148 Van Horn and Hunt — Bournonite Crystals. 



of the massive mineral were detached, and analyzed by Dr. W. 

 R. Yeazey of the Chemical Department of Case School of 

 Applied Science. The results of this analysis follow : 



Theoretical Analysis 

 composition of of 



Bournonite Daly West Combining 

 PbCuSbS 3 Mineral Weights Eatio 



Pb 42-54 43-18 0-209 ] -009 



Cu 1304 13-14 0-207 1-000 



Sb j. 24-64 25-03 0-208 1-004 



S -..- 19-11 19-59 0-611 2-951 



100-00 100-94 



Sp. Gr. 5-829 



The ratio given by the Park City mineral conforms very 

 nearly to the composition required by the formula PbCuSbS,, 

 and, therefore, leaves no doubt that the mineral is actually bour- 

 nonite. Furthermore, the specific gravity, which was found to 

 be 5'829, is about the average obtained from specimens of 

 the mineral at other localities. Before the blowpipe, the 

 mineral gives the usual reactions for sulphur, antimony, lead, 

 and copper. 



Physical Properties. 



The color and streak of the mineral are blackish lead-gray 

 to iron-black. The streak seems to be one of the means of dis- 

 tinguishing it from tetrahedrite at Park City, as the powder of 

 the latter is usually slightly reddish brown. No cleavage is 

 noticeable, and the fracture is almost conchoidal. The mineral 

 is brittle. The hardness is under 3, and the specific gravity, as 

 has been mentioned, is 5 - 829. The luster is a brilliant metallic 

 on a fresh fracture, while that of the crystals is mostly dull. 



Crystallography. 



Special interest is attached to the three bournonite specimens 

 submitted for crystallographic orientation on' account of the 

 unusually large size of the crystals, the dimensions of which 

 have been previously given. In each instance, the specimens 

 examined were broken crystals, and the size of the original 

 complete forms might well have been from 25 to 50 per cent 

 greater than the present dimensions. The faces were for the 

 most part dull and very rough, thus rendering reflections ob- 

 tained in the regular manner with a reflecting goniometer 

 impossible. 



Measurements were secured, however, by means of the con- 

 tact method, and for the means employed, the fair agreement 

 of the observed values with the corresponding calculated angles 



