A new mineral species, terrywallaceite, ideally AgPb(Sb,Bi)3S6, has been found in Mina Herminia, Julcani Mining District, Huancavelica, Peru. It is associated with tetrahedrite, gustavite, barite, and pyrite. Terrywallaceite crystals are lath-shaped, metallic-black, with striations parallel to the elongated direction (the
Terrywallaceite is a member of the lillianite group and isostructural with P21/c gustavite. Its unit-cell parameters are a = 6.9764(4), b = 19.3507(10), c = 8.3870(4) Å, β = 107.519(2)°, and V = 1079.7(1) Å3. The structure of terrywallaceite contains six symmetrically-nonequivalent S sites and five cation sites [Ag, Pb, M1 (=0.82Bi + 0.18Sb), M2 (=0.60Bi + 0.40Sb), and M3 (=0.95Sb + 0.05Bi)]. The pronounced preference of Sb for the M3 site over M2 and M1 in terrywallaceite is consistent with the site occupancy data reported for Sb-bearing gustavite, and suggests an alternative ideal formula for terrywallaceite of AgPb(Sb,Bi)(Bi,Sb)2S6, instead of AgPb(Sb,Bi)3S6.
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