362 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



graphed in polarized light between crossed nicols is shown in Plate 

 12 upper. The flattened analcites adhere to the quartz films which 

 coat the platy calcite and usually show very numerous quartz 

 spicules attached to their surface. This quartz was ground off of the 

 crystal shown in the photograph. Optically the analcite is biaxial 

 and negative with large axial angle and a mean refractive index of 

 1 .486. The biaxial interference figure is extremely sharp and definite 

 and this, together with the geometrical perfection of the division 

 into sectors and the sharpness of the lamellae, makes it hard to accept 

 the idea that the mineral owes its nonisotropic character to anoma- 

 lies explained by internal strain. 



A sample for analysis was selected carefully and its homogeneity 

 and purity were established microscopically. The analysis gave the 

 following: results and ratios: 



Analysis and ratios of analcite from near Challis 

 [E. V. Shannon, analyst] 



Constituent 



Per cent 



Ratios 



Silica (Si0 2 ) 



Alumina (AI2O3) 



Lime (CaO) 



Magnesia (MgO) 



Potash (K 2 0) 



Soda(Na 2 0) 



Water (H 2 0) above 110° C 

 Water (H 2 0) below 110° C 



Total 



57.47 



20.85 



.52 



.21 



.64 



10.94 



8.43 



None. 



99.06 



0.953 

 .204 

 .009) 

 .005 

 .007} 

 .177 

 .4681 



0. 238X4 

 .204X1 



1. 19X4 

 1.03X1 



.199X1 .98X1 

 .234X2 1.17X2 



The above ratios are not satisfactorily close to whole numbers to 

 conform to the analcite formula given by Dana, both water and 

 silica being high. Such variations have been noted in analyses of 

 analcite from other localities and have been variously interpreted. 

 The mineral is somewhat variable in composition. 



The paragenetic position of the analcite is late, as it grew between 

 the laminae of calcite or occupied cavities left by removal of calcite 

 and the analcite crystals inclosed mordemte and quartz. No other 

 mineral is deposited on the analcite and it is probably the youngest 

 mineral represented in the specimens from the Challis locality. 



THOMSONITE (456) 



Orthorhombic. 



Hydrous sodium-calcium aluminium sili- 

 cate, Na 2 0.4Ca0.5Al 2 3 . 

 10 Si0 2 .12H 2 0. 



Several occurrences of fibrous zeolites have been examined from 

 Idaho localities; there has not been enough of the materials for 

 chemical analysis and the optical determinations have not, in all 

 cases, been satisfactory. Four occurrences are mentioned below but 

 in two of these the mineral may not be thomsonite. 



