IF". T. Schaller and F. L. Ransome — Bismite. 173 



Art. XII. — Bismite • by W. T. Schaller and 

 F. L. Ransome. 



In the Goldfield district in Nevada, bismite occurs in the 

 January, Combination, Sandstorm, and probably also in other 

 mines, as minute, pearly scales with brilliant luster and of 

 silvery whiteness. The luster of the scales is almost metallic 

 and suggests at first glance delicate and untarnished crystals of 

 native silver. The mineral is limited to the oxidized zone, 

 is usually accompanied by limonite and is frequently associated 

 with rich ore. It occurs as single glittering scales or specks 

 on the walls of cavities in spongy limonite or rusty ledge- 

 matter, as delicate frost-like films on the same materials and as 

 spongy aggregates with quartz. In the last-named form it is 

 clearly pseuclomorphous after bismuthinite, the material having 

 consisted originally of a mass of bismuthinite prisms held 

 together by a siliceous gangue. The prismatic structure of 

 the bismuthinite is retained as hollow casts in the quartz, lined 

 or partly filled with scales of bismite. 



As natural crystals of bismite do not appear to have been 

 found hitherto and as the mineral has been assigned to the 

 orthorhombic system in Dana's and Hintze's mineralogies on the 

 basis of Nordenskiold's work on artificial crystals in 1860 and 

 1861, considerable interest attaches to the Goldfield occur- 

 rences on account of the crystallinity of the material. 



When examined under the microscope in gently powdered 

 material the crystals appear as thin colorless scales, with occa- 

 sionally a suggestion of hexagonal outline. On account of 

 their tenuity, it is rare that an edge view of a scale is obtain- 

 able. The larger flat-flying scales are dark in all positions 

 between crossed nicols and give a negative uniaxial tigure with 

 convergent light. There are no colored rings and the double 

 refraction is apparently not great. The refractive index, on 

 the other hand, is rather high, being greater than that of 

 anorthite. The mineral has a perfect basal cleavage and the 

 scales, viewed without the analyzing nicol, commonly show 

 delicate interference colors due to the refraction and reflection 

 at the surfaces of the exceedingly thin cleavage sheets. 



Some of the best material obtainable was analyzed with 

 results as follows : 



Insol. in HOI, mostly 12 3 Average 



quartz gangue _ 78*95 78-87 79*01 78*94 



Bi 2 3 __ 17*29 17*00 16*84 17*04 



H 2 (loss on igD.) 3*96 3*96 



Fe 2 3 0*36 0*50 0*21 0*36 



100*30 



Am. Jour. Sci.— Fourth Series, Vol. XXIX, No. 170.— February, 1910. 

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