172 CARNEGIE INSTITUTION OF WASHINGTON. 



When cuprous sulphide is heated in an atmosphere of hydrogen sulphide at 

 various temperatures (below the melting-point), its sulphur content increases. 

 For each temperature the products contain a definite amount of sulphur. 

 This sulphur content increases with the decrease in temperature until at 358° 

 the product becomes cupric sulphide. 



Solid solutions of cuprous-cupric sulphide were also prepared by heating 

 compressed powders of the two sulphides at about 100°. 



The analyses of a number of natural chalcocites from widely varying locali- 

 ties proved that solid solutions sometimes occur in nature. The geological 

 significance of this fact is yet to be discovered. 



Cuprous sulphide is dimorphous. The inversion temperature was found at 

 about 90°, No other inversion could be detected either by the thermal or by 

 the conductivity method. It has been shown that the size of the grains con- 

 siderably influences the inversion temperature. 



Increasing amounts of cupric sulphide, dissolved in cuprous sulphide, raise 

 the inversion temperature. This takes place until cupric sulphide reaches the 

 concentration of about 8 per cent, after which an inversion is no longer 

 observed. 



Crystals of chalcocite were repeatedly formed in the dry way at various 

 temperatures, but only above about 250° were they large enough to permit 

 the determination of their symmetry; all these crystals were isometric. 



Isometric crystals of cuprous sulphide were also obtained from solutions at 

 250° by the reaction of cuprous chloride with sodium sulphide and at 200° and 

 170° by recrystallization of cuprous sulphide in solutions of hydrogen sulphide. 

 Very small crystals, apparently cubo-octahedrons, were also found at 125°. 



Crystals formed by the action of ammonium sulphide on copper, repeatedly 

 described in the literature as orthorhombic cuprous sulphide, have in reality 

 the composition Cu7(NH4)S4. A crystallographic study showed that they 

 are not orthorhombic, but tetragonal. 



Observations of cleavage and parting, combined with a knowledge of the 

 effect of dissolved cupric sulphide on the inversion of cuprous sulphide, are 

 applied to the problem of distinguishing primary from secondary chalcocite ore. 



The possible bearing of some of the data on the origin of chalcocite ores is a 

 matter of much interest. 



Covellite has been prepared in many different ways and a crystallographic 

 and optical study of it made. It is hexagonal and optically negative; the 

 optical dispersion of co is extremely high. coLi<1.0, coNa = 1.45, c<jti = 1.75. 



The specific gravity of two very pure samples of natural covellite has been 



detennined and found to be 4.683 and 4.676 respectively ( : 777— ) • This 



^ "^ V water 4° / 



specific gravity is higher than any previously given. The specific gravity of 



synthetic covellite varied with the fineness of the material used. This was 



probably due to occlusion of air by aggregates of very fine crystals. The 



highest value obtained with synthetic covellite was 4.652 ( ; 75-^ V 



V water 4 / 



Covellite can be heated in an atmosphere of hydrogen sulphide up to 358°; 



at which temperature the covellite is in equilibrium with the gas, and below 



it chalcocite can be completely converted into covellite. 



(31) Die Kupfersulfiden. Eugen Posnjak und E. T. Allen (thermochemische Untersuchun- 

 gen), H. E. Merwin (mikroskopische Untersuchungen). Z. anorg. Chem. (in 



press) . 



A German translation of "The sulphides of copper" (Econ. Geol. 10, 

 491-535, 1915). Reviewed under No. 30 above. 



