Chlorite Group and its Alternative. 197 





Delessite. 





Found. 



Calc. 



Si0 2 



34-00 



33-83 



A1A 



19-34 



19-17 



MgO 



30-24 



30-08 



H 2 



10-36 



10-15 



Aq 



6-06 



6'77 



100-00 100-00 



In reducing the data, and in calculating the formulae, I have 

 commonly assumed R" to be all magnesium or all iron accord- 

 ing as one or the other predominated. Only in diabantite the 

 relation between the Fe and Mg was so simple that it seemed 

 desirable to keep both in view. I have also used relatively 

 simple ratios between component molecules, rather than tne 

 more complex terms which might give a closer agreement be- 

 tween analyses and theory. That is, when Mg„(Si0 4 ) 2 R/ 4 and 

 Mg 3 (Si0 4 ) 2 A 2 stood actually in the relation of 11 to 10, I have 

 computed on the basis of 1:1. Most of the divergences 

 between " found" and " calculated " are due to this cause ; but 

 in the case of cronstedtite, which is least concordant of all, 

 there is probably a small admixture of some more basic com- 

 pound, of the character, possibly, of amesite. 



There now remains to be considered, of all of Tschermak's 

 typical chlorites, only strigovite, rumpfite, amesite, and in part 

 corundophilite. Of these, the first three depend each upon a 

 single analysis, and the available evidence is therefore not con- 

 clusive. To strigovite, the general formula H 4 Fe // 2 Al 2 Si 2 O u 

 seems to apply; which, in the constitutional expression given 

 to it by Tschermak has alreadv been cited. This may also be 

 written Fe s (Si0 4 ) 9 . H(AlH a O s )(A10H), which is of the ortho- 

 chlorite type with the bivalent group AlOH in place of R' 2 . 

 In rumpfite, which has the empirical formula H 28 Mg r Al 16 Si 10 O 65 , 

 the same group AlOH appears, and also a molecule of the 

 form of the last derivative of olivine, Mg(Si0 4 ) 2 R/ 6 . In detail, 

 rumpfite agrees very nearly with the composition 



Mg 2 (Si0 4 ) 2 A 2 H 2 + 2Mg(Si0 4 ) 2 A 2 (A10H) 2 ; 



which gives with the analysis by Firtsch reduced as usual, the 

 following comparison : 



Rumpfite. 





Found. 



Calc. 



SiO, 



31-01 



30-20 



A1A 



42-02 



42 79 



MgO 



13-73 



13-42 



H 2 G 



13-24 



13-59 



100-00 100-00 



