MENZERITE-(Y), A NEW SPECIES, {(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12, FROM A FELSIC GRANULITE, PARRY SOUND, ONTARIO, AND A NEW GARNET END-MEMBER, {Y2Ca}[Mg2](Si3)O12

Menzerite-(Y), a new mineral species, forms reddish brown cores, n = 1.844 (20), up to 70 mu m across, rimmed successively by euhedral almandine containing up to 2.7 wt% Y2O3 and by K-feldspar in a felsic granulite on Bonnet Island in the interior Parry Sound domain, Grenville Orogenic Province, Canada. It is named after Georg Menzer (1897-1989), the German crystallographer who solved the crystal structure of garnet. Single-crystal X-ray-diffraction results yielded space group Ia (3) over bard, a = 11.9947(6) angstrom. An electron-microprobe analysis of the grain richest in Y (16.93 wt% Y2O3) gave the following formula, normalized to eight cations and 12 oxygen atoms: {Y0.83Gd0.01Dy0.05Ho0.02Er0.07Tm0.01Yb0.06Lu0.02Ca1.37Fe0.492+Mn0.07} [Mg0.55Fe0.422+Fe0.583+Al0.35 V0.01Sc0.01Ti0.08](Si2.82Al0.18)O-12, or {(Y,REE)(Ca,Fe2+)(2)}[(Mg,Fe2+)(Fe3+,Al)](Si-3)O-12. Synchrotron micro-XANES data gave Fe3+/Sigma Fe = 0.56(10) versus 0.39(2) calculated from stoichiometry. The scattering power refined at the octahedral Y site, 17.68 epfu, indicates that a relatively light element contributes to its occupancy. Magnesium, as determined by electron-microprobe analyses, would be a proper candidate. In addition, considering the complex occupancy of this site, the average Y-O bond length of 2.0244(16) angstrom is in accord with a partial occupancy by Mg. The dominance of divalent cations with Mg > Fe2+ and the absence of Si at the octahedral Y site (in square brackets) are the primary criteria for distinguishing menzerite-(Y) from other silicate garnet species; the menzerite-(Y) end-member is {Y2Ca}[Mg-2](Si-3)O-12. The contacts of menzerite-(Y) with almandine are generally sharp and, in places, cuspate. It is interpreted to have equilibrated with ferrosilite, augite, quartz, oligoclase, allanite-(Ce), magnetite, ilmenite and fluorapatite, in the absence of almandine, on the prograde path at 7-8.5 kbar and T approximate to 700-800 degrees C, and subsequently dissolved incongruently in an anatectic melt to form almandine, most likely, at P approximate to 8.5-9.5 kbar and T approximate to 800-850 degrees C.