THE MINERALS OF IDAHO 289 



A single small specimen collected by Ransome 6 in the Hercules 

 mine was described as containing a light gray silky asbestiform 

 silicate crystallized with galena. Under the microscope the exceed- 

 ingly fine fibers showed rather low double refraction and parallel 

 extinction. The mineral, as tested by Dr. W. T. Schaller, was not 

 attacked by acids and fused with difficulty. The original specimen 

 collected by Ransome was examined by the writer. It consists of 

 parallel aggregates of pale gray fibers up to 2 cm. long, intergrown 

 with granular galena. It is colorless in thin section with positive 

 elongation. The indices of refraction, which are a = 1.678, 7 = 1.702, 

 birefringence 0.024, indicate this to be ferroanthophyllite, probably 

 purer than that from the Tamarack mine. 



TREMOLITE 



Lime-magnesia silicate, Ca0.3MgO.Si0 2 . Monoclinic. 



Tremolite is a comparatively iron-free amphibole occurring 

 usually as white to grayish fibrous masses. It is often difficultly 

 distinguishable from anthophyllite without an optical examination 

 or an analysis. It is probably of common occurrence in metamor- 

 phosed limestone in Idaho but few specimens have been collected 

 for studv. 



BLAINE COUNTY 



A specimen labeled as from the face of the tunnel of the Red 

 Elephant mine, Wood River region, consists of gray finely fibrous 

 masses of tremolite containing small cubes of pyrite. Under the 

 microscope the material is fibrous with prismatic cleavage and ex- 

 tinction slightly inclined to the elongation. It is biaxial, negative, 

 with 2V medium large, refractive index = 1.623 ±0.003. The 

 country rock here is calcareous shale in which the tremolite is doubt- 

 less a mineral developed during metamorphism. 7 



CASSIA COUNTY 



A specimen labeled "marble from marble quarry, Basin Idaho, 

 T. 14 S., R. 22 E." consists of medium granular gray marble con- 

 taining abundant gray blades and sheaves of tremolite up to 3 mm. 

 thick by 15 mm. long. This mineral is colorless under the micro- 

 scope and is biaxial negative with 2V large. The extinction is 

 about 12°. 



CUSTER COUNTY 



Tremolite is comparatively rare in the contact-metamorphic 

 copper deposits of the Alder Creek (Mackay) district, no good 

 specimens having been reported. It occurs sparingly in the marble 

 but is rare in the diopside rock. 8 



• F. L. Ransome. U. S. Geol. Survey, Prof. Paper 62, p. 99, 1908. 



■ Waldemar Lindgren. U. S. Geol. Survey, 20th Ann. Rept., pt. 3, pp. 202-203, 1900. 



8 J. B. Umpleby. U. S. Oeol. Survey, Prof. Paper 97, p. 5. r ), 1917. 



54347—26f- 20 



