THE MINERALS OF IDAHO 275 



Deen possible to secure any specimens of this material for crystallo- 



graphic study. 



ANDESINE (318) 



SODA-LIME FELDSPAR 



Sodium calcium aluminium silicate. Variable 



solid solution of albite (NaAlSi 2 8 ) and Triclinic. 



anorthite (CaAl 2 Si 2 08) in proportions from 

 Ab 3 Ani to AbiAm. 



Although andesine is of frequent occurrence as a rock making min- 

 eral, especially in the lavas of intermediate composition which are 

 common in Idaho, the occurrences thus far described are of petro- 

 graphic rather than mineralogic interest. The single occurrence de- 

 scribed below is of this character, but furnishes material suitable for 

 mineral specimens. 



FREMONT COUNTY 



The writer is indebted to H. T. Stearns, of the United States 

 Geological Survey, for information on the interesting occurrence of 

 andesine here described, as well as the occurrence of labradorite de- 

 scribed below. The area is covered by a report in preparation by 

 Mr. Stearns on the geology and water resources of the Mud Lake 

 drainage basin, Idaho, to be published as a Water Supply Paper by 

 the United States Geological Survey. The locality is Crystal Butte, 

 about 18 miles north of St. Anthony and 8 miles southeast of Ivan. 

 The rock is a lava, probably an andesite or basalt, which constitutes 

 a fissure eruption with several cones, which Mr. Stearns regards as 

 belonging to a period of igneous activity intermediate between the 

 Tertiary andesites and the Snake River basalts. The rocks are made 

 up of a dense black groundmass containing scattered large and well- 

 formed phenocrysts of transparent plagioclase. These phenocrysts, 

 weathered free from their matrix and strewing the surface of the cone, 

 have given it the name Crystal Butte. 



Most of the specimens examined are loose crystals. These reach 

 a maximum diameter of about 7 cm. They are all tabular to (010), 

 and some of them are twinned on the albite law. They are well 

 bounded by crystal faces and are transparent, except where stained 

 by infiltrated iron oxide or filled with fractures. In color they vary 

 from colorless to pale yellow, and some small pieces are of gem quality. 



A fragment from a typical crystal of this lot when examined 

 optically proved to consist of calcic andesine. In powder under the 

 microscope it is transparent and colorless; biaxial positive with 2E 

 large, dispersion r> v pronounced. No twinning lamellae were seen. 

 The indices of refraction are a= 1.551, jS = 1.555, 7=1.558. These 

 indices identify the mineral as andesine of the composition Ab 53 An 45 . 



