THE MINERALS OF IDAHO 363 



BLAINE COUNTY 



A fibrous zeolite occurs in specimens from Blaine County labeled 

 Snake River lava from Little Smoky Creek northwest of Hailey. 

 Little Smoky Creek is shown on the Sawtooth topographic sheet. 

 The zeolite is fine fibrous and white, radiating from the walls toward 

 the centers of irregular filled cavities up to 8 mm. in diameter, in a 

 dense basaltic rock. 



Under the microscope this material is very finely fibrous and, so 

 far as could be determined, has parallel extinction. Its birefringence 

 is low and it has an opaque appearance due to air inclusions. Aggre- 

 gates of fibers not quite in parallel position give an indistinct inter- 

 ference figure which looks biaxial and positive with 2V large. The 

 fibers all have negative elongation which is X and not Y as they 

 remain negative in various positions. The material is not satis- 

 factory for measurement of the indices of refraction. The mean 

 index is approximately 1.51. This material does not agree in optical 

 orientation with thomsonite but it is nearer thomsonite than to 

 other fibrous zeolites in general properties and, like the similar 

 material from Kootenai County described below, is kept under the 

 thomsonite heading until more is known regarding the optical 

 properties of these minerals. 



BOISE COUNTY 



Associated with the chabazite from Warm Spring Creek in the 

 Bear Valley quadrangle, is a small amount of thomsonite which 

 occurs as round solid amygdules which are radiate fine-fibrous in 

 structure and are white in color with a silky luster. These are 

 much less abundant than the chabazite and usually are only about 

 3 mm. in diameter. A partial analysis made upon a small sample of 

 this material gave the following results: 



Partial analysis of thomsonite from Boise County 



(E. V. Shannon, analyst) 

 Constituent Per cent 



Silica (Si0 2 ) 42,43 



Alumina (A1 2 3 ) 27. 31 



Lime (OaO) Lost. 



Magnesia (MgO) 1. 07 



Alkalies (Na, K) 2 n. d. 



Water above 110°C 12. 85 



Water below 110°C 1. 28 



Total 84.94 



Under the microscope this material is fibrous with the elongation 

 Y as shown when the fragments are rolled between crossed nicols 

 under the gypsum plate. The refractive indices are about a = 1.504, 

 = 1.509, 7 = 1-512. 



