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THE CRYSTAL STRUCTURE OF Si-DEFICIENT, OH-SUBSTITUTED, BORON-BEARING VESUVIANITE FROM THE WILUY RIVER, SAKHA-YAKUTIA, RUSSIA

Evgeny V. Galuskin^1,, Irina O. Galuskina¹, Katarzyna Stadnicka², Thomas Armbruster³ and Marcin Kozanecki⁴

¹ Faculty of Earth Sciences, Department of Geochemistry, Mineralogy and Petrography, University of Silesia, Bdziska 60, 41–200 Sosnowiec, Poland
² Faculty of Chemistry, Jagiellonian University, Ingardena 3, Cracow, 30–060, Poland
³ Laboratory for Chemical and Mineralogical Crystallography, University of Bern, Freiestr. 3, CH–3012 Bern, Switzerland
⁴ Technical University, Department of Molecular Physics, eromskiego 116, 90–924 ód, Poland

E-mail address: galuskin{at}us.edu.pl

The crystal structure of a Si-deficient vesuvianite, space group P4/nnc, a 15.678(1), c 11.828(1) Å, from the Wiluy River, Sakha–Yakutia, Russia, has been refined from single-crystal X-ray data to R = 0.037. Electron-microprobe analyses indicate that this vesuvianite has only ca. 16 Si pfu in contrast to regular vesuvianite with 18 Si pfu. Site-occupancy refinement yielded substantial vacancies at orthosilicate sites Z(1): 25% vacancies, Z(2): 16% vacancies. Vacancies at the tetrahedral site are associated with increased Z(1)–O and Z(2)–O distances, 1.687 and 1.660 Å, respectively. Vacancies and increased Z(1)–O and Z(2)–O bond lengths are consistent with hydrogarnet-type defects, where SiO₄ is replaced by H₄O₄ tetrahedra. The single crystal investigated shows the highest hydrogarnet-type substitution analyzed by structure refinement of vesuvianite. No vacancies were found involving the disilicate groups. Along the c axis, the increased size of Z(1) tetrahedra is balanced by a compression of the adjacent X(3) Ca-bearing dodecahedra. This structural flexibility is the reason why the length of the c axis remains largely independent of the hydrogarnet-type substitution. A corresponding structural flexibility does not exist along the a axis, leading to a systematic increase of a with increasing proportion of vacancies at the Si site. Polarized Raman spectra in the OH-stretching region are interpreted to indicate that hydrogarnet-type defects in vesuvianite lead to a more isotropic polarization of the absorption bands at about 3650 cm^–1.

Keywords: Si-deficient vesuvianite, composition, crystal structure, Raman spectra, hydrogarnet-type defects, Yakutia, Russia.