Prof. J. P. Cooke on the Vermiculites. 251 



H Si K 



i mi i 



O 4 

 H-O-Si-O-R-O-Si-O-K 



6 6 



Si=0 4 §R Ki0 4 =Si 



6 6 



K-O-Si-O-R-O-Si-O-H 



| 1111 I 



O O 4 o 



I mi I 



K Si H 



Hydrous Muscovite . 



Si K 



till I 



O 4 

 O-Si-O-I-O-Si-O-K 



6 o 



Sii0 4 §R Kf0 4 §Si 



6 6 



K-O-Si-O-R-O-SirO 



I mi 



O 4 



1 111) 

 K Si 



Anhydrous Muscovite. 



The new mineral Sterlingite, whose examination has been the 

 occasion 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 Damourite ; and it also agrees with 

 the specimens of this mineral so closely in other physical quali- 

 ties that we cannot regard the small optical angle, observed by 

 Des Cloizeaux on the minute scales of the Pontivy mica, as suffi- 

 cient 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 de- 

 signating it. 



The hydrous micas, of which Sterlingite is a variety, have a 

 special interest in connexion with the subject of this paper, be- 



