Titre du document / Document title

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

Auteur(s) / Author(s)

HIROI Yoshikuni ; GREW Edward S. ; MOTOYOSHI Yoichi ; PEACOR Donald R. ; ROUSE Roland C. ; MATSUBARA Satoshi ; YOKOYAMA Kazumi ; MIYAWAKI Ritsuro ; MCGEE James J. ; SU Shu-Chun ; HOKADA Tomokazu ; FURUKAWA Noboru ; SHIBASAKI Hiroshi ;

Résumé / Abstract

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 λ = 589 nm, has α = 1.631 (1), p = 1.654 (1), y = 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) Å₃, 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₊.

Revue / Journal Title

The American mineralogist   ISSN 0003-004X   CODEN AMMIAY 

Source / Source

2002, vol. 87, n^o1, pp. 160-170 [11 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Mineralogical Society of America, Washington, DC, ETATS-UNIS  (1916) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 503, 35400009471583.0170