The system Ni-As-S; phase relations and mineralogical significance

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Abstract
Phase relations in the ternary system Ni-As-S were determined in rigid silica-glass tubes between 700 degrees and 450 degrees C., with some additional data at lower temperatures. The only ternary phase in the system, corresponding to the mineral gersdorffite for which the accepted formula is NiAsS, has a large variation in its As/S ratio. It is homogeneous from Ni-As (sub 1.77) S (sub 0.23) to NiAs (sub 0.77) S (sub 1.23) at 700 degrees C. Gersdorffite in equilibrium with NiAs 2 (rammelsbergite or pararammelsbergite) has the composition NiAs (sub 1.80) S (sub 0.20) and NiAs (sub 1.72) S (sub 0.28) at 660 degrees and 450 degrees C., respectively. This is a difference of only 2 wt % As. The unit cell edge of gersdorffite between stoichiometric NiAsS and NiAs (sub 1.80) S (sub 0.20' is given by the equation: a = 5.6939 + 0.00047OX, where a is the unit cell edge of gersdorffite (in Aa) containing X weight percent NiAs 2 . Gersdorffite analyses from the literature are reviewed and several new specimens examined to demonstrate that natural gersdorffite also has a large variation in its As/S ratio. Complete solid solution exists between niccolite (Ni (sub 1+ or -x) As) and alpha Ni (sub 1-x) S (the high-temperature modification of millerite) above 595 degrees + or - 5 degrees C. The solvus was determined from 595 degrees to 400 degrees C. along the join from NiAs to NiS. Approximately 1 wt % S substituting for As lowers the polymorphic inversion of rammelsbergite to pararammelsbergite from 590 degrees + or - 10 degrees C. to 475 degrees + or - 25 degrees C. Stable assemblages in the synthetic system correspond closely to the associations found in nature.

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