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 Ca_2.01B_2.00Si_0.98Al_0.02O₇, 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.