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 Fe0.85Mg0.10Mn0.02Al3.01B0.99P0.01Si1.01O9. 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 λ = 589 nm, has α = 1.631 (1), β = 1.654 (1), γ = 1.656 (1), 2VX (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) Å3, Z = 4, Dcalc = 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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