Okayamalite, Ca2B2SiO7, a New Mineral, Boron Analogue of Gehlenite

Satoshi Matsubara, Ritsuro Miyawaki, Akira Kato, Kazumi Yokoyama and Akiyoshi Okamoto
Department of Geology, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku, Tokyo 169, Japan
3-11-5 Ooeda Minami Fukunishi-cho, Nishigyo, Kyoto 610-11, Japan

Abstract: Okayamalite, Ca2B2SiO7, tetragonal, P4¯21m, a = 7.116, c = 4.815 Å, Z = 2, is a new member of melilite group, the boron analogue of gehlenite. Electron microprobe analysis gave CaO 46.28, B2O3 28.50, SiO2 24.24, Al2O3 0.36, total 99.38 wt.%, corresponding to Ca2.01B2.00Si0.98Al0.02O7, a natural counterpart of Ca2B2SiO7 known only synthetically. The strongest lines in the X-ray powder diffraction pattern are 3.479 (40)(111), 2.862 (55)(201), 2.654 (100)(211), 2.129 (20)(301), 1.920 (35)(212), 1.644 (29)(312), very close to those of the synthetic material (a = 7.115, and c = 4.812 ). It is creamy white in colour with an earthy appearance due to the fine grain size. Streak white, cleavage not observed. Hardness ∼5½. Density calculated on the ideal formula is 3.30 g/cm3. It is optically uniaxial negative with ω = 1.700, and ɛ = 1.696. It occurs as patches of a few millimetres across in grey homogeneous-looking aggregate of wollastonite, vesuvianite, calcite and johnbaumite from Fuka mine, Bicchu-cho, Okayama Prefecture, Japan. The patches consist of very fine grains of the mineral up to 30 μm. Okayamalite is considered to be a product after the reaction formula: CaCO3 + CaSiO3 + B2O3 = Ca2B2SiO7 + CO2, arising from boron metasomatism of a wollastonite-calcite aggregate. The name is for the prefecture.

Keywords: okayamalite • new mineral • melilite group • boron • gehlenite • X-ray data • electron microprobe data • optical data • Fuka • Japan

Mineralogical Magazine; October 1998 v. 62; no. 5; p. 703-706; DOI: 10.1180/002646198547936
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)