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LONDON, EDINBURGH, AND DUBLIN

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FOURTH SERIES.]

APRIL 1874.

XXXI. The Vermiculites; their Crystallographic and Chemical Relations to the Micas; together with a discussion of the Cause of the Variation of the Optical Angle in these Minerals. By JOSIAH P. COOKE, Jun., Erving Professor of Chemistry and Mineralogy in Harvard College*.

I

[With a Plate.]

To Professor Tyndall, F.R.S.

Chemical Laboratory of Harvard College,
Cambridge, Feb. 15, 1874.

paper

MY DEAR PROFESSOR, SHALL send you by this mail the advance sheets of a of mine to be published in our Academy Proceedings, giving an account of some new observations, which I am sure will greatly interest you. The mineralogical details of the paper will not probably appeal to you; but they also have importance as bearing on the optical question. You will find that I have succeeded in constructing uniaxial crystals of the hexagonal type from rhombic crystals of 60° and 120° angle, and have also been able to show that, in some cases at least, hexagonal crystals are so constructed in nature. You will find, further, that I have been able to reproduce the optical conditions of a quartz crystal, and in such a way as to be able to point out the mode of molecular structure by which the result is produced. It would be unnecessary to recapitulate these points, which you will find fully stated in my paper, were it not that you might otherwise be repelled by the mineralogical crust with which the optical results are necessarily enclosed.

* From the Proceedings of the American Academy of Arts and Sciences, read December 9, 1873. Communicated by the Author.

Phil. Mag. S. 4. Vol. 47. No. 312. April 1874.

R

In order that you may have the evidence on which my conclusions rest, I send several specimens:

1. A plate of magnesian mica (phlogopite) from Jefferson Co., New York, with optical angle of about 14°.

2. Macle of the same, showing planes of optical axes inclined to each other, on different parts of the plate, at an angle of 60°, and also showing variation of optical angle in passing from one part of the plate to the other.

3. Compound plate essentially uniaxial, made by splitting plate like No. 1 and piling up the lamina in alternating positions at angles of 60°.

4. Plate of Muscovite mica with optical angle between 60° and 70°.

5. Compound plate showing left-handed circular polarization, formed from last by piling thin lamina ( of an inch about) in left-handed spirals.

6. Compound plate showing right-handed circular polarization, formed from films of the same mica as 4 by piling in right-handed spirals. By placing 6 on 5 (point marked a on a) with the edges of the glass of 6 parallel with the pencil lines on 5, you will see, in certain azimuths with the plane of polarization, the Airy spirals beautifully reproduced.

I am sorry that the last two specimens (by far the most interesting) are not more perfect. Neither of them presents constant features at different azimuths with the plane of polarization; but No. 5 is more perfect than No. 6; and a comparison of the two will show how I have approximated to the perfect result in proportion as I have succeeded in making the laminæ at the same time sufficiently thin and of uniform thickness. Plates as good as No. 5 can be readily prepared; but to do better than this, except by accident, involves selecting the lamina by actual measurement with a spherometer; and since the laminæ must also be taken from one and the same piece of mica and cut in parallel positions, you can easily see that, even after succeeding in cleaving the mica into sufficiently thin films, you may work for many days before securing at least twelve laminæ from the same film of uniform thickness. These specimens, however, will illustrate my paper, if not by what they are in themselves, at least by that to which they point and closely approach. My observations were made with a polarizing microscope designed by Des Cloizeaux and made by Soleil. The object is illuminated by a very strongly converging beam of polarized light; and the instrument brings into the field at once the two hyperbolas of a plate of topaz with an optical angle of over 80°. An instrument, however, which will include in the field the two hyperbolas of the plate of Mus

covite mica I send will show the effects I have described with the speciments sent with this.

Introduction. In the American Journal of Science, vol. vii. p. 55 (1824), T. H. 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 distributed through a steatitic base. . . . When heated, it exfoliates prodigiously, the scales opening out into long worm-like threads made up of the separate laminæ. Exfoliation commences at 500° to 600° Fahr., and takes place with so much force as often to break the testtube in which the mineral may be confined. Before the blowpipe it fuses at 3.5 to a greyish-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 colour greyish, 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:

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The results of analysis in column 1, and the portions of the description of the mineral in quotation-marks, above, have been taken from Dana's 'System of Mineralogy,' fifth edition, p. 493; and the atomic ratio which is there deduced is

IV VI II II

Si: R:R: 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. Jefferis, of West Chester, Pa., discovered at the ripidolite locality near that town a peculiar micaceous mineral which exfoliates like the Millbury vermicu

lite, but which, instead of occurring in small laminæ, 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 (Amer. Journ. Sci. (II.) vol. xxxi. p. 369, 1861), was subsequently described as a new species (Amer. Journ. Sci. (II.) vol. xli. p. 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 connexion 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 examinéd; 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 minerals under consideration undergo when heated are analogous to the well-known phenomena presented in the dehydration 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 causenamely, to the escaping of what we call water of crystallization. I also expect to show that the several minerals referred to above are members of a family of hydrous silicates closely allied and parallel to the well-known family of anhydrous silicates called the micas, and that their molecules differ from those of the magnesian micas chiefly in containing a definite number of molecules of water—that is, water of crystallization. I shall call this family of minerals" the vermiculites," using the original name, as "mica" is now employed, to designate a class; and I shall call the three species (or varieties?) of this family Jefferisite, Culsageeite, and Hallite, 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

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