334 Larsen and Steiger — Sulphatic Cancrinite. 



spending to this analysis and the succeeding columns give the 

 analyses and ratios of typical cancrinites for comparison. 



Little can be said regarding the chemical constitution of 

 sulphatic cancrinite further than comparing it with cancrinite, 

 The close agreement of its combining ratios with those of can- 

 crinite, about half of the C0 9 being replaced by S0 4 , has sug- 

 gested the name sulphatic cancrinite. No simple ratios appear 

 in the combining equivalents of analyses of cancrinite. 

 Clarke* has applied the formula Al 2 (Si0 4 ) 3 .Na 4 H(AlC0 3 ) 

 to cancrinite, while Dana considers it to be represented by 

 H 6 Na Ca(]SraCO 3 ).,Al 8 (SiOJ 9 . Clarke considers part of the 

 groups — Al = C0 3 to be partly substituted by — Al = Si0 3 

 and some of the soda replaced by lime. The formula of can- 

 crinite is evidently complicated, our present knowledge throw- 

 ing but little light on its constitution, and were it not for the 

 well-defined crystallographic properties of the mineral suspicion 

 might point to its lack of homogeneity. 



Conclusion. — The data as yet available are not sufficient to 

 show clearly the relation between cancrinite and sulphatic can- 

 crinite. That the two are closely related, both the chemical 

 and optical properties show rather conclusively and they may 

 form a complete isomorphous series from normal cancrinite to 

 a mineral in which all the carbonate is replaced by sulphate. 

 The sulphatic cancrinite described in this paper contains nearly 

 equal parts, molecularly, of S0 4 and C0 3 and may represent 

 an intermediate compound having the same relation to the end 

 members as diopside has in the pyroxene group. 



Sulphatic cancrinite has a much lower birefringence than 

 cancrinite and it is not unlikely that a member of the group 

 somewhat richer in sulphate has ze"ro birefringence and that 

 the pure sulphate member is optically negative. 



* Clarke, F. W., The constitution of the silicates, U. S. Geol. Survey Bull. 

 125, p. 23, 1895. 



