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Barysilite from Garpenberg Norra, Dalarna, Sweden: occurrence and crystal structure refinement

¹ Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstr. 14, A-1090 Wien, Austria
² Department of Mineralogy, Research Division, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

^* E-mail: uwe.kolitsch{at}univie.ac.at

A new occurrence of barysilite, Pb₈Mn(Si₂O₇)₃, at the polymetallic Garpenberg Norra Zn-Pb deposit, Hedemora, Dalarna, Sweden, is described. The mineral, which forms colourless, transparent grains, is characterized by X-ray diffraction and electron-microprobe analyses. The assemblage includes tephroite, zincian jacobsite, manganoan diopside and others. The crystal structure of a barysilite crystal from Garpenberg Norra was redetermined using single-crystal X-ray diffraction data (Mo-K, CCD area detector) and has been refined in space group Rc with a = 9.804(1), c = 38.416(8) Å, V = 3197.8(8) ų, to R1 = 2.32% for 1025 ’observed’ reflections with F_o >4 (F_o). A previous, low-precision structure determination (Lajzérowicz, 1965; R = 20%) is confirmed but improved considerably. The structure contains one distorted MnO₆ polyhedron with six equivalent Mn–O bonds (2.224 Å), one Si₂O₇ disilicate unit with an Si–O–Si angle of 120.9°, and two non-equivalent Pb sites. The Pb1 site has a highly irregular, one-sided coordination with six O ligands, indicating a stereoactive 6s² lone-electron pair on the Pb²⁺ ion, whereas the [6+3]-coordinated Pb2 site is fairly regular, with Pb–O distances of 2.540 (3 x), 2.674 (3 x) and 3.098 (3 x) Å. The Pb2 site contains ~10% of Ca (+Ba) replacing Pb, corresponding to the structural formula Pb1₆(Pb,Ca)²₂Mn(Si₂O₇)₃. This is the first direct proof that not only the M site in barysilite-type Pb₈M(Si₂O₇)₃ compounds can be replaced by divalent cations.

KEYWORDS: barysilite, crystal structure, crystal chemistry, Garpenberg Norra, Sweden