Johachidolite, CaAl[B₃O₇], a mineralogical and structural peculiarity

Authors: Kadiyski, Milen; Armbruster, Thomas; Günther, Detlef; Reusser, Eric; Peretti, Adolf

Source: European Journal of Mineralogy, Volume 20, Number 5, October 2008 , pp. 965-973(9)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

The crystal structure of johachidolite, CaAl[B₃O₇], (Cmma, a = 9.767(2), b = 11.723(3), c = 4.3718(5) Å, V = 408.3(1) Å³, Z = 4) has been refined (R1 = 2.5 %) from a light-green gem-quality crystal from the Mogok gem-mining district in Myanmar. A fragment from the same gem was measured by electron microprobe and Laser-Ablation Inductively-Coupled-Plasma Mass-spectrometry (LA-ICP-MS), and is almost end-member composition with ca. 0.3 wt.% ThO₂ and ca. 1000 ppm Ln probably replacing Ca. Johachidolite has a very dense structure with all cations in maximum oxygen coordination. Boron is tetrahedrally coordinated and the tetrahedra form four- and six-membered rings that link to give a sheet, the only borate layer-structure among minerals with only BO₄ tetrahedra. The interlayer space is filled by octahedrally coordinated Al and ten-coordinated Ca. Johachidolite, forming in boron-rich skarn or pegmatite, has never been synthesized. Other ternary compounds, synthesized from corresponding glass compositions, CaAlBO₄ and CaAl₂B₂O₇, are unknown as minerals and have B (three-coordinate), Al (tetrahedral), and Ca (octahedral) in their lowest known oxygen coordination. Johachidolite displays the structural characteristics of a mineral with high-pressure stability.

Keywords: JOHACHIDOLITE; CAO-AL2O3-B2O3 SYSTEM; SINGLE CRYSTAL X-RAY DIFFRACTION; LA-ICP-MS; CRYSTAL STRUCTURE; BORATE

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2008/0020-1824

Publication date: 2008-10-01

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