High form of pentlandite and its thermal stability

The high-temperature form of pentlandite (Fe4.5Ni4.5S8) was found to be stable between 584 6 3 and 865 6 3 8C, breaking down into monosulfide solid solution and liquid at the later temperature. The phase is unquenchable and always displays the X-ray pattern of pentlandite (low form) at room temperature. High-temperature X-ray diffraction demonstrated that the high form has a primitive cubic cell with a 5 5.189 A ˚ (620 8C) corresponding to a/2 of pentlandite. The high-low inversion is reversible, accompanied by a large latent heat. It is thought to be order-disorder in character. The transition temperature falls with decreasing S content. The high form of pentlandite has a limited solid solution from Fe5.07Ni3.93S7.85 to Fe3.61Ni5.39S7.85 at 850 8C. However its solid solution extends rapidly toward Ni3 6 XS2 in the Ni-S join with decreasing temperature. High-form pentlandite with Fe 5 Ni in atomic percent crystallizes first by a pseudoperitectic reaction between monosulfide solid solution and liquid. The high form (Fe 5 Ni) crystallized from the liquid always has the metal-rich (S-poor) composition in the solid solution at each temperature and coexists with taenite g (Fe,Ni) below 746 6 3 8C. This metal-rich high-form Fe4.5Ni4.5S7.4 breaks down into pentlandite and g (Fe,Ni) at 584 6 3 8C (pseudoeutectoid). These results suggest that in geological processes, such as the formation of Ni-Cu ore deposits, pentlandite can crystallize as the high form from liquid (sulfide magma) at the comparatively high temperatures around 800 8C.