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

¹ Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestr. 3, 3012 Bern, Switzerland
² Laboratorium für Anorganische Chemie, ETH Hönggerberg, HCI, 8093 Zürich, Switzerland
³ Institut für Mineralogie und Petrographie, ETH-Zentrum, 8092 Zürich, Switzerland
⁴ GRS GemResearch Swisslab AG, P.O. Box 4028, 6002 Lucerne, Switzerland

^* Corresponding author, e-mail: thomas.armbruster{at}krist.unibe.ch

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) ų, 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.

Key-words: johachidolite, CaO–Al₂O₃–B₂O₃ system, single crystal X-ray diffraction, LA-ICP-MS, crystal structure, borate.