Okayamalite, Ca 2 B 2 SiO 7 , tetragonal, P421m, a = 7.116, c = 4.815 Aa, Z = 2, is a new member of melilite group, the boron analogue of gehlenite. Electron microprobe analysis gave CaO 46.28, B 2 O 3 28.50, SiO 2 24.24, Al 2 O 3 0.36, total 99.38 wt.%, corresponding to Ca (sub 2.01) B (sub 2.00) Si (sub 0.98) Al (sub 0.02) O 7 , a natural counterpart of Ca 2 B 2 SiO 7 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 approximately 5 1/2. Density calculated on the ideal formula is 3.30 g/cm 3 . It is optically uniaxial negative with omega = 1.700, and epsilon = 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 mu m. Okayamalite is considered to be a product after the reaction formula: CaCO 3 +CaSiO 3 +B 2 O 3 = Ca 2 B 2 SiO 7 +CO 2 , arising from boron metasomatism of a wollastonite-calcite aggregate. The name is for the prefecture.

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