130 E. B. Hurlburt — Alunite from Colorado. 



Art. XYI- — On Alunite, from Med Mountain, Ouray 

 County, Colorado / by E. B. Hurlburt. 



The alunite which will be described in this communication 

 occurs as an aggregate of minute crystals, resembling kaolin, 

 filling pockets and seams in the ore body of the National Belle 

 Mine at Red Mountain. The ore that is mined is chiefly 

 enargite with some tetrahedrite and pyrite. A small quantity 

 of the mineral was observed by Prof. S. L. Penfield on some 

 enargite specimens among material which had been collected 

 by him while engaged in summer work with the United 

 States Geological Survey, and through the kindness of Mr. 

 "W. B. Wilson, superintendent of the mine, an abundant sup- 

 ply was obtained. It was at first supposed to be a well crys- 

 tallized kaolin, similar to that described by Hills,* Cross and 

 Hillebrandf and Reusch;}; from the same mine, but the shape 

 of the crystals and the optical properties did not correspond to 

 the description of the above named authors. Moreover when 

 heated in a closed tube abundant water was given off along 

 with S0 3 and S0 2 and the mineral was determined to be 

 alunite. 



The form of the crystals as seen under the microscope is a 

 combination of rhombohedron and base, shown in vertical pro- 

 jection by the figure. The largest 



\s. crystals were about 0'13 mm in diam- 



" \\ eter and 0'01 mra thick. Professor 



\A Penfield succeeded in measuring one 



\ \ of these minute crystals on the re- 



c X fleeting goniometer but the faces 



/ ) were somewhat rounded so that they 



/ / did not give good reflections, the 



/ / angle of base on rhombohedron was 



^< — V i^ found to be about 54° while Breit- 



haupt gives for alunite o^r, 0001 /^ 

 1011 = 55° 20'. The crystals exhibited normal optical prop- 

 erties and positive double refraction. 



To secure suitable material for analysis the mineral was sus- 

 pended in water and the smaller crystals poured off. The 

 larger ones were then brought into the potassium mercuric 

 iodide solution and further purified, that portion which was 

 taken for analysis, and which constituted the bulk of the 

 material, varied in specific gravity between 2*826 and 2-843, 



* This Journal, TIL xxvii, p. 472, 1884. 

 fBull. Geolog. Survey 20, p. 98, 1885. 

 % Neus Jahrb., 1887, ii, p. 70. 



