A NEW YTTRIUM GARNET SPECIES, {(Y, REE)(CA, Fe2+)2}[(MG,Fe2+)(Fe3+,AL)](Si3)O12, AND TWO NEW COMPONENTS IN METAMORPHIC GARNET: {Y2CA}[Mg2](Si3)O12 AND {Y2CA}[Fe2+2](Si3)O12

A new yttrium garnet (IMA 2009-050, submitted) forms reddish-brown cores (n = 1.844) up to 75 µm across rimmed by euhedral almandine-grossular (Alm₅₉Grs₁₈Prp₉Adr₅Sps₄other₅; < 2 wt% Y₂O₃) in a pyroxene-plagioclase granulite on Bonnet Island in the interior Parry Sound domain, Central Gneiss Belt, Grenville Orogenic Province, Canada. A representative analysis gave {Y_0.83Gd_0.01Dy_0.05Ho_0.02Er_0.07Tm_0.01Yb_0.06Lu_0.02Ca_1.37Fe²⁺_0.49Mn_0.07}[Mg_0.55Fe²⁺_0.42Fe³⁺_0.58Al_0.35V_0.01Sc_0.01Ti_0.08](Si_2.82Al_0.18)O₁₂, corresponding in mole% (rare earth elements combined with Y) to 6.0 {Y₃}[Al₂](Al₃)O₁₂ (YAG), 27.3 {Y₂Ca}[Mg₂](Si₃)O₁₂ (YMg), 17.3 {Y₂Ca}[Fe²⁺₂](Si₃)O₁₂ (YFe), 9.2 Alm, 21.9 Adr, 7.6 {Ca₃}[TiFe²⁺](Si₃)O₁₂, 7.1 {Fe²⁺₃}[Fe³⁺₂](Si₃)O₁₂, and 3.6 other. Single crystal study gave space group Ia-3d, a = 11.9947(6) Å and Mg occupying the octahedral site. Contacts of the new garnet with Alm-Grs are sharp and irregular, showing cusps and embayments. Traverses across the new mineral commonly gave increases in Y, Mg and Ti, but never in Yb, from core to the contact with enclosing Alm-Grs. Y content of the enclosing Alm-Grs decreases towards its rim except for a small increase right near the contact with the new garnet. The new garnet also contacts K-feldspar, allanite-(Ce) (Aln_~70-85) and apatite, but prior to growth of Alm-Grs it presumably equilibrated also with ferrosilite (Fs_56-60En_41-37Wo_~1 plus ~1%MnSiO₃), oligoclase (An₂₇), magnetite, ilmenite and possibly augite during the first stage of metamorphism estimated to be at T ≥ 830 °C, P ≈ 11 kbar. The second stage assemblage included augite, ferropargasite and Alm-Grs + Qtz ± Cpx symplectite. Until now the components YAG and its Fe³⁺ analogue YIG, {Y₃}[Fe³⁺₂](Fe³⁺₃)O₁₂, were the only two recognized substitutions in addition to Na + (Y, REE) = 2Ca in metamorphic garnet. Incorporation of Y + REE via the YMg and YFe components can be related to YAG and YIG by equations of the type 3YMg + 5Al₂O₃ = 2YAG + 3Di + 3En, suggesting YMg and YFe incorporation would be favored in intermediate and mafic rocks, whereas YAG and YIG incorporation would be favored in pelitic and highly oxidized rocks, respectively. Variation of Si with (Y + REE – Na) reported in the literature can be explained by incorporation of YAG or YIG only or by a combination of YAG/YIG with YMg/YFe.