Adachiite, CaFe 3 Al 6 (Si 5 AlO 18 )(BO 3 ) 3 (OH) 3 (OH), a Si-poor member of the tourmaline supergroup, is found in the hydrothermal vein cutting emery from Nabagasako of the Kiura mine, Saiki City, Oita Prefecture, Japan. Adachiite occurs as a constituent (several to 300 µm thickness) of hexagonal prismatic crystals, and forms a zoned structure closely associated with schorl. It is transparent with brownish-purple to bluish-purple color, while the massive aggregate shows black in color.

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... hiite is trigonal, R3m, a = 15.9290(2), c = 7.1830(1) Å, V = 1578.39(4) Å 3 , and Z = 3, determined via single crystal XRD refinement (R 1 = 0.038). Adachiite is the first member of the tourmaline group formed via Tschermak-like substitution.