394 Allen and Crenshaw — Sulphides of Zinc, 



mg data on the crystallography of metacinnabarite. Crystals 

 of this form are black, and when typically developed have 

 six spindle-shaped rays, apparently equal and meeting at right 

 angles. These characteristics indicate skeletal growth par- 

 allel to the axes of the cube. Bnt numerous rays are present 

 in some cases, and though the six can usually be distinguished, 

 the exact relations of the intermediate rays are uncertain. 

 None of the rays exceed -01 mm in length. 



In several preparations — previously described — black prisms 

 were found which, in all but two cases, were shown to consist 

 of transparent double salts coated with amorphous mercuric 

 sulphide. In the two cases no satisfactory determination could 

 be made. 



Summary. 



1. The two sulphides of zinc are enantiotropic : /3-ZnS 

 or sphalerite is stable below 1020°, where it is transformed 

 into a-ZnS or wurtzite. Sphalerite has a density of 4-090, 



(mineral at 25°\ . . 

 ; — )• Wurtzite has a density very slightly less, 

 W 3*161" 3X 4 J 



viz., 4-087. The determinations were made on a very pure 

 analyzed sphalerite, and the wurtzite formed by heating it to 

 the proper temperature. The refractive indices of these forms 

 for sodium light are : sphalerite, n, = 2 , 3088 ; wurtzite, co, = 

 2-356, e = 2-378. 



2. Iron sulphide in solution lowers the inversion point of 

 sphalerite strongly and in a nearly regular manner. The inver- 

 sion temperatures of four analyzed ferruginous sphalerites, the 

 highest containing 17 per cent of iron, were determined. The 

 specific volumes of these sphalerites varied almost rectilinearly 

 with the percentage of iron. The volume increases with the 

 latter, although the specific volume of ferrous sulphide is only 

 about 85 per cent as great as that of zinc sulphide. The 

 refractive indices for sodium light of both sphalerite and 

 wurtzite are raised 0-0033 for each per cent of ferrous sulphide. 



3. Crystals of wurtzite of considerable size were obtained by 

 sublimation at about 1200 o -1300°. Small dodecahedrons of 

 sphalerite were obtained from molten sodium chloride at a 

 little above 800°, while larger dodecahedrons as well as tetra- 

 hedrons crystallized from molten potassium polysulphide at 

 about 350°. From aqueous solutions both sphalerite and 

 wurtzite were obtained at temperatures between 200° and 400°. 

 Below about 200° the products were amorphous. From solu- 

 tions of alkali sulphides (alkaline solutions), only sphalerite 

 formed ; both dodecahedrons and tetrahedrons were obtained. 

 From acid solutions of zinc salts hydrogen sulphide precipi- 

 tates at 250° and above, both sphalerite and wurtzite. In nearly 

 all cases (10 out of 12 experiments) so far as experiments have 



