Hunt and Kraus — Composition of Melanochalcite. 213 



Thus in the above table it will be seen that the three con- 

 stituents are found present in the proportion of 4 parts of 

 tenorite, to 4 of chrysocolla, to 5 of malachite. Expressed in 

 percentages we are dealing with a mixture of 30"8 per cent 

 tenorite, 308 per cent chrysocolla, and 38 - 4 per cent malachite. 



Table 2. 

 Hunt's Analysis 

 Molecular ratios Malachite Chrysocolla 



CuO 1-1177) 0-0808 0-0714 



ZnO 0-0014 \ 



G0 2 0-0404 0-0404 



H 2 0-2488 0-0404 0*1428 



SiO 0-0714 0-0714 



Tenorite 

 0-9669 



Excess 



0-0656 



0-1616 



1-000 



0-2856 

 1-767 



0-9669, 

 5-983 



or 



In the second tabulation it would seem as though we are deal- 

 ing with a mixture of the same constituents but this time in 

 the proportion of 6 parts of tenorite, to l - 75 of chrysocolla, to 

 1 of malachite. Expressed in percentages we would have 68 - 6 

 per cent tenorite, 20 per cent chrysocolla and 11-4; per cent 

 malachite. 



The above computations are based upon the formula of 

 chrysocolla as usually assigned to it in most texts, namely, 

 CuO.Si0 2 .2H 2 0. Foote and Bradley,* however, have shown 

 that the composition of chrysocolla is to be considered a solid 

 solution of copper oxide, silica, and water, and that the water 

 content is variable and frequently exceeds the two molecules 

 usually assigned to it. If this view is accepted the excess of 

 water noted in the tabulations could easily be explained. It is 

 also very probable that a portion of this surplus is due to 

 moisture held hygroscopically. 



Additional evidence to support the supposition that melano- 

 chalcite is a mixture is to be found in a study of the very fine 

 powder when examined under a polarizing microscope. When 

 viewed between crossed nicols the material appears hetero- 

 geneous, being composed of a dark, opaque substance, together 

 with particles which are doubly refractive. A portion of the 

 doubly refractive material extinguishes normally while still 

 other particles possess anomalous colors and are not influenced 

 by a rotation of the stage. This portion is probably chryso- 

 colla under strain. When a drop of very dilute, cold acid is 

 applied to a small portion of the powder spread over an object 



* This Journal, xxxvi, 180-184, 1913. 



