Constitution of certain Micas and Chlorites. 388 



the final result. If we deduct the spinel from the composi- 

 tion of each mineral, the soluble portion is expressible as a 

 mixture of olivine and magnesian garnet ; two species which 

 are among the commonest progenitors of the chlorite group. 

 That olivine and garnet are actually formed is certainly not 

 proved ; but it seems highly probable that the chlorites studied 

 do split up in the manner indicated, yielding water, spinel, 

 garnet and olivine as the final products of decomposition. 

 Other chlorites may behave differently ; and it is quite likely 

 that the species or varieties rich in iron may diverge widely 

 from the types considered here. Speculation upon this theme 

 would be premature. 



As to the chemical structure of the leuchtenbergite and 

 clinochlore, the evidence now available is quite in harmony 

 with the general theory of the chlorite group recently advanced 

 by one of us.* Again including the Pennsylvania ripidolite, 

 the three chlorites reduce to compositions as follows : 



Clinochlore, Mg M (SiOJ M (AlH s O a )„(MgOH)„H I0 



Leuchtenbergite, Mg 49 (Si0 4 )^AlH 2 2 ) 42 (MgOH) 37 H 22 

 Ripidolite, Mg 49 (SiO 4 ) 50 (AlH 2 O 2 ) 38 (MgOH) S7 H 27 



Condensing, by union of the univalent factors, these be- 

 come — 



Clinochlore, Mg 50 (SiO 4 ) 5 „R' 105 



Leuchtenbergite, Mg 49 (SiO 4 ) 50 R' ]01 

 Ripidolite, Mg 49 (SiO 4 ) 50 R' 1()2 ; 



or, in general, all three examples have nearly identical compo- 

 sition, and conform to the typical expression — 



Mg 2 (Si0 4 ) 2 R' 4 , 



in accordance with the theory which represents them as sub- 

 stitution derivatives of the normal salt Mg 4 (Si0 4 ) 2 . We may 

 interpret the minerals in detail as was done in the case of our 

 earlier discussion of the West Chester ripidolite, or we may' 

 regard them as mixtures of the two salts — 



Mg 2 (Si0 4 ) 2 (AlH 2 2 ) 2 (MgOn) 1 H 

 I and Mg.(Si0 4 ),(AlH 1 0,) 1 (MgOH) a H ' 



In either case the juxtaposition of the groups A1U 2 2 and 

 MgOH renders the formation of spinel intelligible. 



In the waluewite, or more properly xanthophyllite, we have' 

 the first example of a true brittle mica met with in the course 

 of our investigations. Being the most basic known member 

 of the clintonite series, it has peculiar interest ; and it deserves 

 a somewhat detailed discussion. In the empirical formula 

 deduced from our analysis, 



*This Journal, March, 1892. 



