Constitution of certain Micas and Chlorites. 385 







Found, reduced. 



Calculated. 



SiO„ 





16-97 



"17-02 



aiA 





44-14 



44-71 



CaO 





13-50 



13-00 



M^O 





20-92 



20-63 



H 2 





4-47 



4-64 





100-00 



100-00 



Spinel 



formed, 43-96 



43-94 



Reckoned on a similar basis, clintonite, according to the 

 analysis published by Sipocz, may be written — 

 A10 2 R". Si0 4 . H 8 + (A10 a RV Si0 4 



and brandisite is of the same general character, with a small 

 admixture of the first of the three xanthophyllite molecules 

 given in our structural expression. Whether chloritoid is a 

 true member of this group, seems to be uncertain ; but if it is, 

 its composition may be written A10 2 Fe . Si0 4 . H . AlOH ; the 

 last factor being a bivalent group related to the univalent 

 A10 2 H 2 . 



As for the ultimate structure of the group A10 2 R", it may 

 be written in two ways, thus : 



-A1<^>R" or -R'-O- Al = O 



The first form corresponds to the group — Al(OH) 2 , the second 

 to the formula commonly assigned to spinel, 



M X>-A1 = 



In either case two of the groups AlC^R" coalesce to form one 

 molecule of spinel, with removal of a single magnesium atom. 

 Such a coalescence is easiest comprehended when the second 

 form of the radical is chosen ; but the first form is more har- 

 monious with the constitution of the mica series in general. 

 If the xanthophyllite compound (A10 2 R // ) 4 Si0 4 be regarded 

 independently of the micas, it is most simply written as fol- 

 lows, with its splitting up into olivine and spinel as indicated : 



O-Mg-O-Al-0 O Mff M 0-Al = 



^<0_Mg-0-Al=:0 fe, f0 >M(r M 0-Al = 



If, however, for the sake of preserving harmony with the mica 

 formulae we adopt the first expression for — A10,R', then 

 spinel would seem to have the constitution 



M g<o> A1 -°- A1=0 



