Abstract
High-form pentlandite with Fe4.94Ni4.06S8.00 first crystallizes by pseudoperitectic reaction between liquid and monosulfide solid solution (SS) at 870°C (865C for Fe=Ni in at. %) in the Fe-Ni-S system, and forms a limited SS including Fe4.5Ni4.5S8.0 at 850°C. The SS grows rapidly to extend its field toward the Ni-rich side with decreasing temperature and connects with β2 in the Ni-S boundary at 806C. After the peritectic reaction, successive crystallization of most metal-rich high-form pentlandite from liquid directly continues to the pseudoeutectic temperature (746°C for Fe=Ni) with decreasing temperature. The high-form SS extended from the Ni-S boundary is maintained at 700 and 650°C and coexists with monosulfide SS, liquid (870 to 739°C), β1 SS (800 to 503°C) and/or γ(Fe,Ni) (762 to 579°C).
High-form pentlandite (Fe=Ni, 46.74 at. % S) transforms into pentlandite (low-form) at 615C. However, this transition temperature decreases continuously to 584°C (Fe=Ni, 46.10 at. % S) with decreasing S content. This inversion is reversible. The high form which is unquenchable has a primitive cubic cell with a=5.187(3)A (Fe=Ni, 46.74 at. % S) at 620C. Pentlandite belongs to cubic Fm3m with a=10.100(1)A ( Fe=Ni, 46.74 at. % S) at 25°C. The transition is thought to be an order-disorder inversion from the supercell (low form) to the subcell. The Fe-rich end of the high-form pentlandite SS breaks down to pentlandite and γ at the pseudoeutectoid (584°C for Fe=Ni). This reaction occurs successively using up the Fe-rich end of the SS with decreasing temperature. Consequently, the high-form SS retreats toward the Ni-rich side reducing its field and finally disappears at 503°C and Fe:6.47, Ni:50.00 and S:43.53 at %(ternary eutectoid) to break down into a mixture of pentlandite, high-form godlevskite and ternary β1. High-form pentlandite SS coexists with pentlandite SS at temperatures from 615 to 503°C. Pentlandite and heazlewoodite assemblage found in the Ni-Cu ores is first formed at 498°C. Ternary β1 decomposes into a mixture of pentlandite, heazlewoodite and γ at 484°C and Fe:5.07, Ni:55.94 and S:38.99 at. % (ternary eutectoid).
In the geological processes such as the formation of Ni-Cu ore deposits, it is thought as a result that pentlandite crystallized primarily as the high form by pseudoperitectic reaction between liquid (sulfide magma) and monosulfide SS, successively direct crystallization from liquid along solidus and pseudoeutectic at temperatures from 870 to 739°C (Fe-Ni-S system) and inverted to the low form (pentlandite) during cooling. Furthermore, it has also been ascertained experimentally that pentlandite appears due to exsolution and/or breakdown of monosulfide SS, high-form pentlandite SS and β1 SS below ∼600°C.
