The phase relations between Fe4.5Ni4.5S8 and Co9S8 in the system Fe-Ni-Co-S at temperatures from 400° to 1100°C

Phase relations along the join Fe4.5Ni4.5S8-Co9S8 in the system Fe-Ni-Co-S were investigated by the evacuated-silica-glass-tube method. Euhedral crystals of high-form cobalt pentlandite (Co9S8) and a member of the solid solution containing 50 mole % Co9S8 were also synthesized by both I-2 vapor-transport and NaCl-KCl flux methods. The phases produced in this study were examined by ore microscopy, SEM, EPMA, high-temperature X-ray diffraction and DTA. A continuous solid-solution (low form) between pentlandite and cobalt pentlandite transforms to a high-form solid-solution at temperatures from 615degrees +/- 3degreesC (pentlandite) to 831degrees +/- 3degreesC (cobalt pentlandite). Except for end-member compositions, there is a narrow two-phase field with both low- and high-form solid-solutions. The high-low inversion is reversible. High-form solid-solution melts incongruently to liquid and monosulfide solid-solution at temperatures from 865degrees +/- 3degreesC (high-form pentlandite) to 930degrees +/- 3degreesC (high-form cobalt pentlandite), and then remnant monosuffide solid-solution melts completely at temperatures from 982degrees +/- 5degreesC for (Fe,Ni)(1-x)S to 1069degrees +/- 5degreesC for Co1-xS. These lines of evidence suggest that in geological processes such as the formation of Ni-Cu(-Co) ore deposits, pentlandite, Co-bearing pentlandite and Ni-bearing cobalt pentlandite can crystallize as the high form by peritectic reaction between monosulfide solid-solution segregated first from liquid (sulfide magma) and residual liquid at relatively high temperatures around 800degrees to 900degreesC. Pentlandite (Fe4.5Ni4.5S8.0), cobalt pentlandite (Co9S8) and the member of the solid solution with 50 mole % Co9S8 (Fe2.25Ni2.25Co4.50S8.00) are cubic, Fi (3) over barm, with a equal to 10.0608(2), 9.9287(2) and 9.9925(2) Angstrom, respectively, at room temperature. On the other hand, the high forms have a primitive cubic (pc) cell with a equal to 5.335(2) Angstrom for Fe4.5N4.5S8.0, 5.171(2) Angstrom for Co9S8, and 5.251(2) Angstrom for the composition with 50 mole % Co9S8 at 850degreesC, corresponding to a/2 of the low form. The inversion of low- and high-form solid-solution is of order-disorder type, from supercell (low form) to subcell.