MENZERITE-(Y), A NEW SPECIES,{(Y, REE)(Ca, Fe2+) 2}(Mg, Fe2+)(Fe3+, Al) O12, FROM A FELSIC GRANULITE, PARRY SOUND, ONTARIO, AND A … (original) (raw)

Menzerite-(Y), a new mineral species, forms reddish brown cores, n = 1.844 (20), up to 70 mm across, rimmed successively by euhedral almandine containing up to 2.7 wt% Y 2 O 3 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 , the German crystallographer who solved the crystal structure of garnet. Single-crystal X-ray-diffraction results yielded space group Ia3d, a = 11.9947(6) Å. An electron-microprobe analysis of the grain richest in Y (16.93 wt% Y 2 O 3 ) gave the following formula, normalized to eight cations and 12 oxygen atoms: {Y 0.)O 12 , or {(Y,REE)(Ca,Fe 2+ ) 2 }[(Mg,Fe 2+ )(Fe 3+ ,Al)](Si 3 )O 12 . Synchrotron micro-XANES data gave Fe 3+ /SFe = 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) Å is in accord with a partial occupancy by Mg. The dominance of divalent cations with Mg > Fe 2+ and the absence of Si at the octahedral Y site (in square brackets) is the primary criterion for distinguishing menzerite-(Y) from other § the canadian mineralogist silicate garnet species; the menzerite-(Y) end-member is {Y 2 Ca}[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 � 700-800°C, and subsequently dissolved incongruently in an anatectic melt to form almandine, most likely, at P � 8.5-9.5 kbar and T � 800-850°C.