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American Mineralogist; February 1998; v. 83; no. 1-2; p. 133-140
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High form of pentlandite and its thermal stability

Asahiko Sugaki, and Arashi Kitakaze

4-30-503 Kadan, Aoba-ku, Sendai, Japan
Tohoku University, Japan

The high-temperature form of pentlandite (Fe (sub 4.5) Ni (sub 4.5) S 8 ) was found to be stable between 584+ or -3 and 865+ or -3 degrees C, 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 = 5.189 Aa (620 degrees C) 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 Fe (sub 5.07) Ni (sub 3.93) S (sub 7.85) to Fe (sub 3.61) Ni (sub 5.39) S (sub 7.85) at 850 degrees C. However its solid solution extends rapidly toward Ni (sub 3+ or -x) S 2 in the Ni-S join with decreasing temperature. High-form pentlandite with Fe = Ni in atomic percent crystallizes first by a pseudoperitectic reaction between monosulfide solid solution and liquid. The high form (Fe = Ni) crystallized from the liquid always has the metal-rich (S-poor) composition in the solid solution at each temperature and coexists with taenite gamma (Fe,Ni) below 746+ or -3 degrees C. This metal-rich high-form Fe (sub 4.5) Ni (sub 4.5) S (sub 7.4) breaks down into pentlandite and gamma (Fe,Ni) at 584+ or -3 degrees C (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 degrees C.

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