240 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



of slightly different axial ratios. That the present mineral is ara- 

 gonite and not another carbonate of the same group was established 

 by qualitative tests, which proved the absence of barium, strontium, 

 and lead. The faces measured are above referred to the nearest 

 established form where possible. The previously undescribed forms 

 here given are of doubtful authenticity. More material from the 

 locality would be necessary to completely elucidate the peculiarities 

 of this aragonite. 



CUSTER COUNTY 



Aragonite is found in the oxidized ore from the Beardsley mine- 

 Bayhorse district. It forms sheaves of acicular crystals and coarse 

 fibers associated with calcite. Some of the aragonite is associated 

 with psilomelane, which occasionally has replaced it, giving pseudo- 

 morphous rosettes of psilomelane after aragonite. 



LEMHI COUNTY 



Aragonite occurs in the ore of the Pittsburg-Idaho mine, Texas 

 district, as branching plant-like forms which show no measurable 

 crystals. A specimen of this material is illustrated by Umpleby. 61 



CERUSITE (281) 



Lead carbonate, PbC0 3 . Orthorhombic. 



Cerusite is second only to galena in importance as an ore mineral 

 in the argentiferous lead ores of Idaho. It forms from the galena 

 under the action of percolating ground waters and the amount of the 

 ore which has been thus altered to cerusite varies greatly in different 

 •districts. Vast quantities of carbonate ores were mined from the 

 Coeur d'Alene district, but these ores are largely exhausted in this 

 district, but in the mining districts of the central and southern parts 

 of the State cerusite is still more important than galena. 



The oxidized ores are known to miners and operators of mines in 

 the State as " carbonate ores." They vary greatly in appearance. 

 When pure the cerusite is white or gray with a greasy luster, but 

 much of the ore mined is greatly contaminated with silica and oxides 

 of iron and manganese and varies through various shades of gray and 

 brown to yellow or black. The appearance is often earthy and the 

 only index to the lead content is the high specific gravity. In addi- 

 tion to the iron and manganese oxides, there often occur, with the 

 cerusite, yellow ocherous lead antimonate, bindheimite, and blue 

 and green carbonates of copper, azurite, and malachite. 



In cavities in the massive cerusite there very frequently occur 

 crystals, often very brilliant in luster and colorless or white. The 

 cerusite crystals present a great variety of form and most of them are 

 twinned in groups. These are the "crystallized lead" of the miners 



•i J. B Umpleby. U. S. Qeol. Survey, Bull. 528, pi. 10, 1913. 



