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Ominelite, (Fe,Mg)Al₃BSiO₉ (Fe²⁺ analogue of grandidierite), a new mineral from porphyritic granite in Japan

¹ Department of Earth Sciences, Chiba University, Yayoicho, Inage-ku, Chiba 263-8522, Japan
² Department of Geological Sciences, University of Maine, Orono, Maine 04469, U.S.A.
³ Department of Crustal Studies, National Institute of Polar Research, Kaga, Itabashi-ku, Tokyo 173-8515, Japan
⁴ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.
⁵ Department of Geology, National Science Museum, Hyakunincho, Shinjuku-ku, Tokyo 169-0073, Japan
⁶ Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, U.S.A.
⁷ Hercules Research Center, 500 Hercules Road, Wilmington, Delaware 19808, U.S.A.
⁸ Overseas Activities Department, Metal Mining Agency of Japan, Toranomon, Minato-ku, Tokyo 105-0001, Japan

Correspondence: ^* E-mail: yhiroi{at}earth.s.chiba-u.ac.jp

Ominelite, (Fe,Mg)Al₃BSiO₉, is the Fe²⁺ analog of grandidierite. The mineral occurs as elongated and euhedral to equant and anhedral grains in close association with sekaninaite (Fe-dominant analogue of cordierite), garnet, biotite, andalusite, topaz, alkali feldspar, plagioclase, muscovite, quartz, dumortierite, schorl, zircon, ilmenite, apatite, monazite, and pyrite in a porphyritic granite of Miocene age exposed along the Misen River in Tenkawa, Yoshino, Nara Prefecture, Japan (34°12'40'’N, 135°53'40'’E). Temperatures <700 °C and pressures below 4 kbars are suggested for the formation of ominelite and associated sekaninaite, topaz, andalusite and dumortierite. The Al-rich minerals could be either magmatic or restitic in origin. A representative electron microprobe analysis of ominelite is SiO₂ 19.34, TiO₂ <0.01, Al₂O₃ 48.85, FeO 19.37, MnO 0.43, MgO 1.33, CaO <0.01, P₂O₅ 0.13, B₂O₃ 10.91 wt%, total 100.36 wt%, corresponding to Fe_0.85Mg_0.10Mn_0.02Al_3.01B_0.99P_0.01Si_1.01O₉. Mohs’ hardness is about 7. No cleavage is observed. Its color is blue, and the streak is pale blue. It is pleochroic X = Z = pale blue-green and Y = colorless. Optically, it is biaxial (–) and, at = 589 nm, has = 1.631 (1), ß = 1.654 (1), = 1.656 (1), 2V_X (meas.) = 31.5 (6)°. Y = c (prism elongation direction). Dispersion is v >> r. Major lines in the powder pattern [d in Å, (I), (hkl)] are 5.57(m)(020), 5.21(vs)(200), 3.73(m)(121), 3.51(m)(130), 2.97(s)(101), 2.79(s)(040), 2.18(s)(150, 421, 312). Space group is Pbnm. Lattice parameters are a = 10.343 (2), b = 11.095 (1), c = 5.7601 (8) Å and V = 661.0(2) ų, Z = 4, D_calc = 3.169 g/cm³. Refinement of the structure confirms that ominelite is isostructural with grandidierite with no detectable substitution of Al by Fe³⁺.

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