F. W. Clarke— Theory of the Mica Group. 387 



case the composition of a mica becomes reducible to the one 

 general type, under the proposed theory of substitution. 

 Representing the groups Si0 4 and Si 3 8 by the common symbol 

 X, the micas all fall within limits indicated by the formulae 

 A1 3 X 3 R' 3 , and A1X 3 R' 9 . We may test this principle and the 

 preceding formulas by application to actual examples, taking 

 the different micas group by group. In general, however, I 

 must omit the details of the individual analyses discussed, as I 

 hope to publish them more fully hereafter. 



Muscovite. 



This mica, the most typical and most abundant of all, is also 

 the simplest chemically. It is best represented by formula 

 No. 1, which, in its special application becomes ordinarily 

 Al 3 (Si0 4 ) 3 KH 2 , with some variation in the ratio of K to H. In 

 most cases muscovite contains small amounts of magnesia and 

 ferrous iron ; and if these are deducted, as shown in formula 

 No. 6, the residue agrees still better with formula No. 1. 

 Fluorine is often present in small quantities, and appears to 

 vary in relation to hydrogen, being lowest when the latter is 

 high, and the reverse. Hence it is probable that the group 

 A1F 2 replaces H rather than K. This is shown more clearly 

 among the lepidolites, in which fluorine reaches a maximum, 

 while the proportion of water is almost insignificant. 



Some muscovites, however, vary from the normal compound 

 in that they contain more silicon and less oxygen ; thus 

 approaching somewhat to lepidolite. These micas, which 

 Tschermak has called " phengites," are represented by him. as 

 mixtures of Al 6 R/ 6 Si 6 24 with H 8 Si 10 O 24 in the ratio of three to 

 one. It is simpler, however, to follow out the analogy offered 

 by the feldspar group, and to assume the existence in muscovite 

 of the isomorphous compound Al 3 (Si 3 8 ) 3 KH 2 . True, this 

 compound has not been found by itself in nature, and so far its 

 assumption is objectionable. But the compound H 8 Si 10 O 24 is 

 also non-existent, is different in type from ordinary muscovite, 

 and is not easily conceivable as a definite entity. The alterna- 

 tive which I offer for it is therefore, it seems to me, more 

 philosophical and more satisfactory ; and it accounts completely 

 for all the oxygen variations in muscovite. For the sake of 

 brevity, however, we may well retain the name of phengite in 

 our vocabulary, and may speak of micas containing the Si 3 E 

 groups as more or less phengitic. 



Lepidolite. 



In this species, the most phengitic of all the micas, we find 

 little water, high fluorine, and a very notable proportion of 

 lithia. It always occurs with muscovite, and commonly im- 



