A new natural phase in the system Mg₂SiO₄-Mg₂BO₃F-Mg₂BO₃(OH): composition, paragenesis and structure of OH-dominant pertsevite

Authors: Galuskina, Irina O.; Kadiyski, Milen; Armbruster, Thomas; Galuskin, Evgeny V.; Pertsev, Nikolai N.; Dzierżanowski, Piotr; Wrzalik, Roman

Source: European Journal of Mineralogy, Volume 20, Number 5, October 2008 , pp. 951-964(14)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

Si-bearing, OH-dominant pertsevite Mg₂(BO₃)_1−x(SiO₄)_x(OH,F)_1−x (x = 0.2−0.3) from contact-metasomatic kotoitites and kotoite marbles at East Verkhoyan'ye region (northern Siberia) and at Gonochan, Dzhugdzhur Ridge (Russian Far East) was studied by electron microprobe (EMP) techniques, including boron analyses, and Raman spectroscopy. At both localities borate mineral-assemblages are very similar. Minerals associated with OH-dominant pertsevite (East Verkhoyan'ye region) are ludwigite, kotoite, szaibelyite, REE-bearing sakhaite, olivine and minerals of the humite group. Humite-group minerals and olivine were found to be slightly B₂O₃-bearing (ca. 1 wt.%). OH-dominant pertsevite, described here for the first time, formed at the expense of kotoite and hydroxylclinohumite and has a rather heterogeneous composition with respect to Si, B, F and OH between different grains and even within individual grains.

The crystal structure of one selected grain of OH-dominant pertsevite (Mg_1.95Fe_0.04Mn_0.01)(BO₃)_0.75(SiO₄)_0.25[(OH)_0.45F_0.30], space group Pnma, a = 20.494(3), b = 11.890(2), c = 4.5880(6) Å, from the East Verkhoyan'ye region, has been determined by single-crystal X-ray methods yielding a strongly disordered distribution of SiO₄ tetrahedra and triangular BO₃ units within a hexagonal close-packed anion (O, OH, F) arrangement with Mg occupying half of the octahedral voids.

Keywords: OH-DOMINANT PERTSEVITE; MAGNESIOBORATE; CHEMICAL ANALYSIS; CRYSTAL STRUCTURE ANALYSIS; RAMAN SPECTROSCOPY; KOTOITE MARBLE; BORON-BEARING SKARN

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2008/0020-1821

Publication date: 2008-10-01

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