The extent of solid solution in zinkenite, robinsonite and meneghinite has been determined from electron-microprobe-established compositions of their synthetic analogs in pertinent assemblages in the course of a phase-equilibrium study of the system Cu 2 S-PbS-Sb 2 S 3 . All three solid-solution fields are elongate more or less parallel to the PbS-Sb 2 S 3 binary join, and are relatively broader at their PbS-rich ends. The positions of the PbS-rich ends are practically insensitive to variations in temperature. Robinsonite and meneghinite are not stable at 300 degrees C. Plots of molar proportions of Sb 2 S 3 versus PbS suggest a common scheme of substitution, 3Pb (super 2+) <-->2Sb (super 3+) . In PbS-rich compositions of the solid solutions, the proportion of Cu is substantial, and Cu must be incorporated on the left-hand side of the substitution scheme. Natural compositions of meneghinite cluster at one point in the Cu 2 S-PbS-Sb 2 S 3 ternary system, but zinkenite and robinsonite have variable compositions. The most appropriate formula for zinkenite appears to be 6PbS.7Sb 2 S 3 . Likewise, 4PbS.3Sb 2 S 3 is the preferred formula for robinsonite.

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