388 F W. Clarke— Theory of the Mica Group. 



planted upon the latter, in such a way as to clearly indicate its 

 later formation. In composition it is regarded by Tschermak 

 as a mixture of Al.K.S^O^ with F 24 Si 10 O e , the former being 

 about half replaced by the corresponding AlgLi.SijO^, and the 

 latter in part by H 8 Si 10 O, 4 . The objections to this interpreta- 

 tion have already been pointed out. 



Under the new mode of interpretation, lepidolite becomes 

 much simpler. In every case, if we eliminate traces of mag- 

 nesia and iron, as was done under muscovite, the residue corre- 

 sponds sharply to a mixture of the two molecules A1X 3 R' 9 with 

 A1 3 X 3 R' 3 , X representing Si 3 O e and Si0 4 indiscriminately. In 

 the purest lepidolites these molecules are in the ratio 1 : 1, 

 corresponding to Al 2 X 3 B/ 6 ; but in general the second molecule 

 is slightly in excess, due to small admixtures of normal musco- 

 vite. In the discussion of fourteen published analyses of 

 lepidolite the ratio Si 3 8 : Si0 4 varies from 1:1 to 1:3; and 

 the sum of Li + A1F 2 appears to be directly related to the pro- 

 portion of Si 3 8 . In brief, expanding the general formulae 

 given above, typical lepidolite may be expressed by the two 

 compounds Al 3 (Si0 4 ) 3 KHLi and Al(Si 3 O e ) 3 K 3 Li 3 (AlF 2 ) 3 in equal 

 molecules. All the variations may be accounted for by admix- 

 tures of muscovite. 



Two other highly fluoriferous lithia micas, cryophyllite and 

 zinnwaldite, may properly be considered here. For the former 

 we have Riggs's analysis, and for the latter, analyses by Ber- 

 werth and Rammelsberg. From these analyses we get the 

 following empirical formulae : 1, Riggs, cryophyllite ; 2 and 

 3, Berwerth and Rammelsberg, zinnwaldite. 



1. Al 186 Fe" 94 K a56 Li S24 H 46 (AlF a ) 178 (Si s 8 ) m (Si0 4 ) 1 , B . 



2- Al 3S9 Fe" ]Sfi K 236 Li 2IB H 102 (AlF 2 ) 209 (Si 3 O 6 ) 161 (SiO 4 ) 312 . 



3- Al 244 Fe" 16T K a4a Li 2a4 H ll6 (AlF a ) S00 (Si s O 8 ) 156 (SiO 4 ) 30a . 



Condensing these formulae they become : 



1. Al 186 Fe 94 R' 904 X 412 . 



2. Al 239 Fe 166 R' 766 X 463 . 

 3- Al 244 Fe 167 R 782 X 462 . 



Expanding again, these formulae give : 



1. 31(AlX 3 Fe" 3 R' 3 ) + 81(AlX 3 R' 9 ) + 25(Al 3 X s R' 3 ). 



2. 62 " +49 " +43 " 



3. 56 " +53 " +45 " 



The component molecules here correspond essentially to Nos. 

 6, 3, and 1 of the fundamental series, the last one in each 

 expression being muscovite. The method of factoring here 

 shown I have adopted throughout my investigation of the 

 micas, and I have found very few trustworthy analyses to 



