THE MINERALS OF IDAHO 433 



over 8 or 10 claims and very common at most prospects in the. camp. 

 This is an earthy appearing pale yellow-green material. Under the 

 microscope it is greenish brown in color and transparent and is com- 

 pletely crystalline with moderately high birefringence and a finely 

 aggregate structure. The index of refraction varies slightly between 

 1.76 and 1.7S. Some of the smaller grains are minute prisms with 

 inclined extinction, Z:c = 20°, and these have a = approximately 1.76 

 and A = much higher, possibly 1.79. The mineral dissolves readily 

 in dilute hydrochloric, acid. It contains abundant ferric iron and 

 arsenic and considerable copper and no ferrous iron, nickel, or 

 cobalt. 



BAYLBONITE (629) 



Hydrous copper-lead arsenate, Monoclinic(?) 



*3CuO.PbO.As 2 5 .2H 2 0(?) 



A single specimen from Custer County contains a mineral which 

 may be bayldonite although the identification is incomplete. 



CUSTER COUNTY 



A specimen of galena-tetrahedrite ore from the Ramshorn mine, 

 Bayhorse district, contains a few small cavities lined with white 

 cerusite crystals. On the cerusite rest minute acicular yellow prisms 

 of mimetite, previously mentioned, and a large part of the mimetite 

 prisms are coated or replaced by a lusterless mossy friable olive- 

 green to yellow-green mineral. Under the microscope this green 

 material is seen to be made up of aggregated minute grains and cry- 

 stals of a transparent mineral of pale green nonpleochroic color and- 

 index of refraction very much above 1.82. Some of the grains show 

 crystal outlines which appear as though they were short prisms with 

 oblique terminations resting on the b pinacoid. These give optical 

 properties apparently indicating the mineral to be biaxial positive 

 with 2 V large, dispersion r>v weak. The extinction is inclined and 

 the data obtained make the optical orientation Z = b, Y: c^=28°. 

 These data do not agree entirely with the optical orientation and 

 dispersion given for bayldonite, but they are approximate only, owing 

 to the nature of the material, and may be incorrect. The mineral 

 gives an arsenic mirror when heated with charcoal in a closed tube. 



EVANSITE (645) 



Hydrous aluminium phosphate, Amorphous. 



3Al2O3.P2O5.l8iU). 



Specimens sent to the United States Geological Survey by C. R. 

 Potts from Goldburg in Custer County were identified as evansite 

 and described by Dr. W. T. Schaller. 30 



3 » Waldemar T. Schaller. U. S. Geol. Survey, Bull. 490, p. 94, 1911. 



