Binary behavior along the join Cu2S‐Sb2S3, is observed for compositions between stibnite and chalcostibite, i.e., XCu2S = 0 to 0.50 at 895 K. For compositions more Cu2S-rich than XCu2S = 0.50, nonbinary behavior manifested itself as anomalously large exothermic heats of mixing in the high-temperature (895 K) calorimetric study of solid and liquid phases along this join by Bryndzia and Kleppa (1988). This nonbinary behavior is attributed to the precipitation of a solid Sb‐Cu alloy with Xsb ≃ 0.92. This alloy is believed to be the same as the phase reported as pure Sb metal by previous investigators of the Cu‐Sb‐S system. Sb metal was interpreted by these investigators to form by subsolidus breakdown of tetrahedrite or skinnerite.

A textural and compositional examination of liquidus and subliquidus phase relations on the join Cu2S‐Sb2S3 shows that the alloy is the earliest phase to crystallize, consistent with its precipitation from Cu2S‐Sb2S3 melts at 895 K. Early precipitation of an Sb‐Cu alloy from a bulk composition with XCu2S ≈ 0.75 on the quasi-binary join Cu2S‐Sb2S3, would result in the residual composition entering the ternary solid-solution field of tetrahedrite and forming an assemblage of Sb metal, skinnerite, and tetrahedrite.This is exactly what has been documented by Karup-Møller and Makovicky (1974) in a natural skinnerite- bearing sample from the type locality in Greenland. The apparent rarity of skinnerite in nature appears to be controlled by a combination of two factors: (1) the low probability of successfully quenching from high temperature a bulk composition corresponding to skinnerite and (2) the numerous subsolidus peritectic breakdown reactions involving skinnerite conspire to remove much of any that crystallizes. For these reasons, natural samples of appropriate bulk composition often contain tetrahedrite as the dominant phase, with subordinate amounts of Sb metal (± minor famatinite and/or chalcostibite), but rarely any skinnerite.

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