Johachidolite, CaAl[B (sub 3) O (sub 7) ], a mineralogical and structural peculiarity

The crystal structure of johachidolite, CaAl[B (sub 3) O (sub 7) ], (Cmma, a = 9.767(2), b = 11.723(3), c = 4.3718(5) Aa, V = 408.3(1) Aa (super 3) , 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.% ThO (sub 2) 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 BO (sub 4) 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, CaAlBO (sub 4) and CaAl (sub 2) B (sub 2) O (sub 7) , 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.