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The new mineral baumstarkite and a structural reinvestigation of aramayoite and miargyrite

¹ Institut für Mineralogie und Kristallographie, Universität Wien, Althanstrasse 14, A-1090 Vienna, Austria
² Institut für Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria
³ Department of Mineralogy, The Natural History Museum, London SW7 5BD, U.K.

Correspondence: ^* E-mail: herta.silvia.effenberger{at}univie.ac.at

Baumstarkite is a new mineral found coating miargyrite from the San Genaro mine, Huancavelica Department, Peru. It is triclinic and the third naturally occurring modification of AgSbS₂ besides monoclinic miargyrite and cubic cuboargyrite. The composition is usually close to the ideal formula. However, some grains of baumstarkite show zoned lamellae with As contents up to 11.5 wt% and accords to Ag₃(Sb, As)₂SbS₆. Baumstarkite is isotypic with aramayoite [end-member composition Ag₃Sb₂BiS₆; solid solutions require the extended formula Ag₃Sb₂(Bi, Sb)S₆]. Single-crystal X-ray structure investigations were performed for baumstarkite [type locality, a = 7.766(2), b = 8.322(2), c = 8.814(2) Å, = 100.62(2), ß = 104.03(2), = 90.22(2)°, Z = 2{Ag₃Sb₃S₆}, space group P, R1(F) = 0.057, wR₂(F²) = 0.128], aramayoite [Armonia mine, El Quevar, Argentinia: a = 7.813(2), b = 8.268(2), c = 8.880(2) Å, = 100.32(2), ß = 104.07(2), = 90.18(2)°, Z = 2{Ag₃Sb₂S₆}, space group P, R1(F) = 0.034, wR2(F²) = 0.084], and miargyrite associated with baumstarkite type material [a = 12.862(3), b = 4.409(1), c = 13.218(3) Å, ß = 98.48(2)°, Z = 8{AgSbS₂}, space group C2/c, R1(F) = 0.031, wR2(F²) = 0.082]. The space-group symmetries of aramayoite and miargyrite were revised, and the refinements unambiguously showed that the three investigated minerals are centrosymmetric.

In baumstarkite and aramayoite each three atomic sites are occupied by Ag and M = As, Sb, Bi, respectively. The Ag atoms have two short bonded ligands (Ag-S is 2.51 to 2.58 Å). The M1 and M2 sites are [3 + 3] coordinated and are predominantly occupied by (Sb, As) atoms (M-S = 2.44 to 2.54 Å and > 3.09 Å). The [2 + 2 + 2] coordination of the M3 atom differs in the two mineral species: the two shortest bond lengths in baumstarkite are smaller (2.51 Å) than in aramayoite (2.64 Å) to allow for the different sizes of the Sb and Bi atoms, respectively; the medium bond lengths are similar (2.75 to 2.82 Å) and the longest bond lengths are >3.02 Å. Considering only the nearest-neighbor environments, baumstarkite and aramayoite feature zigzag chains parallel to [010], which are linked together to form layers parallel to (001). In miargyrite [2 + 2] and [2] coordinated Ag atoms are linked by SbS₃ pyramids to form a three-dimensional network.

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