394: Allen, Crenshaw, and Merwin — 



I. Introduction. — Determinative Influence of Alkalinity 

 and Acidity on Crystalline Form. 



In the course of an investigation of the disulphides of iron, 

 it was found by the authors that the stable form, pyrite,* is 

 the only crystalline form which is obtained by the action of 

 alkali polysulphides on ferrous salts. The first product of this 

 reaction at room temperature is a mixture of amorphous ferrous 

 sulphide and sulphur, from which the disulphide gradually 

 forms. The formation is very slow at low temperatures, and, even 

 at 100°, the product appears to be at first amorphous, but it 

 crystallizes in time to pyrite with excess of reagent — a trans- 

 formation which could not be effected with natural marcasite 

 under similar conditions. From acid ferrous solutions, on the 

 other hand, by the action of sulphur and hydrogen sulphide the 

 unstable form marcasite is obtained. The marcasite is com- 

 monly admixed with pyrite, the quantity of which may be 

 reduced by raising the initial concentration of the free acid ; or 

 increased by raising the temperature. 



The same regularity was observed to hold good for the 

 sulphides of zinc and mercury,t i. e. only the stable forms, 

 sphalerite and cinnabar, were obtained from alkaline solutions ; 

 while only from acid solutions could the corresponding unstable 

 forms, wurtzite and metacinnabar, be obtained. This peculiar 

 regularity seemed to us of sufficient interest to form the 

 subject of a separate investigation. There has never been any 

 doubt about the products of the alkaline solutions, but a more 

 thorough study of the products of the acid solutions seemed 

 desirable, more especially as so little is known about the genesis 

 of unstable forms. We have been obliged to confine ourselves 

 to the investigation of marcasite and wurtzite, for, while our 

 synthetic black sulphide of mercury is an unstable crystalline 

 form and conforms to the above rule in crystallizing from acid 

 solutions, it could not be absolutely identified with the natural 

 metacinnabar, and its formation has thus far been brought 

 about only within a very limited range of conditions. 



II. Marcasite and Pyrite. 



A. Preparation. — The marcasite-pyrite mixtures were pre- 

 pared by heating in sealed tubes, a 5 per cent solution of 

 hydrous ferrous sulphate,:); containing a measured quantity of 

 sulphuric acid, sulphur and hydrogen sulphide. A Jena glass 

 tube of about 17-18 mm internal diameter and closed at one end 



* This Journal, xxxiii, 169, 1912; Zs. anorg. Chem., lxxii, 201, 1912. 

 f This Journal, xxxiv, 341, 1912; Zs. anorg. Chein., lxxix, 125, 1912. 

 % In one series of experiments ferrous chloride and hydrochloric acid were 

 taken. 



