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

Autor: Heinz-Juergen Bernhardt, Olaf Medenbach, Biljana Lazic, Edward S. Grew, M. Darby Dyar, Andrew J. Locock, Samuel W. Bell, Thomas Armbruster, Jeffrey H. Marsh, Martin G. Yates
Rok vydání: 2010
Předmět:
Zdroj: The Canadian Mineralogist. 48:1171-1193
ISSN: 1499-1276
0008-4476
DOI: 10.3749/canmin.48.5.1171
Popis: Menzerite-(Y), a new mineral species, forms reddish brown cores, n = 1.844 (20), up to 70 μm 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 (1897–1989), the German crystallographer who solved the crystal structure of garnet. Single-crystal X-ray-diffraction results yielded space group Ia 3 d , a = 11.9947(6) A. 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 0.83 Gd 0.01 Dy 0.05 Ho 0.02 Er 0.07 Tm 0.01 Yb 0.06 Lu 0.02 Ca 1.37 Fe 2+ 0.49 Mn 0.07 } [Mg 0.55 Fe 2+ 0.42 Fe 3+ 0.58 Al 0.35 V 0.01 Sc 0.01 Ti 0.08 ](Si 2.82 Al 0.18 )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+ /∑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) A 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) are the primary criteria for distinguishing menzerite-(Y) from other 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.
Databáze: OpenAIRE
Externí odkaz: