364 BULLETIN" 131, UNITED STATES NATIONAL MUSEUM 



ELMORE COUNTY 



Associated with the chabazite described above from 5 miles below 

 Glenns Ferry are scattered small spherulites of a colorless mineral in 

 radiated blades. These are only a millimeter or two in diameter 

 and are inconspicuous. Under the microscope the mineral is in 

 coarse flat fibers, the elongation of which is Y. The axial plane i& 

 thus across the length and the mineral is biaxial positive with 2V 

 medium, dispersion r<v perceptible. There is a perfect cleavage 

 perpendicular to the acute bisectrix. The extinction is parallel. 

 The refractive indices are a = 1.522, /3= 1.525, 7 = 1.537, birefringence 

 7 — a = 0.015. These values are very close to those obtained for 

 thomsonite by Wherry. 



KOOTENAI COUNTY 



Associated with stilbite, laumontite, prehnite, etc., on a specimen 

 from Post Falls is a radiated fine fibrous mineral which forms centers 

 around which stilbite occurs in parallel position continuing the 

 fibers. Under the microscope this material is very fine fibrous with 

 low birefringence and the grains are aggregates of fibers which 

 give sweeping extinction. The elongation of the fibers is definitely 

 negative and that this is X and not Y was definitely established* 

 The birefringence is low and the extinction apparently is parallel 

 although, in such material, a few degrees inclination would be hard 

 to detect. The fibrous material is rendered somewhat opaque by 

 the presence of numerous air inclusions. The determination of the 

 indices of refraction is rendered difficult by the character of the 

 material. The 7 index was estimated in one lot at 1.518 and in an- 

 other lot a was estimated at 1.505. The average for /3, the mean 

 index, is probably about 1.510. 



This fibrous mineral is easily soluble in hot dilute hydrochloric acid 

 and gelatinizes. It reacts for silica, alumina, and lime, and not for 

 potash or barium. It fuses easily coloring the flame red and yellow. 



It is obvious that the mineral described from the four localities 

 above varies decidedly in properties, but the material is not sufficient 

 to definitely clear up the matter of their identity. The composition 

 of thomsonite has recently been discussed by Wherry and its optical 

 properties by Gordon. 73 More work needs to be done to definitely 

 clear up the optical properties of these minerals. The first, from 

 Blaine County, has about the indices of faroelite, with elongation X. 

 The second, from Boise County, has about the indices of mesolite,. 

 with the elongation Y as in thomsonite. The third, from Elmore 

 County, has the indices given by Wherry for thomsonite and agrees 

 with the data given for the optical orientation of thomsonite by 

 Larsen. It agrees also with echellite. The fourth and last, from 



« Amer, Min., vol. 8, pp. 121-127, 1923. 



