Hunt and Kraus — Composition of Melanochalcite. 211 



Art. XI. — Note on the Variable Composition of Melano- 

 chalcite; by W. F. Hunt and E. H. Kraus. 



The literature relating to the unusual mineral melanochal- 

 cite is very meager, in fact, our. previous knowledge has been 

 restricted to that of a single paper by the late Professor G. A. 

 Koenig.* When, therefore the Mineralogical laboratory of 

 the University of Michigan received from the Ward's Natural 

 Science Establishment several excellent specimens of this rare 

 mineral, a chemical investigation of the material was deemed 

 very desirable. 



The material examined was, as far as could be ascertained, 

 from the same general locality as that described by Koenig, 

 namely from Bisbee, Arizona. The lustrous, pitchy black 

 mineral forms bands of varying thickness from a few mil- 

 limeters to several centimeters, which surround a nucleus 

 of compact cuprite. Chrysocolla and malachite occur in thin 

 thread-like veins penetrating the melanochalcite, and often as 

 a coating upon its outer surface. In order to compare the 

 mineral on hand with that previously described, fragments of 

 the same material which formed the basis of Koenig's article 

 were kindly loaned by the Michigan College of Mines, through 

 the courtesy of Professor A. E. Seaman and President F. W. 

 McNair. While in outward appearance the material pre- 

 viously described and that recently obtained appear identical, 

 considerable difference was noted, however, in their physical 

 properties as well as in their chemical composition. Koenig 

 described his material as giving a coffee-brown streak and 

 possessing a specific gravity of 4/141. The material here in- 

 vestigated gave a decidedly brownish-black streak and pos- 

 sessed a specific gravity of 4 - 704. The darker streak as well as 

 the higher specific gravity of the specimen analyzed in this 

 laboratory can readily be explained, as noted below, by its 

 higher CuO content. 



Sufficient material w r as on hand to obtain several grams of the 

 jet black mineral entirely free from admixture of cuprite and 

 other foreign matter. Si0 2 was recovered after two evapora- 

 tions with HC1 and corrected by volatilizing with hydrofluoric 

 acid. CO„ and H 2 were determined in a single operation by 

 heating in a combustion furnace, while a current of dry and 

 C0 2 -free air was slowly passed through the apparatus. The 

 water was caught in a CaCl 2 U-tube while the C0 2 was received 

 in a potash bulb. The increase in weight in the U-tube and 

 potash bulb checked very closely with the loss in weight in the 

 *This Journal (4), xiv, 404-409, 1902. 



