from the Zillerthal, by Shafhäutl (No. 1), and one of a mica, from Fahlun, by H. Rose (No. 2);—

Si

Al. Fe. Cr. 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

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 3:21 and 4:3: 1. Corresponding to these, it is probable that there are two varieties of anhydrous mica, containing an excess of SiO2, 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 previous paper (Amer. Journ. Sci. (11.) vol. xii. p. 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,

H404 Si and H2=02=SiO;

and he there described a mica whose atomic ratio was very closely that of the second type, and which he called Cryophyllite. Now the simplest theory of the relation of the hydrous to the anhydrous Muscovites 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 (H2-02-SiO), the hydrous micas contain sufficient basic hydrogen to bring all the silicon atoms into the condition of the normal hydrate (H404 Si). The two graphic formula which follow indicate more clearly than any other language can, the relations we have attempted to describe. In these formulæ R stands for the double atom in the radical of the sesquioxides, having the quantivalence of six :

The new mineral Sterlingite, whose examination has been the occasion of this discussion, is remarkable as being a very welldefined example of a hydrous mica occurring in large crystals, and exhibiting very marked characters. It does not materially differ in composition from Damourite; and it also agrees with the specimens of this mineral so closely in other physical qualities that we cannot regard the small optical angle, observed by Des Cloizeaux 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 connexion with the subject of this paper, be

cause they illustrate the characteristics of basic water, which will be contrasted with those of water of crystallization in our description of the following 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†.

Jefferisite, of West Chester.-This well-known mineral, found in the serpentine at West Chester, Pa., was, as I have said, carefully analyzed by Professor Brush, of Newhaven, 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 relations I have studied.

The crystals of Jefferisite cleave like mica, affording thin but unelastic laminæ. 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 14 inch high by 2 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 Plate III. fig. 6, 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 have measured angles on different plates, of 27°, 24°, and 10°, and observed many intermediate conditions. Owing to the deep yellow colour,

*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 corundum, 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.

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 two hyperbolas, due to the thinness of the plate. The dispersion of the axes is but slight, and only perceptible in the thicker lamina when p<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 twinning is the cause of this most remarkable variation in the optical angle, as will be explained at length in connexion 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:-(1) the results of the analysis of Jefferisite by Professor Brush; (2) the same results, calculated for the anhydrous mineral; (3) the results 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 :

Fe. Mg. Ca. K. II.

1.26

28

(1) 37.10 17.57 10.54 19.78 8.18 3.16

19.65 0.56 0.43 13.76=100.87 7.86 16 07 12.23

Si. Äl. Fe. Fe. Mg. Ča. K.

(2) 42.94 20.33 12.20 1.46 22.75 22.90 9.47 3.66 32

66 •50

=100.84

9.10

19 08

Fe. Mn. Mg.

Si. Al. (3) 42.58 21.68 10.39 75 10·27

1·04 8·45 3.35 ·51=99.02 22.71 10.10 3.12 17 4.11 30 1.43 2.98

Ca. K. H.

F.

The general symbol of Jefferisite deduced from (1) would be

II VI

R4, R2. O20. Si5. 6 H2O.

A comparison of the results given in (2) and (3) will show that the anhydrous Jefferisite corresponds very closely in its chemical constitution 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 connexion that the Biotites present a very wide variation in the ratio between the amounts of the protoxide and sesquioxide radicals which the various varieties contain. The limits usually assigned to this variation correspond to the ratios

R:R:Si=1:2:3, and R: R: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 common class of Biotites, which have the ratio 1: 1: 2.

But this resemblance in chemical constitution only appears when we compare the Biotite 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 (124 per cent.) which the crystallized mineral contains?

To aid us in forming a conclusion on this point, we have the following evidence:

First. As the above analysis shows, the water is united in definite and atomic proportions amounting to six molecules to every five molecules 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 observations 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 has