Effect of Temperature ', Acidity, etc. 429 



of sulphides, including pyrite or mareasite. Mr. Siebenthal 

 has recently submitted some specimens of mareasite imbedded 

 in calcite from Crystal Cave, Joplin, Missouri ; and Mr. J. B. 

 Umpleby a specimen of wurtzite also imbedded in calcite. 

 The nature of these crystals indicates that they did not develop 

 in free space, but that they crystallized simultaneously with cal- 

 cite.* If so, the mareasite and wurtzite must have crystallized 

 from solutions containing no stronger acid than carbonic acid, 

 and probably at the ordinary temperature. It should be borne 

 in mind that all the solutions used in our experiments, though 

 dilute in some cases, were incontestably acid or incontestably 

 alkaline, so that the specific influence of each is unquestionable. 

 The nearest approach to the above conditions we have been 

 able to employ are given on page 404, where we found that 

 pure mareasite, or at any rate very nearly pure mareasite, was 

 obtained from a solution containing 0*015 per cent sulphuric 

 acid. Between this degree of acidity and very slight alkalinityf 

 at ordinary temperatures, there exists a narrow field which has 

 not been explored, nor does it present a promising field for 

 exploration. We only know that in solutions containing only 

 hydrogen sulphide, and therefore close to neutrality (reaction 

 with ferric hvdroxide suspended in water), pvrite is obtained 

 at 150°4 



We have also shown that wurtzite, in the temperature inter- 

 val where it could be crystallized, was formed from acid solu- 

 tions, and the concentration of the acid demanded for the 

 inhibition of sphalerite grows less with descending tempera- 

 ture as it does with pyrite. 



C. Paragenesis of pyrite ivith calcite. — Pyrite also may 

 be contemporaneous with calcite. A specimen of both marea- 

 site and pyrite in calcite was obtained by Mr. Siebenthal and 

 examined by Dr. Merwin.§ The latter believes that we have 

 here two distinct generations of iron disulphide, the mareasite 

 before the pyrite, and each contemporaneous with calcite. It 

 would be unwise to attempt a discussion of the problem here 

 presented without further evidence, but it might be suggested 

 that calcite has been formed in the laboratory by precipitation 

 with alkali carbonate, as well as from carbonic acid solutions, 

 and its occurrence in recent crystals in the slightly alkaline 

 waters of the sulphur springs above cited proves also that 

 pyrite and calcite can form from the same solutions. 



It is also possible that in some cases pyrite and mareasite 

 form together in nature, though the specimen in question does 

 not appear to have formed in that way. Our experiments 



* H. E. Merwin, The simultaneous crystallization of calcite and certain 

 sulphides of iron, copper and zinc, this Journal, (4), xxxviii, p. 355, 1914. 

 fThe waters of the sulphur springs just cited were almost neutral. 

 {This Journal, (4), xxxiii, 181, 1912; Zs. anorg. Chem., lxxvi, 215, 1912. 

 §Loc. cit. 



