THE MINERALS OF IDAHO 



349 



heulandite, and snow-white masses of mordenite. The white masses 

 are very compact and tough and on fracture show the texture of 

 porcelain and are lusterless. When ground for analysis these were 

 found to be pure with a very finely felted fibrous structure. The mean 

 index of refraction of the material analyzed is 1.473. The analysis 

 gave the results of column 4. 



The four analyses made on the mordenites from Idaho are below 

 stated and their average compared with the composition calculated 

 from the formula (Na 2 , Ca) O.Al 2 3 .9Si0 2 .6H,0, a 1 : 1 ratio between 

 soda and lime being assumed. 



Analyses of mordenite from Cxister County 

 (Earl V. Shannon, analyst) 



Silica (Si0 2 ) 



Alumina (AI2O3)- 



Lime (CaO) 



Magnesia (MgO) 



Soda (Na 2 0) 



Potash (KjO)... 

 Water (H3O) 



Total 



The analyses indicate quite conclusively that the correct formula 

 for all of the several modifications of the Idaho mordenite is as above 

 given. The small inclined extinction indicates that the mineral is 

 monoclinic. Ptilolite is orthorhombic and hence gives parallel 

 extinction. Since the fibers are very thin and the birefringence is low 

 an extinction obliquity of several degrees might readily evade observa- 

 tion. Such a small angle might also be attributed to lack of adjust- 

 ment between the cross hairs and the nicols of the microscope. After 

 specially adjusting the microscope Mr. Ross found the Idaho material 

 to give, as the mean of about 20 good measurements, an extinction 

 inclination of 3K° measured from the elongation of the fibers. 



The Idaho mordenites are interesting as furnishing a complete 

 gradation between the physically ver}^ dissimilar compact mordenite 

 from Nova Scotia and the cottonv fibrous "flokite" from Iceland. 



HEULANDITE (438) 



Hydrous lime alumina silicate, approxi- 

 mately 5H 2 O.CaO.Al 2 3 .GSi0 2 . 



Monoclinic. 



Heulandite has been noted from two localities in the State, one in 

 Adams County, in the Seven Devils district, where it occurs in very 

 small amount with chabazite and stilbitc in a contact-metamorphic 

 ore deposit, and one in Custer County near Challis, where heulandite 

 accompanies other zeolotes in amygdaloida! lava. 



