430 Allen, Crenshaw, and Merwin — 



show conditions of temperature and acidity under which the 

 two minerals may be formed at will together and apparently 

 simultaneously. 



Summary. 



1. Our former results on the genesis of marcasite and wurtz- 

 ite have been reinvestigated, the former conclusions have 

 been confirmed and new data determined. The specific influ- 

 ence of acidity and alkalinity on the crystalline form of the 

 sulphides investigated has been much more rigorously demon- 

 strated. Only from acid solutions were the unstable forms 

 obtained. The sulphides were prepared by the action of 

 hydrogen sulphide and sulphur on acidic solutions of zinc salts 

 and by hydrogen sulphide and sulphur on acidic solutions of 

 ferrous salts. The unstable forms were usually mixed with 

 the corresponding stable forms, viz. : sphalerite and pyrite, and 

 the composition of the mixtures was determined, approxi- 

 mately for the zinc sulphides, by microscopic estimation ; and 

 within 1 to 2 per cent by the Stokes method for the iron disul- 

 phides. 



2. As previously found, the higher the maximum tempera- 

 ture of experiment, other conditions remaining unchanged, the 

 greater the quantity of the stable form, pyrite or sphalerite, 

 obtained in the product. 



3. As previously concluded, the higher the percentage of 

 acid in the solution, other conditions remaining unchanged, 

 the greater in general the quantity of the unstable sulphide, 

 marcasite or wurtzite. The relation between the percentage 

 of marcasite and the average acidity was practically linear for 

 maximum temperatures of 200° and 300.° There are also 

 indications of a similar relation in the case of wurtzite. In 

 the case of wurtzite, however, the final acid was found to be 

 the determining factor, since at 300° and 325° wurtzite appears 

 to change into sphalerite when heated with sufficiently dilute 

 acid. The temperature-acid field in the case of zinc salts may 

 be divided by two boundary curves into three sub-fields, a 

 high acid field in which only wurtzite is obtained, a low acid 

 field where only sphalerite is obtained, and an intermediate 

 field where mixtures of the two are obtained. 



4. No crystalline zinc sulphide could be obtained from 

 hydrochloric acid solutions, but the iron disulphides were crys- 

 tallized from them, and always contained much more marca- 

 site for an equivalent quantity of acid ; i. e., hydrochloric acid 

 has a much greater influence on the crystalline form than an 

 equivalent quantity of sulphuric acid, which should be the case 

 if the hydrogen ion concentration were the real determining 

 factor. 



