384 Clarke and Schneider — Experiments upon the 



Al 86 Ca 84 Mg M H e0 (SiO 4 ) a8 O 174 , 



the most noticeable feature is the great excess of oxygen com- 

 pared with the possible hydroxy 1 and the small amount of 

 Si0 4 . This excess is accounted for in part by the general 

 formula which we have heretofore assigned to the clintonite 

 group, 



A1-0 >R 

 ^SiO=R' 3 ; 



which formula, however, needs some extension along a new 

 line. In that formula the group of atoms — A10 2 R" appears, 

 an equivalent obviously of the group A10 2 H 2 ; and this group 

 may fairly be repeated among the components of B/ 3 . With 

 such a group, the extreme member of the clintonite series 

 should have the composition (A10 2 R") 4 Si0 4 ; and a molecule of 

 that type is theoretically capable of splitting up into olivine 

 and spinel in the ratio of 1:2, when each mineral is given its 

 lowest possible formula : 



(A10 2 R") 4 Si0 4 = R" 2 Si0 4 4- 2R'A1 2 4 



In the case of xanthophyllite, if such a molecule is con- 

 tained in it, the olivine formed should be partly monticellite. 

 At Monzoni, monticellite is found associated with spinel ; and 

 brandisite, a member of the clintonite series, occurs in the 

 same region. In fact, all three of the true clintonite micas are 

 commonly associated with spinel, a fact of much interest when 

 considered in its relations to our experiments. In the light of 

 the foregoing argument the composition of waluewite and its 

 manner of decomposition by heat, may be represented as fol- 

 lows: 



5A/o> Ca +5a/o> M S 



^SiO =H a . A1H 2 2 ^SiO =(A10 2 Mg) 3 



+ 1A1— >( " a 



\SiO-(A10 2 Ca) 3 



These three molecules are all of the general type heretofore 

 assigned to the clintonite series, and two of them are presum- 

 ably capable of yielding spinel. Reducing the original analysis 

 to 100 per cent with consolidation of like bases as usual, we 

 get the following comparison between experiment and theory ; 

 the calculated composition being in accordance with the struc- 

 tural formula above cited. 



