Mineral Sulphides of Iron. 181 



FeS0 4 + H 9 S + S = FeS a + H„S0 4 . The quantity is smaller the 

 lower the temperature. It appears to bear no relation to the 

 solubility of the sulphide, for at room temperature this quantity 

 is only a small fraction of 1 per cent. At 200° it lies between 

 3*5 per cent and 5 per cent ; at least this is true for periods of 

 a few weeks. 



Crystals of marcasite. — Measurable crystals of marcasite 

 were obtained by the slow action of hydrogen sulphide on fer- 

 ric sulphate or chloride at several temperatures up to 300° 

 (see p. 174). The general problem of making measurable crys- 

 tals is one of the most troublesome in synthetic mineralogy. 

 As yet we have no light on the means of controlling the 

 number of nuclei which form in the process of crystallization. 

 In general, the more soluble minerals are obtained in larger 

 crystals. Likewise, larger crystals are obtained from a medium 

 in which they are more soluble. In preparing well-formed 

 marcasite crystals, some unaccountable failures were met with, 

 though generally the products obtained by the method previ- 

 ously described contained a number of crystals which were 

 measurable. The marcasite crystals were like the natural 

 mineral in color and luster and. the axial ratio deduced from 

 the angular measurements was a :b:c = 0*7646 : 1 : 1*2176 as 

 compared with 0*7580 : 1 : 1*2122 for natural marcasite (Gold- 

 schmidt). The striations which marked the crystals agreed 

 with orthorhombic and not with regular symmetry (see III, 

 Crystallographic Study). 



Formation of pyrite.* — While the product of the action of 

 sulphur and hydrogen sulphide on ferrous salts is largely mar- 

 casite, the percentage increasing with the quantity of free acid 

 present, pyrite is the principal product where the solution 

 remains neutral or but slightly acid. 



Action of hydrogen sulphide on ferric hydroxide. — Freshly 

 precipitated ferric hydroxide is instantly blackened by hydrogen 

 sulphide. The product is a mixture of ferrous sulphide and 

 sulphur, as shown by the following. Freshly precipitated fer- 

 ric hydroxide was washed free of soluble salts, suspended in 

 water and treated for some time with hydrogen sulphide. A 

 portion of the black amorphous precipitate dissolved in cold 

 dilute acid with evolution of hydrogen sulphide, leaving a 

 residue of amorphous sulphur. Another portion was first 

 digested with ammonium sulphide solution. After filtering 



* For former syntheses of pyrite, see Wohler (Ann., xvii, 260, 1836). 

 Wohler heated an intimate mixture of Fe 2 3 , S, and NH 4 C1 till the NH 4 C1 

 was sublimed. He obtained some small brass yellow tertrahedra and octa- 

 hedra. Senarmont (loc. cit.) obtained FeS 2 by heating ferrous salt solutions 

 with alkaline polysulphides. Geitner (Ann. 129, 350, 1864) heated metallic 

 iron with a solution of sulphur dioxide to about 200°. His product may 

 have been marcasite. See also Doelter (Zs. Kryst., xi, 30, 1886). 



