Decomposition of Iron Pyrites. 395 



form of pyrite. An example of this has already been given in 

 the reference to Freiberg. 



However, under other conditions of alteration — I suspect, low 

 pressure and ordinary temperature of surface waters, and it may 

 be, as Mene suggests, under conditions of chemical double de- 

 composition — the iron disulphide, derived from the original 

 pyrrb.otite, has at once assumed the form of marcasite. The 

 common occurrence of this mineral in calcareous beds, e. g., the 

 chalk of England, the Trenton limestone of New York, the car- 

 boniferous limestones of Wisconsin and Illinois, etc., suggests 

 the possibility that the presence of calcium carbonate, under 

 certain conditions, may determine the crystallization of iron di- 

 sulphide as marcasite, rather than pyrite, in a way similar to 

 that of the crystallization of calcium carbonate as aragonite, 

 rather than calcite, in the presence of strontium carbonate, gyp- 

 sum, or salts of lead, as shown by Credner. 



A reference is here called for to the view advanced by Breit- 

 haupt, 71 who endeavored to account for the origin of the differ- 

 ent physical properties of the two dimorphous minerals, pyrite 

 and marcasite, by connecting them with those of the dimorphous 

 forms of sulphur itself, first ascertained by Mitscherlich. But 

 later investigation has even reversed the analogy attributed by 

 Breithaupt to some of the physical properties of the sulphur 

 forms, e. g., that of specific gravity; and little remains except 

 that the form S«, i. e., of native sulphur, and marcasite both 

 happen to be ortho-rhombic and homoeomorphous. 



Thirdly, by some process not yet understood, pyrite is often 

 transformed afterwards into its dimorphous associate, marcasite. 

 A discrimination of this kind has been recently made by Prof. 

 A. Liversidge, 72 between iron pyrites of primary and secondary 

 metamorphic origin, in his description of the occurrence of con- 

 cretions of pyrites at a mine, near Eydal, in New South Wales. 

 Here, he states, the pyrite was originally crystallized in cubes 

 through a slaty shale, afterward dissolved and redeposited "in 

 the form of nodules of marcasite, i. e., the rhombic and less 



11 Erd. Jour. f. pr. Obem. (1835), IV, 257. 



« Jour, and Proc. of Royal Soc. of New So. Wales (1884), XVIII, 47. 



