410 Clarke and Schneider — Experiments upon the 



pression E /,/ 4B E ,, 79 (Si0 4 ) 4S (OH) 127 reduces to(AlH 2 2 ) 48 (R"OH) sl 

 K" 4a (Si0 4 ) 42 ; in which R"OH is mainly Fe ;/ OH and B" is 

 almost entirely Mg. This, generalized, becomes R, // 42 (Si0 4 ) 4a E,' 79 , 

 which is quite nearly the olivine-serpen tine type of formula. 

 A mixture of such molecules in which B/ is satisfied by 

 MgOH, FeOH, and A1H 2 2 in the ratio of 1:3:6; would 

 have the subjoined composition ; which is comparable directly 

 with the results of analysis. 





Found. 



Calculated. 



Si0 2 



25-40 



24-88 



AiA 



Fe 2 3 



22-80 I 

 2-86 \ 



25-37 



FeO 



17-77 



17-91 



MgO 



19-09 



19-90 



H 2 0, essential 



11-26 



11-94 



99-18 100-00 



If the first column were recalculated to 100 per cent, with 

 the ferric iron reduced to its equivalent in aluminum, the 

 agreement would be even closer. In brief, prochlorite seems 

 to have a constitution strictly analogous to that of ripidolite ; 

 although, on account of its high proportion of ferrous iron it 

 behaves differently towards gaseous hydrochloric acid. The 

 leuchtenbergite evidently has a similar structure ; but the im- 

 purities in the sample analyzed preclude us from discussing 

 this species more in detail. Just as the micas are derived by 

 substitution from normal aluminum salts, so the chlorites are 

 derived from normal magnesium silicates ; and, in a very curi- 

 ous way the two series seem to approach each other. Thus a 

 compound having the chloritic formula Mg 2 (Si0 4 ) 2 (A10 2 H 2 )H 2 , 

 if halved, may be written as if it were a derivative of alu- 

 minum orthosilicate analogous to some of the more basic 

 hydromicas ; and the close physical similarity between the 

 two groups is thus remarkably emphasized. 



5. The micas. 



In this group only three examples were studied, all of the 

 magnesian or ferro-magnesian class. A. Phlogopite from Bur- 

 gess, Ontario. The ordinary, slightly brownish, broadly foli- 

 ated mica, somewhat resembling muscovite. B. Phlogopite 

 from Edwards, St. Lawrence County, New York. The pecu- 

 liar, non-fluoriferous variety, superficially resembling brucite, 

 described by Penfield and Sperry ; whose analysis is thoroughly 

 confirmed by ours. C. A nearly black, broadly-foliated iron 

 mica from Port Henry, JSTew York. Commonly regarded as 

 a lepidomelane. Analyses as follows : 



