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

Authors: Kolitsch U.; Holtstam D.

Source: Mineralogical Magazine, Volume 66, Number 2, 1 April 2002 , pp. 353-363(11)

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Abstract:

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 alpha

, CCD area detector) and has been refined in space group R&3macr;c with a = 9.804(1), c = 38.416(8) Å, V = 3197.8(8) Å³, to R1 = 2.32% for 1025 'observed' reflections with F_o >4 sigma

(F_o). A previous, low- precision structure determination (Lajzérowicz, 1965; R = 20%) is confirmed but improve 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×), 2.674 (3×) and 3.098 (3×) 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

Language: English

Document Type: Research article

Publication date: 2002-04-01

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