308 MINERALOGY 



Pyrrhotite. FeS, Fe n S n+ i ; Magnetic sulphide of iron ; Fe = 

 61.6, S = 38.4; Hexagonal; Type, Ditrigonal or Dihexagonal 

 Polar (?) c = 0.7402 ; 0001 A *&& = 45 7' ; Common forms, 

 c(0001), m(1010), s(1011); TwmAing plane s; Cleavage, c dis- 

 tinct, a less so; Brittle, fracture uneven; H. =3.5-4.5; G. = 

 4.58-4.64 ; Color, bronze-yellow ; Streak, grayish black ; Luster, 

 metallic; Magnetic. 



B.B. In O. F. on coal yields sulphur dioxide and in R. F. black- 

 ens and becomes strongly magnetic. Yields an iron reaction with 

 the fluxes; some may show cobalt, nickel, or copper. Dissolves in 

 hot HC1, yielding hydrogen sulphide. 



General description. Usually massive with a distinct part- 

 ing, granular, or disseminated. Crystals are rare, generally small 

 six-sided tablets, combinations of the base and a prism ; the prism 

 faces are horizontally striated. Other forms have been described 

 as a series of pyramids, but are unusual in occurrence. Artificial 

 crystals show a polar development. Pyrrhotite darkens on expo- 

 sure, often iridescent, easily oxidizing to hematite or limonite. 



Chemical analyses show a great variation in the proportions of 

 iron and sulphur, there being more sulphur than is required for the 

 formula FeS by an amount equal to an atom of sulphur in excess ; 

 from this it has been suggested that the series may be represented 

 by the formula Fe n S n+ i, commonly FenSi2, though analyses show 

 a variation from Fe 6 S 6 to Fei 6 Sn in the natural mineral. Both 

 nickel and cobalt may form an analogous series of sulphides, as in 

 each case minerals related in the same manner have been described, 

 while pyrrhotite contains small quantities of these metals, and in 

 fact the larger portion of the nickel of commerce is derived from 

 pyrrhotite. When the percentage of nickel is above 6 per cent., the 

 mineral is known as pentlandite (FeNi)S. 



Troilite, FeS, is the massive ferrous sulphide occurring in me- 

 teorites. Pyrrhotite may occur as a magmatic segregation, as is 

 the case of the Norway deposits, or as a minor accessory, connected 

 with gabbro, diorite, basalts, and norites, or with other ferromagne- 

 sian rocks. Such magmas dissolve sulphides which on solidifica- 

 tion separate very early, a process somewhat analogous to the 

 formation of a matte in the smelting furnace. The large deposits 

 at Ducktown, Tennessee, and at Sudbury, Ontario, are impregna- 



