amounting to six molecules to every five mole- 
cules of silicon in the molecules of the mineral, that is sufficient to 
convert all the silicon into a hydrate, assuming that the five silicon 
atoms in this hydrate are joined to each other by the smallest possible 
number of bonds. 
Secondly. While both the crystallographic and the chemical relations 
of Jefferisite to the other vermiculites, and to the magnesian micas, 
indicate that the mineral is an orthosilicate, the amount of basic radical, 
exclusive of the water, is amply sufficient to saturate the atomicity of 
the silicon. 
Thirdly. It was observed by Professor Brush — and his observa- 
tions have been fully confirmed by ourselves — that the water is given 
off at a comparatively low temperature, — about 300° C.; and, as every 
mineralogist knows, this dehydration is attended with that remarkable 
exfoliation which is characteristic of the vermiculites, and indicates a 
complete disintegration of the molecular structure. This exfoliation is 
wholly different from the phenomena which the so-called hydrous micas 
present under like conditions. In these last — which, as we suppose, 
contain hydrogen as a part of the basic radical of their molecules — a 
very high temperature is required to expel the water, and the loss is 
attended by no such marked change of volume and disintegration. 
The conclusion that we draw from these facts is, that the combined 
water is in the same condition in Jefferisite as in the zeolites, and in 
many crystallized salts, — a condition which lias long been known in 
chemistry as water of crystallization. We shall not here attempt to 48 
PROCEEDINGS OF THE AMERICAN ACADEMY 
"discuss what are the relations of the water thus combined to the mo- 
lecular structure of the mineral. This question is still in suspense, and 
we are persuaded that our science is not yet in a condition to solve the 
problem. All that we can at present do is to classify the phenomena 
presented by the exfoliation of the vermiculite minerals with the efllo- 
rescence of Glauber’s salt or the intumescence of alum. 
If, then, I am justified in the inferences I have drawn, JefFerisite 
differs chemically from Biotite mica chiefly in containing water of 
crystallization, very much in the same way that gypsum differs from 
anhydrite, or common salt from the crystals with two molecules of 
water, which form in brine at a low temperature. In these Inxlies, 
however, the forms of the crystals of the hydrous are wholly different 
from those of the anhydrous salts, while the crystallographic relations 
of JefFerisite are very similar, if not identical, with those of the mag- 
nesian micas. It is, however, also true that the crystalline structure of 
the micas seems to admit of a very wide variation of chemical com- 
position. In cryophyllite, which has all the crystallographic charac- 
ters of the micas strongly developed, is the atomic ratio 3:4:14; 
while in the Biotite, whose analysis was cited above, it is 2:3:5. 
The first is an acid, and the last a neutral silicate. The other varieties 
of mica have a composition intermediate between these extremes; 
and it might be said, if such a variation of composition is compatible 
with the crystalline structure exhibited by this group of minerals, it 
would not be surprising if the structure were sufficiently elastic to admit 
of the insertion of the water of crystallization without great alteration 
of external form. 
It may be further urged that the normal mica crystal (a rhombic 
prism with angles of 60° and 120°) is a form which appears to be 
related to a mode of molecular structure common to a great many sub- 
stances, and the phenomena which will be described in this paper seem 
to sustain this opinion. Still, we do not care to theorize about the 
subject. Our object is simply to make prominent the two points: 
1st. That the crystallographic structure of JefFerisite is identical with 
that of the magnesian micas. 2d. That the chemical constitution of 
the anhydrous mineral is closely allied to that of Biotite. 
Culsageeite or the Vermiculite of the Jenks Mine, North Carolina.— 
Through the kindness of Colonel C. W. Jenks, the proprietor of the 
corundum mines on the Culsagee River in Macon County, N. C., I 
have had an opportunity of examining a variety of vermiculite which at 
that locality forms an important part of the matrix of the corundum. The 
associations of the mineral in North Carolina are very similar to those 49 
OF ARTS AND SCIENCES, 
in Pennsylvania. Professor J. Lawrence Smith, in liis recent article 
in Am. Jour. Sci. (111. vi. 180), says that “in the development in 
North Carolina the corundum ” (with which the vermiculite is associated) 
“occurs in chrysolite or serpentine rocks, and outside of serpentine it 
has not been found. These chrysolite rocks belong to a regular system 
of dikes which have been traversed for the distance of about one hundred 
and ninety miles. This system of dikes lies on the north-west side of 
the Blue Kidge, and has a strike parallel to the main mass of the 
ridge, and has an average distance from the summit of the ridge of 
about ten miles. The serpentine appears at intervals along this whole 
line of one hundred and ninety miles,” and “is usually enclosed in a 
hard crystalline gneiss.” In the serpentine the corundum has beep, 
found at several localities along a distance of forty miles. Colonel 
Jenks writes me: “ The workmen have opened a new vein they call the 
gem vein. It is walled with chrysolite, and the fissure is from one to four 
feet wide, which fissure is filled with ripidolite, and in this the corundum 
crystals are imbedded.” As regards the association of the ripidolite and 
vermiculite, he also writes: “ They occur both alone and intermingled, 
and also in layers, like the leaves of a book.” Again: “The ripidolite 
maintains a uniform character of texture and appearance, whether 
associated with the vermiculite or alone. In the vein at the bottom of 
the shaft I have spoke of, ami elsewhere, it is of uniform hardness, 
texture, &c., but in it the corundum seems to have lost cohesion, — in 
some localities falling to pieces, and crumbling out of the ripidolite 
matrix. But, when exposed to the air and sunshine for a day or two, 
the corundum regains its cohesion, and you cau then chip oft' the ripid- 
olite from it. It is just the opposite with the vermiculite when alone 
the matrix. Sometimes it occurs in masses, several feet long, formed 
of scales as large as your hand, easily separating like mica from the 
corundum, which in this matrix always seems to retain its cohesion. 
Then, again, the vermiculite will occur in masses of many bushels, all 
broken up and disintegrated, like very coarse bran, in which the crystals 
of corundum lie like eggs in a box of sawdust. The great crystal 
(weighing 31G i>ounds) was thus de|>osited, At other places the entire 
mass seems to have changed to the color of white Castile soap, with 
spec*ks of green in it, the corundum crystals all falling apart until 
exposed, when they harden.” This untechnical language of an unpro- 
fessional but accurate observer conveys a very vivid idea of the mode 
of association of these minerals. The specimens so kindly sent me by 
Colonel Jenks confirmed in all respects his observations. In several 
cases the corundum was still imbedded in both the ripidolite and the 50 
PROCEEDINGS OF THE AMERICAN ACADEMY 
vermiculite, and the scales of these minerals not unfrequently penetrated 
the corundum crystals in such a way as to indicate that the corundum 
had crystallized around them The ripidolite is 
firm in texture, and closely resembles the variety 
from West Chester. It occurs in hexagonal 
plates, which have a striation similar to that I for- 
merly described as characteristic of the Pennsyl- 
vania specimens (Am. Jour. Sci., II. xliv. 201, 
1867), and which is illustrated by Fig. 2. 
Professor Smith, loc. cit., gives analyses of 
two varieties of ripidolite — one compact, the 
other friable — as follows :— 
Fig. 2. 
Si 
A1 
Fe 
Mg 
H 
27.00 
21.60 
16.63 
22.00 
12.30 = 99.53 
Compact Ripidolite from North Carolina. 
Friable Ripidolite from North Carolina. 
Si 
A1 
Fe 
Mg 
H 
29.15 
10.50 
23.50 
25.44 
10.04 = 98.63 
A determination of the silica in the compact mineral made by Mr. F. 
Gooch, student in the Laboratory of Harvard College, gave 27.25 per 
cent of Si02, and 11.93 ff.,0, as a mean of two closely agreeing deter- 
minations. But in the specimens sent me by Colonel Jenks I did not 
recognize the friable variety. 
The vermiculite occurs in close contact with the ripidolite, and is 
frequently interlaminated with it, but the two are always perfectly 
distinct, thus entirely disproving the theory that the vermiculite was 
derived from the ripidolite by weathering. It occurs in large plates 
having more or less of an hexagonal outline. Some of those received 
from Colonel Jenks were five inches in diameter. It has a greenish- 
yellow color, which is very much lighter than that of the West Chester 
variety. The plates are strongly marked by lines crossing at angles 
of 60° and 120°, like those from West Chester; but these lines are 
more marked in the North Carolina variety. This variety is also much 
more friable than the other, and readily breaks in directions parallel to 
these lines,—yielding rhombic plates with angles of 120° and 60°, and 
more readily hexagonal or triangular plates, produced by the truncation 
of the 60° angle of the rhombic plate, on a line parallel to the shorter 
diagonal of the fundamental rhomb. The plates most readily break OF ARTS AND SCIENCES. 
51 
parallel to this diagonal, and, like the specimens from West Chester, are 
frequently jointed in this direction. Like other micaceous minerals, the 
plates cleave readily parallel to the basal plane, yielding very thin folia*, 
exceedingly flexible, but not elastic. The optical characters are the same 
as those of the West Chester variety, — strong negative double refraction 
yielding a biaxial ring system, with uniform distribution of colors, and 
very variable optical angle. I have measured angles from about MO® to 
about 13°. The angle often varies widely in different parts of the same 
plate. Thus I have measured on different lamina*, from a single plate 
not exceeding 3 inches in diameter, the three angles 30°, 24"*, and 13°; 
and again I have noticed a similar variation on one and the same 
lamina. Indeed, the phenomena which I observed were almost identical 
with those I had previously observed on plates of ripidolite from Texas, 
Pennsylvania, loc. cit. On moving the lamina just referred to parallel to 
itself, in front of the polarizing microscope, the optical angle varied as I 
passed from one side of the field to the other. Beginning with a value 
of about 30°, the angle decreased to about 13°. Moving the plate 
still further, I found a region of indistinctness, and then the axes opened 
again, — the new plane making an angle of 12U° to the old. I had 
evidently here a macling precisely similar to that 
I had previously descril>ed in the Texas ripidolite. 
and shown in Fig. 3, where the lines of shading 
represent the position of the plane of the optical 
axes. This represents what we may call an ideal 
made; for I have seldom been able to trace more 
than three individuals on the same plate, and. as a 
rule, these are very unequally developed. On many 
of the specimens of the North Carolina vermiculite, the macling is exter- 
nally marked by the eminent cleavage or jointing parallel to the shorter 
diagonal of the rhomb section, and in several of the specimens. 1 have 
examined it was quite symmetrical. A study of these specimens led me 
to an explanation of the cause of the remarkable variation of the optical 
angle, which 1 believe not only applies to the vermiculites and ripido- 
lites, but also, in many cases at least, to the micas. It would be expected 
that the several members of such macles as Fig. 3 represents would 
penetrate each other, and I therefore made a series of experiments to 
ascertain what would be the effect of the interfoliation of lamina* in 
which the planes of the two sets of optical axes had the same relative 
position as in the several mejnl>ers of the made. To that end I divided 
a plate, which presented the largest optical angle I had observed, into 
as thin laminae as possible, and then superimposed them in the relative 
Fi<r. :t 52 
PROCEEDINGS OF THE AMERICAN ACADEMY 
position I have mentioned, so that the planes of the optical axes should 
be inclined at an angle of 60°. The result was — when the thickness 
of the plates in each position were nearly equal — that a symmetrical 
ring system was obtained, in which the optical angle was about 13°,— 
the smallest I had measured; and, by varying the.relative thickness, 
intermediate degrees of optical divergence were produced. By now 
introducing lamina} into the compound crystal, in the position of the 
third member of the made, — that is, with the plane of the optical 
axes in the j>ositiou of the third diagonal of the hexagon, — the 
apparent angle could be reduced still further, so that the plate 
was apparently uniaxial, Although these experiments were suffi- 
cient to show that the macling was an adequate explanation of the 
apparent variation of optical angle I had observed in the plates of 
ripidolite and Jefferisite, they also raised the question how far the 
effect I had obtained in my experiments might be due to the circum- 
stance, that, on account of the deep color of these minerals, it is only 
possible to experiment on very thin plates, with which, of course, the 
rings of interference are very wide, and the hyperbolas proportionally 
indefinite. I therefore next made a similar experiment with a well- 
known phlogopite mica from Jefferson County, N. Y., whose crystals are 
very distinctly macled after the type of Fig. 3 or o, the plates presenting 
a variation of optical angle similar to that I have described, the normal 
axial divergence being about A very clear portion of one of 
these plates was first cut into a regular hexagon* one of whose diagonals 
was in the plane of the optical axes. This hexagonal plate was then 
split into twelve laminte, which were superimposed with the interven- 
tion of balsam, and in alternating positions, like the members of a 
made, — the optical plane in each of the laminae making an angle of 
60° with that of the lamina above or beneath it. The result was an 
essentially uniaxial plate, differing from a plate of uniaxial mica only 
in small irregularities in the contour of the rings, such as the lamination 
would he expected to produce. On repeating now this experiment 
with a Muscovite mica having a wide optical angle about 63°, I obtained 
a most remarkable and unexpected result, — a structure presenting 
optical phenomena similar to those of a plate of quartz cut perpen- 
dicularly to the principal axis. At the first trial I obtained a compound 
mica plate showing a disk of color in the centre of the field whose tint 
changed during the rotation of the analyzer, — of the polarizing micro- 
scope,— like a plate of left-handed quartz ; and, on superposing a plate 
of right-handfed quartz, the spirals of Airy at once appeared. The 
rings, however, were wholly broken up, and appeared only in irregular OP ARTS AND SCIENCES. 
53 
patehes of color. This irregularity was due to the unequal thickness 
of the lamina} employed, and I found it impossible to split up one and 
the same hexagonal plate of mica into lamina*, which were sufficiently 
uniform for the purpose, llut very satisfactory results were obtained 
in the following way. I selected for the purpose the very clear and 
easily cleavable mica from Grafton, N. H.; and, after a few trials, suc- 
ceeded in cleaving off very thin plates of considerable size and nearly 
uniform thickness. Selecting one of these plates, I first divided it by 
means of a parallel ruler into strips, and from these strips cut out the 
hexagonal lamina; by means of a steel pattern carefully made. The 
thin mica can be cut with perfect accuracy by a sharp knife on a plate 
of glass. The hexagonal lamina} thus obtained, though coming from 
different parts of the mica plate, were optically parallel to each other ; and 
by drawing at the outset a line, with a sharp point, near the correspond- 
ing edges of the several strips, this line served as a guide for placing the 
hexagonal lamina;. From laminae thus prepared, plates were made show- 
ing the familiar system of rings as perfectly as could be expected. The 
best results were obtained with plates consisting of from twelve to twenty- 
four lamina;; and the character of the resulting plate, whether left or 
right handed, was found to depend on the order of the spiral arrange- 
ment. If in building up the pile the marked side of each successive 
lamina is turned through an angle of 60° in the direction of the motion of 
the hands of a watch, the result corresponds to left-handed quartz if turned 
in the reverse direction to right-handed, and on superposing two dis- 
similar plates thus prepared I obtained again the spirals of Airy in great 
perfection. Thus, then, it appears that, even with micas of the widest 
optical angle, we can build up a structure which is optically uniaxial.* 
* The great difficulty in preparing these plates is to obtain thin films of mica 
of uniform thickness, which are of sufficient size to yield a dozen or more lami- 
na;; and the more nearly we have succeeded in preparing such a film, by 
splitting sheets of mica, the more closely we have been able to imitate the 
phenomena seen under like conditions with a plate of quartz. We have been 
able to work with films which measured with a spherometer only of an inch 
in thickness, and have not obtained good results with those which were much 
thicker, and when thinner than this the mica cannot readily be cut into shape. 
The least inequality in the thickness of the several laminae composing the same 
plate more or less mars the effect; and, although some of the striking features 
seen with quartz may remain, such as the succession of colors on revolving the 
analyzer, and even the spirals of Airy, yet the more delicate phases of the phe- 
nomena disappear. The plate changes color when revolved in its own plane, 
the rings lose their circular form and become confused, and the violet cross 
disappears. Moreover, as regards the conditions which determine the phase of 
the circular polarization, the law stated above can only be affirmed with cer- 54 
PROCEEDINGS OF THE AMERICAN ACADEMY 
The theory which I have formed to account for these facts is as follows. 
We may conceive that the molecule 
of mica is an ellipsoid, whose ellip- 
tical section, through the longer 
axis, can be inscribed in the rhomb 
of 60° and 120°. Assume now 
that these molecules have polarity, 
the rhombic prism would be the 
normal result of their association, 
as represented in Fig. 4. We 
may, however, conceive that three of these molecules may become 
associated by alternate single poles to form a nucleus, such as repre- 
sented in Fig. 5. Such a group once formed would be in a condi- 
tion of great stability, resulting from the 
concurrent action of the several poles; 
and if now each of the molecules devel- 
ops into a crystal, the result would be a 
made of a form which is very common in 
the mineral kingdom. If six molecules 
unite in a similar way to form a nucleus, 
as in Fig. 6, we should also have stable 
equilibrium (although less firm than 
before), and the result of a symmetrical 
development would be a made such 
as has been represented in Figs. 2 and 
3. Whether this more complex arrange- 
ment is necessary in order to explain the phenomena presented by 
Fig. 4. 
Fig. 5. 
tainty of plates consisting of laminae which very nearly fulfil the conditions we 
have described. Very small inequalities of thickness renders the effect irregu- 
lar, and made it at first difficult to discover the law. Our experiments have 
been a series of approximations; and, although we may never be able with our 
rude appliances to compete with nature in the manufacture of uniaxial crystals, 
yet we have approached so near to the perfect result as to be able to point out 
with confidence one way, at least, by which the effects seen in natural crystals 
may be produced. We have usually cut the lamina.* into regular hexagons, but 
equilateral triangles might more easily be cut, and would probably give as good 
results ; for, although the errors of position might not be so well distributed, we 
have found that a slight variation in the relative position of the laminae injures 
the result to a far less degree than the least inequality in their thickness. It 
still remains to make careful quantitative measurements of the effeots produced 
under determinate conditions; and, as can easily be seen, the subject opens a 
wide field for mathematical analysis as well as physical investigation. OF ARTS AND SCIENCES. 
55 
the vermiculites and micas, I do not feel confident. It is seldom 
that more than three individuals 
can be distinguished on a given 
plate; and the very unequal de- 
velopment of the several individ- 
uals, and the indefiniteness of the 
lines of demarcation, resulting from 
the phenomena which have been 
described, render what would seem 
to be a characteristic feature of the 
more complex group not neces- 
sarily a certain indication of the 
structure. 1 refer to the fact, very 
constantly noticed, that the plane 
of the optical axes is parallel to 
the nearest hexagonal edge, as 
shown in Figs. 2 and 3. In Figs. 5 and 6 this same plane is parallel 
to the shorter axis of the ellipse; and it can easily be seen that if 
either of the individual of Fig. 5 were developed over any large 
portion of the space of its neighbors, the optical plane might appear 
parallel to the adjacent edge. 
Having made the two suppositions, as above, to explain the phe- 
nomena of twinning, which have been long familiar and externally 
visible, it will not, we trust, appear unreasonable if we make a third 
supposition to explain the phenomena first described in this paper. 
We may conceive that the ellipsoidal molecules, instead of grouping 
together on the same plane, become associated by their alternate poles, 
one over the other, as represented in Fig. 7. 
Molecules so associated, developing later- 
ally, would produce the laminas of a mica 
plate in the relative position in which we 
have placed them in our artificial crystals, 
with only this difference, that the laminae 
would be indefinitely thin, and in exact 
position ; and the effect of such compound 
molecules in modifying the elasticity of 
the crystalline structure must be, in most 
respects at least, like that of single mole- 
cules, symmetrical on all sides of one line 
or axis,— in other words, they must produce a structure similar to that 
of uniaxial crystals. Under what further conditions the grouping of 
Fig. 6. 
Fig. 7. 56 
PROCEEDINGS OF THE AMERICAN ACADEMY 
the molecules, in right or left handed spirals, determines the phenomena 
of right or left handed circular polarization, and what bearing the new 
facts may have on the received theory of these phenomena as they 
appear in quartz, must be left for further analysis to discuss. 
I pass next to consider the composition of the Culsagee vermiculite, 
and I give below, at (1), (2), and (3), the results of three analyses, 
made by myself, together with the corresponding oxygen ratios. 
The pulverized mineral, after it has been exfoliated by heat, is easily 
and perfectly decomposed by hydrochloric acid. In analysis (1), after 
the separation of the silica, the alumina and ferric oxide were sepa- 
rated from the magnesia by ammonia, with the usual precautions. In 
(2) and (3) the bases were converted into nitrates and separated by De- 
ville’s method. In each case the magnesia was weighed as pyrophos- 
phate, and the alumina and ferric oxide were weighed together. 
All three analyses were made with material rendered anhydrous 
by ignition until the weight was constant, and each is represented 
by three distinct determinations namely, the weight of the silica, 
the sum of the weight of the alumina and ferric oxide, and the weight 
of the magnesic pyrophosphate. The oxides of iron and the water were 
once for all determined on separate portions of the dried but not exfoliated 
mineral, and therefore appear of the same value in all the analyses. 
The determination of the water was the only difficulty which the 
analysis of this mineral presented. It is by far the most hygroscopic 
silicate I have ever examined, when once dry absorbing wrater from 
comparatively dry air with almost as much avidity as chloride of cal- 
cium. In two experiments with different portions of the same powder, 
the material was heated in an air bath, at 100° C., for seventy-two hours 
before the weight became constant; and in each case the weight was 
compared at intervals of about six hours. The total loss in the first 
experiment was 10.27 per ceut, and in the second 10.19 per cent. The 
mineral thus dried lost when ignited 10.84 per cent. Another portiou 
of the same powder, which had been dried over sulphuric acid for more 
than two months, lost when ignited 11.09 per cent. This close agreement 
indicates that all the water lost in drying, either at 100° or over sulphu- 
ric acid, is hygroscopic; and the conclusion is confirmed very greatly by 
the fact that the mineral in thus drying does not change its aspect in the 
least degree, and rapidly reabsorbs the water when exposed to the air. 
On the other hand, w'hen the mineral is ignited it swells up to many 
times its volume, like other members of the vermiculite family, and under- 
goes what is evidently a profound alteration in its molecular structure.* 
* In his valuable paper on corundum, above referred to (p. 44), Dr. Genth OP ANTS AND SCIENCES. 
57 
Si 
A1 Fe 
Fe Mg 
11 
[37.58 
19.73 5.95 
0U38 25.13 
11.09 = 100.06 
(1) 
20.04 
9.19 1.78 
0.14 10.05 
9.86 
20.04 
10.97 
10.19 
9.86 
2.03 
1.11 
1.03 
1. 
[37.43 
19.75 5.95 
0.58 25.58 
11.09 = 100.38 
(2) 
10.96 
9.20 1.78 
0.14 10.23 
9.86 
19.96 
10.98 
10.37 
9.86 
2.03 
1.11 
1.05 
1. 
[37.10 
20.22 5.95 
0.58 25.07 
11.09 = 100.01 
(3) ■ 
19.79 
9.42 1.78 
0.14 10.03 
9.86 
19.79 
11.20 
10.17 
9.86 
2. 
1.13 
1.03 
1. 
lias published two analyses of the Culsagee vermiculite, made in his laboratory 
by Mr. Chatard and Dr. Koenig. Their results agree very closely with those 
given above, except in the amount of water ; and since, as Dr. Genth states, the 
analyses were made with material which had been dried in vacuo over sulphuric 
acid for only four days, it is obvious that they have estimated as combined 
water a large portion of what we, for reasons stated, have regarded as hygro- 
scopic moisture. Reducing their results to the same standard as our own, 
the close agreement becomes evident. We give, for comparison, first the 
results, as published ; second, the same reduced to a water percentage of 11%; 
third, the corresponding oxygen or atomic ratios. 
Si 
5:1 Fe 
Fe 
tfi 
Mg 
H 
a 
33.93 
17.38 5.42 
0.50 
0.35 
23.43 
19.17 = 100.18 
« 
< 
37.33 
19.12 5.94 
0.55 
0.38 
25.77 
11.09 
< 
B 
19.91 
8.91 1.78 
0.12 
0.07 
10.31 
9.86 
19.91 
10.69 
10.50 
9.86 
2.02 
1.06 
1.05 
1. 
33.77 
17.56 5.61 
0.50 
22.48 
20.30=100.22 
37.65 
19.58 6.26 
0.56 
25.06 
11.09 
w 
20.08 
9.12 1.88 
0.12 
10.02 
9.86 
20.08 
11.00 
10.14 
9.86 
« 
2.04 
1.11 
1.03 
1. 
The close agreement of these results with our own, obtained by different 
processes and with different specimens of the mineral, indicates a remarkable 
constancy of composition for a micaceous mineral. I have confirmed Dr. Genth’s 
observation of the presence of a very small amount of nickel in the mineral, 
and I also detected in it a trace of copper. 58 
PROCEEDINGS OF THE AMERICAN ACADEMY 
It is evident from these analyses that the atomic ratio of the mineral 
is 2 :1 :1 :1, and its formula may therefore be written: — 
H3,R.012.Si3.3H20. 
By referring to what has already been said of the relation of Jefferi- 
site to the Biotites, it will be seen that, while that mineral corresponds 
to a less common variety of this species, the Culsagee vermiculite cor- 
responds to its more usual type. We give below, at (l),the results of 
an analysis of Vesuvian Biotite, by Chodnef; and, at (2), the same, 
assuming that a portion of the iron is in the ferrous condition, as is 
well known to be the case, in order to show that values within the 
probable error of the analysis would give the ratio 2:1:1. Here, of 
course, the alkali takes the place of a portion of the magnesia of the 
vermiculite. 
Si 
A1 
Fe 
Fe 
Mg 
Ca 
K 
0) 
40.91 
17.19 
11.03 
19.04 
0.30 
9.96 = 
= 98.43 
(2) 
40.91 
17.19 
7.03 
4.00 
19.04 
0.30 
9.96 = 
= 98.43 
21.82 
8.30 
2.11 
.89 
7.62 
0.09 
1.69 
21.82 
10.41 
10.29 
2.12 
1.01 
1. 
This new variety of vermiculite is so well marked, and the compo- 
sition so definite, that I have thought best to designate it by the name 
Culsageeite. As regards its other characters, it has a specific gravity 
of 2.225 (taken in alcohol), and about the hardness of talc. Before 
the blowpipe the exfoliated mineral fuses readily to a white enamel, 
but does not fuse in the flame of a Bunsen lamp. 
To this variety of vermiculite belongs, as I have already intimated, 
the original mineral from Millbury, analyzed by Crossley. There can 
be no question as to the general accuracy of Crossley’s results; and, 
assuming that all the iron is in the ferric condition, they give, as I 
have shown, almost precisely the atomic ratio 2: 1 : 1 : 1. The only 
question that can arise is in regard to the condition of the iron. I 
have therefore made an assay of the iron by the accurate method I 
formerly described in the Am. Jour. Sci., xliv. 347, 1867. One hun- 
dred parts of the massive mineral gave 7.40 per cent ferric oxide, and 
3.86 per cent ferric oxide, which corresponds to a total of 10.56 ferrous 
oxide. Crossley found 10.02 ferrous oxide, but he separated with great 
care the vermiculite from the steatite with which it is mixed; and this 
steatite was probably the source of the greater part of the small quan- 
tity of ferrous oxide found in our assay, which, though not made with OP ARTS AND SCIENCES. 
59 
pure material, shows conclusively that the condition of the iron in the 
Millhury vermiculite is not different from that in other varieties of the 
same family of minerals. 
Hnllite. — Several years since this variety of vermiculite was sent 
to me by Mr. Johu Hall, of Philadelphia, by whom it was originally 
discovered. The examination then made showing that the mineral was 
a new variety, if not a new species, of the vermiculite family, I gave 
to it the name of Hallite, in recognition of the mineralogical services 
of Mr. Ilall, who not only discovered the mineral, but has carefully 
worked the locality and observed the associations in which it is there 
found. A preliminary notice of the mineral under this name was 
published at the time by Professor Leeds, of Hoboken; but the in- 
teresting relations which the mineral bears to the subject of this paper 
have made a further examination desirable. 
Hallite occurs in large rough six-sided prisms, with easy micaceous 
cleavage. There are two varieties, differing markedly in color, green 
and yellow; and I am indebted to Mr. Hall for the following facts in 
regard to the locality and associations of this species. Mr. Hall writes : 
“ The mineral is found at East Nottingham, in the serpentine formation 
of south-eastern Pennsylvania, three miles south of Oxford, in Chester 
County; and 1 know of no other locality. I think the green and 
yellow varieties are very closely related, and may possibly pass from 
one into the other; but I have no positive proof that they do. The 
crystals are found in nests or pockets, and the two colors are not 
found in the same nests. The green crystals are imbedded in a stea- 
tite earth or base of the same color as the crystals, and the yellow in 
a yellow earth ; and sometimes nests, containing the opposite varieties, 
are oidy a few feet apart in seams of the serpentine rock.” 
As the following analyses show, the two varieties have essentially 
the same composition, and the only difference that could be detected 
was in the degree of oxidation of the iron. The yellow crystals appeared 
to be more weathered than the green, and on the last the green color 
frequently fades out towards the centre of crystals, thus giving indica- 
tions of a metamorphosis by which one variety may pass into the other. 
Under the microscope these scales of the mineral show a remarkable 
appearance. Between the greenish or nearly colorless plates are seen 
elongated scales of a yellow mineral resembling closely in color thin 
scales of Jefferisite. They are more or less spear-shaped in form, al- 
though usually very narrow, and lie accurately in parallel lines, which 
cross at angles of G0° and 120°, like the magnetic oxide of iron in the 
Muscovite from Pennsbury, Pa., or the microscopic crystals in the 60 
PROCEEDINGS OF THE AMERICAN ACADEMY 
Biotite of South Burgess of Canada ; and the phenomenon of asterism, 
seen so beautiful with the plates of the last, can also be seen with thin 
laminae of Hallite. It was impossible to free the mineral from this 
admixture, but specimens were selected for analysis as free from it as 
possible. It was also impossible to determine its exact nature. The 
scales had not a definite form, but there was a tendency to a rhombic 
shape, which is well described by the term “ spear-shaped and though 
the material is so widely distributed through the crystal, the total mass 
must be very small. 
This mineral is not so hygroscopic as Jefferisite, and no difficulty was 
found in drying the material for analysis. When ignited, it exfoliates 
like other species of vermiculite, but not nearly to so great an extent as 
Jefferisite. After ignition it is decomposed by hydrochloric acid. The 
specific gravity of the green variety, mean of four determinations, 
2.398; that of the yellow variety, mean of two determinations, 2.402. 
Before the blowpipe fuses with difficulty to a brown enamel. The 
following analyses were made by Mr. C. E. Munroe, Assistant in the 
Laboratory of Harvard College :— 
Green Variety of Hallite. 
Si 
A1 
Fe 
Fe 
Mg 
K 
H 
(1) 
35.97 
7.61 
8.83 
1.13 
31.34 
0.43 
14.32 = 99.63 
(2) 
35.80 
7.29 
8.73 
1.13 
31.56 
0.49 
14.33 = 99.33 
(Mean.) 
35.89 
7.45 
8.78 
1.13 
31.45 
0.46 
14.33 = 99.49 
19.14 
3.47 
2.63 
0.25 
12.58 
0.08 
12.74 
19.14 
6.10 
12.91 
12.74 
3. 
0.96 
2.02 
2. 
3. 
1. 
2. 
2. 
Yellow Variety 
of Hallite. 
Si 
A1 
Fe 
Fe 
Mg 
K 
H 
0) 
35.17 
7.74 
9.76 
0.32 
31.61 
0.56 
14.65 = 99.81 
(2) 
35.34 
7.42 
9.61 
0.32 
31.41 
0.65 
14.91 = 99.66 
(Mean.) 
35.26 
7.58 
9.68 
0.32 
31.51 
0.61 
14.78 = 99.74* 
18.81 
3.53 
2.90 
0.10 
12.60 
0.10 
13.14 
18.81 
6.43 
12k) 
13.14 
3. 
1.03 
2.04 
2.09 
3. 
1. 
2. 
2. 
®2 ' 
0;v.. /Si, 
g . \2H20. • 
* Trace of manganese. OF ARTS AND SCIENCES. 
61 
It will lie seen from the above analyses that, although the atomic 
ratio between all the basic radicals and the silicon is the same as in 
Culsageeite, Jefferisite, and liiotite, the ratio between the. protoxide 
and sesquioxide radicals is very different. In this respect the mineral 
resembles the phlogopite micas, in which also the protoxide radicals 
preponderate; and the symbol given above for Ilallite, less the water, 
is identical with that given by Professor Dana as the more probable 
formula of the phlogopites. 
The opacity produced by the interspersed material made it difficult 
to determine the optical characters of the mineral, as the rings produced 
with polarized light could only be seen with very thin plates, and the 
cross was therefore ill defined; so that, although in some cases there 
appeared to be a separation of the hyperbolas, the plates could not be 
distinguished from uniaxial. On one specimen the hexagonal form was 
very perfect, and the crystal presented the planes of a rhombohedron 
having an angle over the basal edge of about 122°, resembling the 
crystals of liiotite from Greenwood Furnace. Mr. Hall informs me 
that these more perfect crystals have only been found in one pocket of 
the serpentine. 
The distinction, however, between the phlogopites and the biotites is 
not fundamental, either chemically or physically. Chemically, both 
species are orthosilicates; that is, the atomic ratio between the silicon 
and the sum of the basic radicals is 1:1. The species differ in com- 
position only in the relative proportion of the sesquioxide and protoxide 
II VI 
radicals. In the phlogopite the ratio of R to is probably normally 
2:1; but of the published analyses the value varies between that ratio 
and the ratio 3 : 2. In the biotites the same ratio is probably normally 
1:1; but here, again, the different analyses which have been made give 
values varying between 5 : 3 and 1 : 2. In like manner the optical 
distinction between the phlogopites and biotites, of which so much has 
been made, is equally indefinite. Between a so-called phlogopite, like 
that from Jefferson County, N. Y., with an angle of about 15°, and the 
apparently uniaxial plates of biotite from Vesuvius, there is every possible 
gradation — sometimes, as I have shown, on one and the same mica plate ; 
and I have endeavored in this paper to explain the cause of this varia- 
tion. With the Vesuvian biotites themselves, — if the specimens in the 
uiineralogical cabinet of Harvard College are fair representatives of 
the mineral from this locality, — it is only occasionally that we find a 
perfectly uniaxial plate. More commonly there are distinct evidences 
of twinning, and on the borders of the hexagonal plat'® may be dis- 62 
PROCEEDINGS OP THE AMERICAN ACADEMY 
covered a biaxial structure of which the optical plane is parallel to 
different edges of the hexagon on different parts of the plate. 
It must, however, he remembered that, as by the process of twinning 
we have described the structure of the magnesian micas approaches 
that oflnaxial crystals, rhombohedral and other planes characteristic of 
the hexagonal system begin to appear on the crystal. This is illus- 
trated not only by the crystals of Biotite from Vesuvius and from 
Greenwood Furnace, N. Y., hut also by the more perfect crystals of 
Ilallite from Chester County, Pa. In other words, the process of twinning 
we have illustrated in this paper produces hexagonal crystals in external 
form as well as in optical characters ; and the question naturally arises, 
May not the hexagonal crystals of other minerals he formed in a similar 
way ? — that is, may they not be developed from twinned molecules, 
which, though in their aggregate producing an hexagonal structure, 
singly would develop into biaxial crystals ? Bearing on this point, we 
have discovered some very remarkable evidence. 
We have in our possession a plate of Elba tourmaline cut perpen- 
dicular to the axis, in which the polarizing microscope shows on 
different zones a separation of the hyperbolas, which amounts in 
some positions to eight degrees; and in moving the plate across the 
field the optical divergence varies precisely as on plates of phlogopite 
and vermiculite. There is certainly no external evidence of lamination 
on tourmaline crystals, for the mineral is remarkably compact, and the 
crystals have not even a basal cleavage : but it will be remembered how 
readily some of the varieties pass by alteration into micas of the mag- 
nesian type; and this change to a foliated structure, in which the 
lamination is parallel to the base of the original hexagonal crystal, 
may be facilitated by a grouping of the molecules of the tourmaline, in 
the manner represented by Fig. 7. 
We have also a plate of amethystine quartz, in which a beam of 
parallel polarized rays of light exhibits a twinning almost as symmetrical 
as that shown in Fig. 5, — the three zones being most beautifully mapped 
out by the alternating bands of right and left handed quartz, which are 
such a familiar phenomenon of these crystals ; but, besides this, in 
each of these zones, near the border of the plate, can be distinguished 
a biaxial structure with an optical divergence of several degrees; and, 
on one other plate of amethyst we have had an opportunity of exam- 
ining, we have also seen under the polarizing microscope the biaxial 
curves at one or more points. 
These facts most distinctly suggest the theory that the optical phe- 
nomena of quartz are produced by a molecular structure similar to that OF AHTS AND SCIENCES. 
63 
by which we have obtained identical phenomena in our artificial plates 
of mica, and that the two orders of crystals are aggregates of compound 
molecules, whose parts are twinned together iu the one case in right- 
handed, and in the other in left-handed spirals, and, lastly, that the 
simple molecule, if developed normally, would produce a biaxial 
structure.* This theory is most markedly in harmony with the chemi- 
cal relations of silica. The compound Si0.2 is the only one of the 
tetrad oxides which crystallizes in the hexagonal system; and ever 
since, by the study of the organic compounds of silicon, the quadriva- 
lent character of the element has been made evident, this fact has been 
a striking anomaly in our chemical classification. Assume, however, 
that the molecule Si02 would develop normally into a rhombic struct- 
ure, and that the hexagonal form of quartz is solely a result of molec- 
ular twinning, and the anomaly disappears. The molecule SiO., may 
be approximately of the same form as the molecule Ti0.2, in Krookite; 
but, having the exact dimensions and polar conditions which favor the 
mode of molecular twinning, described above and represented by Fig. 7, 
it may always develop into hexagonal shapes. 
Are, then, all hexagonal forms thus closely related to the rhombic 
systems of crystals ? And do all molecules of the dimensions and 
polar conditions illustrated by the figures of this article — that is, 
those which correspond to the rhomb of 60° and 120° — usually 
develop into hexagonal forms ? May not the whole difference between 
an hexagonal and a rhombic form arise from a slight difference of dimen- 
sions, which determines a molecular macHng in the one case, and a 
normal development of the single molecules in the other ? These 
questions point out most interesting lines of investigation, and will 
recall to the mineralogist a number of facts bearing upon the subject. 
Allow me to refer to two of the most striking and most obvious. 
On the crystals of ehrysoberyl, the rhombic angle is 119° 4G'; 
and every mineralogist is familiar with the hexagonal macling, similar 
* Since the above was in type, we have received Am. Jour. Sci., IV., Feb- 
ruary, 1874, containing a description of the rhombic silica which Professor Mas- 
kelyne, of the British Museum, has discovered iu the meteorite of Breitenbach. 
This new species of silica, which Professor Maskelyne calls Asmanite, has the 
form of a right rhombic prism, with an angle of 120° 20', and the crystals are 
optically biaxial; but while the specific gravity of quartz is 2.6, that of Asmanite 
is said to be 2.245. It is perhaps to be expected that such a molecular macling 
as we have described would determine an increase of density, since thereby three 
molecules coalesce to form one; or it is possible that the remarks made in regard 
to caleite beyond apply also to quartz ; but still the marked difference remains 
to be explained. 64 
PROCEEDINGS OF THE AMERICAN ACADEMY 
to Fig. 2, which is so very characteristic of this species.* Corundum 
differs chemically from chrysoberyl, in that a portion of the alumina 
in the former is replaced hy glucina in the latter. Corundum has a 
perfect hexagonal form, and, fundamentally, may not the only crystal- 
lographic difference be that, in consequence of the replacement, a 
rhomb of 120° changes to a rhomb of 119° 4G'? Now we have a 
plate of macled chrysoberyl, showing the normal wide divergence of 
the optical axes at certain points on its borders, and a nearly uniaxial 
structure at the centre, where there is an obvious interpenetration 
between the individuals of the made, and where the superposition of 
the several laminae is most beautifully shown by a polarized beam of 
parallel rays. We have also a section of a corundum crystal, present- 
ing phenomena similar to those seen with the plate of tourmaline, 
described above. Further, we have observed like phenomena on a 
section of phenacite; and, although the last mineral contains silica, 
yet if the molecules of Si02 are crystallographically equivalent to 
those of Al2 031 it may be that the molecular structure both of phena- 
cite and of beryl is more closely allied to that - of chrysoberyl and 
corundum than the received theory of their chemical constitution would 
indicate. 
We would not convey the impression that in all these crystals the 
appearances we have described are strongly marked, or that they have 
passed wholly unnoticed hitherto. Every one who has become familiar 
with the optical properties of crystals must have noticed that, with 
many always regarded as uniaxial, there is not unfrequently in some 
positions a small separation of the cross into the hyperbolas, which are 
characteristic of biaxial structure. But these irregularities, although 
long known, have never been satisfactorily explained. They have 
been hitherto residual features not accounted for by the received theory 
of crystalline structure, which explains so satisfactorily the general 
order of the phenomena observed with the polariscope. We have 
endeavored in this paper to trace their true significance: first, by 
showing that the appearances we are discussing are precisely similar 
to the effects which can be obtained by known means with mica 
plates; and, secondly, by observing on different specimens of various 
minerals every intermediate stage between the unmistakable effects 
of twinning on plates of mica or vermiculite, and the delicate phases 
of the same phenomena, seen with sections of crystals of tourmaline, 
corundum, or phenacite. One other illustration of our theory. 
* See also Dana’s System of Min., 5th ed., Figs. 154, 155, p. 156. OF ARTS AND SCIENCES. 
65 
The rhombic angle of witherite (native baric carbonate) is 118° 30', 
and the all but universal hexagonal macling of crystals of this species is 
a well-known fact.* The rhombic angle of aragonite (the corresponding 
form of calcic carbonate) is 11 G° 10', and the much greater divergence of 
this angle from 120° determines, as is also known, a style of macling 
which is usually quite different from that of witherite. In the isomeric 
calcite, however, we have the type of all hexagonal forms. Hitherto 
the crystalline forms of calcite and aragonite have been regarded as 
being as widely separated as possible, and a comparison of these two 
well-known mineral species has furnished one of the most striking 
illustrations of demorphism. But may not, after all, the comparatively 
small physical differences between these two minerals correspond to a 
crystallographic difference no greater, fundamentally, than the difference 
between the rhomb of 116° 10' and the rhomb of 120° ? 
The macles of chrysoberyl and witherite are illustration of a general 
truth, fully recognized in mineralogy, that all rhombic crystals, whose 
angles approach 120°, tend to form hexagonal macles. The optical 
phenomena described in this paper certainly suggest the theory that 
a perfect hexagonal form and structure may be the result of a more 
fundamental and molecular macling, which results when the angle is 
exactly 120°. 
Observations of Senarmont.—The only previous observations which 
we have been able to find bearing on the subject of this paper are 
those of the late eminent mineralogist, II. de Senarmont, of Paris. In 
a well-known paper (Ann. Chim. et Phys. 3? xxxiii. 391), Senarmont 
showed that salts which were both geometrically and chemically iso- 
morphous might have very different optical relations; for example, 
that, while the biaxial crystals of two such salts might have the same 
bisectrix, the plane of the optical axes in the crystals of one might be 
perpendicular to the corresponding plane in those of the other, lie 
further proved, by crystallizing together two salts so related, that in 
the crystals of the isomorphous mixtures thus obtained the optical 
angle varied with the varying proportions of the constituents, between 
the extreme conditions in the crystals of either salt; and by trial he 
succeeded in forming from two biaxial salts crystals which, in mono- 
chromatic light at least, appeared uniaxial. In a later paper (Ann. 
Chim. et Phys. 3? xxxiv. 171), Senarmont applied the principles, 
which he had thus experimentally verified, to explain the variation of 
the optical angle of the micas. In this paper he seeks to prove, first, 
* See figures, Dana’s System of Min., p. 697. 66 
PROCEEDINGS OP THE AMERICAN ACADEMY 
that all micas may be referred to a right rhombic prism with angles of 
approximately 60° and 120° ; secondly, that, while 
in some micas the plane of the optical axes is 
parallel to the shorter diagonal of the rhomb, as in 
Fig. 8, in others it is parallel to the longer diago- 
nal, as in Fig. 9. Interpreting these facts by the 
results of his experiments on isomorphous salts, lie 
draws the inference that there are, cry stall ©graph- 
ically at least, but two species of mica which are 
geometrically isomorphous but optically distinct, 
that these are represented by the varieties which 
have the widest optical angle between the axes in 
either plane, and that all other varieties with opti- 
cal angles varying from 0° to 70° in either direc- 
tion are isomorphous mixtures of the two optically 
distinct conditions of the mineral. 
The observations described in this paper, al- 
though they prove that another cause may also determine the variation 
of optical angle in micaceous minerals, do not necessarily invalidate this 
beautiful theory of Senarmont. The variations observed with other min- 
erals, not only on different specimens, but with the same specimen at dif- 
ferent temperatures, and which are so beautiful seen with the orthoelase 
from Wehr in the Eifel, and with crystals of selenite, indicate that such 
variations may be determined by conditions of molecular structure 
wholly independent of the macling here described. We have shown 
that the macling does produce the Variation in certain cases, and it 
must remain for future investigation to assign the limits of the influence 
which this cause may exert. We would only remark in conclusion that, 
although in the 57 varieties of micas examined by Senarmont he did not 
note a single instance in which the position of the plane of the optical 
axes — with reference to the diagonal of the rhomb — was different 
on different parts of the same specimen, or even on different speci- 
mens from the same locality, he does describe and figure several 
remarkable macles of muscovite mica similar to those of vermiculite 
described above and represented by Fig. 5. In one instance the plane 
of the optical axes is parallel to the shorter, and in the other to the 
longer diagonal of the rhombic prism; but in both cases it has the 
same relative position in the several individuals of the macles, the 
three planes forming with each other angles of 60°. On the plate of 
another made a difference of optical angle was observed on different 
portions of the plate, and this effect was probably similar to that we 
Fig. 8. 
Fig. 9. OP ARTS AND SCIENCES. 
67 
have been studying in this paper. On another plate, from the same 
crystals he observed a superposition of the laminae of the different 
individuals of the made; and the following language by which this 
phenomena is described, and which is the only reference made to it, 
shows how closely he came to the results recorded in this paper: — 
“ Une partie seulement du cristal made est commune aux trois 
lames; mais ici certaines plages (pointillees sur la figure) ont un carac- 
tere optique tout particulier ; elles ne cessent jamais de developper des 
couleurs dans la lumiere polarisee, quelle que soit d’ailleurs l’orienta- 
tion de la lame cristallisee. II est evident que dans ces plages il y a 
sujterposition de lames appartenant a des cristaux orientes, les uns 
comme le petit, les autres comme le grand cristal, de faejon que quand 
les unes ont leurs sections principales dans le plan de polarisation, les 
sections principales des autres font avec ce plan des angles de 60 
degres. 
“ Cet exemple prouve done, non-seulement que les micas peuvent se 
grouper lateralement sans que leurs clivages cessent d’etre paralleles, 
mais que des feuillets superposes peuvent raeme appartenir a des cris- 
taux dont l’orientation differe de 60 degres. Un pareil mode de 
groupement, qui ne trouble ni la regularity de la cristallisation, ni la 
transparence, semble indiquer que l’arrangement moleculaire de ces 
prismes rbomboidaux differe tres-peu de celui qui conviendrait au 
prisme hexagonal regulier.”