JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV APRIL 19, 1914 No. 8 



MUS^ERALOGY .—Cnjstallized chrysocolla from Mackay, Idaho. 

 Joseph B. Umpleby, Geological Siu'vey. 



Crystallized chrysocolla has been found only in a few places, 

 and descriptions of its optical properties do not agree. ^ It seems 

 worth while, therefore, to describe briefly some exceptionally good 

 material from the Empire copper mine, situated three and one- 

 half miles southwest of Mackay, Idaho. The specimen analyzed 

 was picked from the ore bins of the Copper Bullion tunnel during 

 an examination of the deposits by the writer in 1912. Copper 

 silicate ore has afforded most of the production from these de- 

 posits, but very little of the material is crystallized. 



The chrysocolla is characteristically brown in color and con- 

 tains much iron, but locally it is bluish green and free from iron. 

 The latter variety only is described in this paper. 



The crystalUzed chrysocolla at Mackay occurs as mammillary 

 crusts, as small masses of irregular shape, and as acicular crystals 

 imbedded in other copper minerals. The acicular crystals are 

 microscopic in size and in the thin section appear both as radi- 

 ating groups and narrow bands of closely packed individuals 

 oriented normal to the sides of the bands. The massive variety 

 offered the best material for analysis, but even with it consider- 

 able difficulty was experienced in separating a sufficient amount 

 for chemical examination. Within distances of a few millimeters 



1 A description of chrysocolla from the Belgian Congo, by H. Buttgenbach 

 (Ann. Soc. geol. publ. Congo Beige, p. 31-70, 1913) gives the following optical 

 properties: fibres, uniaxial + , mean index of refraction 1.39 (average of 6 readings 

 from 1.33 to 1.48); mostly second order colors. Lindgren (Prof. Paper 43, U. S. 

 Geol. Survey, p. 113, 1904) states that chrysocolla from Clifton-Morenci, Arizona, 

 is negative with double refraction about like augite. 



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