Crystals of pyrrhotite from nine localities of different geological conditions have been examined by the reflection microscope and X-rays. The results indicate abundant and wide occurrence of three different pyrrhotites, 4C, nC (intermediate pyrrhotites), and 2C (troilite). The 4C and 2C types are stoichiometric in composition with Fe 7 S (sub 8>) and FeS, respectively. The nC pyrrhotites have a composition range approximately from Fe 9 S 10 to Fe 11 S 12 and usually show nonintegral types of superstructure. The 5C and 6C types with integral n-values are specialcases of nC pyrrhotites.Intergrowth of two types is commonly found. Only one type of nC pyrrhotite usually occurs in one locality with the integral type of 4C or 2C, such as 4C + 4.84C and 5.7C +2C. Intergrowth between nC pyrrhotites with different n-values is extremely rare. Only in the Kohmori mine different nC pyrrhotites with n-values ranging from 5.36 to 5.80 have been found together with 4C and 2C.The 4C and 2C types are stable at low temperature because of abundant and wide occurrence, constant and stoichiometric composition, and ordered crystal structures. Because of different nonstoichiometric compositions of the nC pyrrhoittes coexisting with the 4C or 2C types from various localities, and of partially disordered crystal structures, nC pyrrhotites, including 5C and 6C types, are considered to be metastable at very low temperatures.

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