GEOPHYSICAL LABORATORY. 151 



determined. The specific influence of acidity and alkalinity on the crystal 

 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 on acidic solutions 

 of zinc salts and by hydrogen sulphide and sulphur on acidic solutions of fer- 

 rous salts. The unstable forms were usually mixed with the corresponding 

 stable forms, viz, sphalerite and pyrite, and the composition of the mixtures 

 was determined, approximately for the zinc sulphides, by microscopic esti- 

 mation; and within 1 to 2 per cent by the Stokes method for the iron disulphides. 



As previously found, the higher the maximum temperature of experiment, 

 other conditions remaining unchanged, the greater the quantity of the stable 

 form, pyrite or sphalerite, obtained in the product. 



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° C. 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° C. 

 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 subfields: 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. 



No crystalline zinc sulphide could be obtained from the hydrochloric-acid 

 solutions, but the iron disulphides were crystallized from them, and always 

 contained much more marcasite for an equivalent quantity of acid, i. e., 

 hydrochloric acid has a much greater influence on the crystal form than an 

 equivalent quantity of sulphuric acid, which should be the case if the hydro- 

 gen ion concentration were the real determining factor. 



The acid concentration required to give rise to pure marcasite or pure 

 wurtzite falls with the temperature and is close to neutrality for marcasite at 

 ordinary temperature, and probably so for wurtzite. 



Several conditions other than acidity and temperature were varied in the 

 formation of wurtzite, where the process was necessarily more complicated; 

 these were zinc concentration, addition of sodium sulphate to the solutions, 

 and hydrogen-sulphide pressure. None of these had any influence, except as 

 they affected the acidity. 



At temperatures of 25° and 200° C. from sulphuric-acid solutions and at 

 300° C. from hydrochloric-acid solutions we obtained a product containing 95 

 per cent of marcasite comparable with the purest natural marcasite we have 

 had in our hands. Since this determination depends on the quantity of iron 

 dissolved from the mineral under definite conditions, and different natural 

 specimens vary somewhat, it may be that this product is pure synthetic 

 marcasite. 



Some new data on the genesis of the natural minerals are cited. 



(32) Einflussvon Temperatur und Sauregrad auf die Bildung von Marcasit (FeS2) und 

 Wurtzit (ZnS); einen Beitrag zur Enstehung instabiler Formen. E. T. Allen 

 und J. L. Crenshaw. Mikroskopische Studien von H. E. Merwin. Z. anorg. 

 Chem., 90, 107-149 (1914). 



A German translation of "The effect of temperature and acidity on the 

 formation of marcasite (FeS2) and wurtzite (ZnS) : a contribution to the genesis 

 of unstable forms" (Am. Jour. Sci. (4), 38, 393-431: 1914). Reviewed 

 under No. 31 above. 



