F. W. Clarke— Theory of the Mica Group. 389 



which it does not apply. Even the exceptions are apparent 

 only, and can be met by a modification of the process which is 

 somewhat more generalized. Whether the separate compon- 

 ents of K/ and of X can be distributed with certainty among 

 the several molecules is an outstanding question, but not, I 

 believe, an unanswerable one. 



Phlogopite. 



In order to ascertain the composition of this mica I have 

 discussed the four analyses cited by Tschermak, (I. c), and 

 three of Rammelsberg's.* All of these are fluoriferous, and 

 all but one are phengitic, the exception in the latter case being 

 the Edwards, N. Y., mineral, which Tschermak regards as 

 somewhat altered. I have also considered the new phlogopite 

 from Edwards, recently described by Pentield,f which is re- 

 markable as being free from fluorine. 



In general, phlogopite agrees quite closely with the formula 

 AlX 3 Mg 3 B' 3 , but there are often variations which are not easy 

 to explain, and which are greater than can be accounted for by 

 presumable errors of analysis. They may be due in part to 

 impurities arising from the great alterability of the mineral ; 

 but I am inclined to trace the difficulties to other sources. 



In order to satisfy the conditions of the formulae proposed 

 by me, it is necessary that the aluminum atoms, apart from 

 A1F S , shall be in number at least one-third of X. But in 

 several phlogopites the alumina is too low to fulfil this require- 

 ment ; and the difficulty is best obviated by assuming an MgF 

 group in place of A1F 2 . In Penfield's phlogopite, however, 

 there is no fluorine, and yet the alumina is considerably — 

 about two per cent — too low. If we suppose in his mineral, 

 which came from a talc mine, a small admixture of talc, the 

 residue agrees with the theoretical formula, X being wholly 

 Si0 4 . But if that impurity is absent we must assume that the 

 mica differs from ordinary phlogopite by containing the group 

 — Mg— OH instead of the usual MgF. The composition of 

 the mineral then reduces to the uniform type. I am now dis- 

 posed to believe that phlogopite differs from the other micas 

 in that it contains these special groups MgF and MgOEL, both 

 as part of B/ ; but the supposition is not yet fully justified. 

 It is, however, I think, susceptible of experimental investiga- 

 tion, and a laboratory research upon the problem is now being 

 carried out under my direction. Apart from that, the sup- 

 position is strengthened by the composition of certain vermicu- 

 lites ; some of which have certainly been formed by the alter- 

 ation of phlogopite. Three of these interesting minerals are 



* Wied. Annal., ix, 129. f This Journal, III", xxxvi, 329. 



