Prof. J. P. Cooke on the Vermiculites. 255 



long been known in chemistry as water of crystallization. We 

 shall not here attempt to discuss what are the relations of the 

 water thus combined to the molecular 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 efflorescence 

 of Glauber's salt or the intumescence of alum. 



If, then, I am justified in the inferences I have drawn, Jeffer- 

 isite 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 tempera- 

 ture. In these bodies, 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 magnesian micas. It is, how- 

 ever, also true that the crystalline structure of the micas seems 

 to admit of a very wide variation of chemical composition. In 

 cryophyllite, which has all the crystallographic characters of the 

 micas strongly developed, the atomic ratio is 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 varie- 

 ties of mica have a composition intermediate between these ex- 

 tremes ; 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 ap- 

 pears to be related to a mode of molecular structure common to 

 a great many substances ; and the phenomena which will be de- 

 scribed 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 : — (1) that the crystallographic 

 structure of Jefferisite is identical with that of the magnesian 

 micas; (2) that the chemical constitution of the anhydrous 

 mineral is closely allied to that of Biotite. 



Culsageeite, or the Vermiculite of the Jenks Mine, North Caro- 

 lina. — Through the kindness of Colonel C. W. Jenks, the pro- 

 prietor 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 in Pennsylvania. 



