A new mineral. skinnerite (Cu3SbS3 with minor amounts of silver), is identical with synthetic Cu3SbS3(the B'phase of Skinner, Luce, and Makovicky, 1972). A reversible phase transfoimation takes place in both materials at about l22°C.The high tbmperature modification (B) is orthorhombic (a = 7.8l Å,b = lO.25 Å,c = 6.61 Å, space group Pnma, Z = 4); the low temperature modification (B') is monoclinic (a = 7.81 Å, b = 10.25Å, c = 13.27 Å, β = 90°21´, space group P21/c, Z = 8). The latter becomes polysynthetically twinned on (001). Optical data, microindentation hardness, etching results, and Guinier powder data are listed. Skinnerite occurs intergrown with senarmontite, valentinite, native antimony, tetraliedrite, chalcostibite, galena, and traces of loellingite. These minerals form ore aggregates less, than I cm in size disseminated in analcime-natrolite veins cutting naujaitic rocks in the southern part of the Ilimaussaq intrusion. The geological and mineralogical evidence indicatesthat the ore aggregates crystallized at about 400°C from metal-rich residual magmatic fluids markedly deficient in sulfur.

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