Mineral Sulphides of Iron. 193 



which separates from solution, but also that comparatively 

 slight differences in conditions may give rise to one or the 

 other. Further, that the two minerals may have formed 

 at the same time in some instances. The synthetic experi- 

 ments which have been described proved that the minerals 

 very commonly formed together,* as polymorphic forms which 

 are monotropic are apt to do. Cold solutions which were suffi- 

 ciently acid gave marcasite ; w T arm or hot solutions, either neu- 

 tral or alkaline, gave pyrite, and intermediate conditions gave 

 mixtures. 



II. Pyrrhotite. 



Composition. — Special interest attaches to the composition 

 of pyrrhotite, which, despite much discussion, is still an unset- 

 tled question. The various formulae,! Fe 7 S 8 , Fe n S 12 , Fe n S n + 1 

 and FeS which have been assigned to it by various authors 

 rest on widely varying analytical data. The analytical 

 methods have no doubt been at fault, but the more important 

 question concerns the physical homogeneity of the substance. 

 This has been unusually troublesome. Pyrrhotite almost 

 always occurs in the massive condition, a circumstance which 

 naturally arouses suspicions of its purity, while its opacity 

 makes it impossible to put the question to an optical test. 



Many years ago Lindstrom \ subjected all the known analyses 

 of this mineral to a careful critique. Those which for any 

 reason, such as defective analytical methods or impure mate- 

 rial, appeared unconvincing were rejected. § In the remainder, 

 the ratiof of iron to sulphur was calculated and found to vary 

 from 1:1*06 up to 1:1*19. Some years later Habermehl**lf inves- 

 tigated the same question. He crushed pyrrhotite to a fine 

 powder, covered it with water, and endeavored, by means of a 

 strong horseshoe magnet, to separate it into fractions varying 

 in magnetic intensity. Such fractions as he obtained in this 

 way showed no systematic difference in composition. Haber- 

 mehl used in his experiments the pyrrhotite from Bodenmais. 



A very satisfactory general discussion of the question of 

 admixtures in pyrrhotite is also given in Habermehl's paper. 

 He decided that pyrrhotite could not contain free sulphur 

 because carbon disulphide removes none from it, neither could 



* It may be, however, that pyrite was formed first and was succeeded by 

 marcasite as the acidity of the solution increased. 



\ Sidot judged from experimental work with Fe 3 4 and H 2 S that pyrrho- 

 tite should have the formula Fe 3 S 4 , C. E. lxvi, 1257, 1868. 



X Ofv. Ak. Stockh., xxxii, No. 2, 25, 1875. 



§ 13 analyses out of 43 were thus rejected. 



| In the calculation of the ratios Lindstrom took, in place of the small 

 percentage of nickel found in many of these analyses, the equivalent of iron. 



Tf" Ber. Oberhess. Ges. fur Natur- und Heilkunde, xviii, 83, Giessen, 1879. 



