104 REPORTS ON INVESTIGATIONS AND PROJECTS. 



the usual method of cooHng curves. This fact explains the wide variations 

 between existing determinations by different observers. The melting tem- 

 perature is not subject to this limitation, and appears with reasonable prompt- 

 ness at 1088° with a probable uncertainty of 2°. The densities and some of 

 the optical constants of the crystals were determined for the preparations 

 used. 



(23) Minerals and rocks of the composition MgSiOs-CaSiOs-FeSiOs. Robert B. Sos- 



man. Jour. Wash. Acad. Sci., i, 54. 191 1. 



This paper is a partial report on a study from the statistical standpoint of 

 certain rocks and minerals having a composition corresponding to a mixture 

 of the metasilicates of magnesia, lime, and ferrous iron. The method of 

 treatment is already familiar to mineralogists and petrologists through the 

 publications of Vogt. 



(24) The mineral sulphides of iron. E. T. Allen, J. L. Crenshaw, and John Johnston; 



with Crystallographic study by E. S. Larsen. (In press.) 



The formation of iron disulphide was accomplished (i) by the action of 

 hydrogen sulphide on ferric salts, or the action of sulphur and hydrogen 

 sulphide on ferrous salts; (2) by the addition of sulphur from solution to 

 amorphous ferrous sulphide or pyrrhotite; (3) by the action of soluble poly- 

 sulphides on ferrous salts ; (4) by the action of soluble thiosulphates on fer- 

 rous salts according to the equation 4M2S2O3 -|- Fexs = 3M2SO4 + FeSg + 

 2Mx -|- 3S. The first three methods may be generalized as the action of 

 sulphur on ferrous sulphide: (i) in acid solutions; (2) in nearly neutral 

 solutions; and (3) in alkaline solutions, since in (i) we may assume that 

 ferrous sulphide first forms by the action of hydrogen sulphide on the fer- 

 rous salt, and in (3) we know that polysulphides first precipitate a mixture 

 of ferrous sulphide and sulphur. Marcasite was obtained with certainty 

 only by method (i) ; low temperatures and free acid favor its formation. 

 A solution containing about i per cent of free sulphuric acid at 100° gives 

 pure marcasite. Less acid solutions at higher temperatures give mixtures of 

 marcasite and pyrite. The other methods give pyrite, which, under certain 

 conditions, may be mixed with amorphous disulphide. It is possible that 

 some marcasite may be formed by method (4). 



Marcasite and pyrite were identified in the above products : (i) by micro- 

 scopic examination and crystallographic measurement; marcasite crystals 

 with the proper axial ratio were prepared for the first time : (2) by Stokes's 

 oxidation method. This method serves also for the analysis of mixtures of 

 the two minerals. 



Marcasite changes to pyrite with evolution of heat. The change proceeds 

 very slowly at 450° and is not accelerated by pressures even of 10,000 atmos- 

 pheres. Marcasite is monotropic toward pyrite. This is in accord with the 

 greater inclination of marcasite to oxidize, its assumed greater solubility, and 

 the fact that its formation is conditioned by the composition of the solution 

 from which it crystallizes. 



The fact that marcasite never occurs as a primary constituent of magmas, 

 while pyrite sometimes does, is explained by the fact that marcasite can not 

 exist above 450°. The formation of pyrite in deep veins and hot springs is 

 explained by the fact that the waters from which it came contained no strong 

 acid.^ The marcasite of surface veins was probably formed from cold acid 

 solutions, while mixtures of marcasite with pyrite were probably conditioned 

 by higher temperature (up to 300° or 400°), or the presence of less acid, or 



