Tetrahedrite forms a stable cone-shape solid solution at 500°C in the quaternary Cu-Fe-Sb-S composition tetrahedron, extending from its base with a narrow composition field on the Cu-Sb-S face to the top of composition Cu10Fe2Sb4S13. With the composition change from Cu12.3Sb4S13 to Cu10Fe2Sb4S13 in the solid solution, the cell edge, melting temperature, and Vickers micro-hardness increase regularly from 10.3260 to 10.3835 A, from 573° to 661 °C, and from 300 to 380 kg/mm2, respectively, whereas the density decreases from 5.00 to 4.81. Although iron-free tetrahedrite Cu12Sb4Sl3 decomposes below 250°C, iron-bearing tetrahedrite such as Cu11.5Fe0.5Sb4S13 persists till room temperature.

Tetrahedrite solid solution is the only stable quaternary phase inside the Cu-Fe-Sb-S system and coexists with all the binary and ternary sulfide phases of the Cu-Fe-Sb-S system at 500°C, except covellite.

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