Adachiite, a Si–poor member of the tourmaline supergroup from the Kiura mine, Oita Prefecture, Japan

Abstract

Adachiite, CaFe₃Al₆(Si₅AlO₁₈)(BO₃)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. Adachiite is trigonal, R3m, a = 15.9290(2), c = 7.1830(1) Å, V = 1578.39(4) ų, and Z = 3, determined via single crystal XRD refinement (R₁ = 0.038). Adachiite is the first member of the tourmaline group formed via Tschermak–like substitution.