PHASE RELATIONS IN THE Fe-Ni-S SYSTEM FROM 875 TO 650 °C

High-form pentlandite of composition Fe4.93Ni4.06S8.01 first crystallizes by peritectic reaction between liquid and monosulfide solid-solution at 870 +/- 3 degrees C (865 +/- 3 degrees C for high-form pentlandite with Fe=Ni in at.%) in the Fe-Ni-S system; it forms a limited solid-solution from Fe5.08Ni3.93S7.99 to Fe3.81Ni5.24S7.96 including the ideal composition Fe4.50Ni4.50S8.00 at 850 degrees C. The solid solution grows rapidly to extend its field toward the Ni-rich side with decreasing temperature and connects with beta(2) (44.7 at.% S) in the Ni-S join at 806 +/- 3 degrees C (peritectic between liquid and Ni1-xS). Below the peritectic temperature, high-form pentlandite (metal-rich composition) crystallizes directly from liquid. This crystallization continues to the eutectic (746 +/- 3 degrees C for the high form with Fe=Ni) with decreasing temperature, and the high-form solid solution grows still more. Crystallization of the monosulfide solid-solution and high-form pentlandite solid solution from liquid finishes at 762 +/- 3 degrees C and 739 +/- 3 degrees C, respectively. The high-form pentlandite solid solution extended from the Ni-S join is maintained at 700 and 650 degrees C and coexists with monosulfide solid-solution and/or with liquid (870 to 739 degrees C), and/or with beta(1) or its solid solution (800 to 503 degrees C), and/or with gamma (762 to 579 degrees C). In geological processes such as the formation of Ni-Cu ore deposits, pentlandite can crystallize as the high form from liquid (sulfide magma) by peritectic and eutectic reactions at comparatively high temperatures from 870 to 739 degrees C (Fe-Ni-S system). Pentlandite is also produced by the breakdown of the high form at the pseudoeutectoid (or ternary eutectoid). Pentlandite can further be formed by exsolution from monosulfide solid-solution below 600 degrees C, and from the ternary beta(1) phase.