Incorporation of Y and REEs in aluminosilicate garnet: Energetics from atomistic simulation
MetadataShow full item record
Yttrium and the rare-earth elements (Y+REEs) are incorporated into aluminosilicate garnet as trivalent ions replacing divalent Mg, Fe, Mn, or Ca (“M2+”) in dodecahedral sites, which requires some form of coupled substitution to maintain electroneutrality. We compare the energetic costs of competing coupled-substitution schemes, using lattice dynamics calculations to assess defect energies and exchange energies for each scheme. Substitutions with relatively low energetic costs introduce menzerite-like components via the exchange vector [YM–1•(Mg,Fe)Al–1], or alkali components via the exchange vector [Y(Na,Li)M–2]. Substitutions with substantially higher energetic costs introduce a vacancy component via the exchange vector [Y2oM–3], or the yttrogarnet (YAG) component via the exchange vector [YM–1•AlSi–1], or a component with octahedral Li via the exchange vector [Y2M–2•LiAl–1]. Energetic costs decrease significantly as the host-garnet unit-cell dimension expands, decrease very modestly as temperature rises or pressure falls, and decrease substantially with the contraction in ionic radius across the lanthanide series. These results, combined with critical re-examination of arguments cited in favor of each substitution scheme in natural occurrences, suggest that Y+REE incorporation in natural garnet is dominated by coupled substitutions that introduce menzerite and alkali components, that the YAG substitution plays only a subsidiary role, and that the other schemes are likely to be of very minor importance.
Showing items related by title, author, creator and subject.
High-resolution geochemical record of fluid–rock interaction in a mid-crustal shear zone: a comparative study of major element and oxygen isotope transport in garnetRaimondo, T; Clark, Chris; Hand, Martin; Cliff, John; Harris, C (2012)Garnet grains from an intensely metasomatized mid-crustal shear zone in the Reynolds Range, central Australia, exhibit a diverse assortment of textural and compositional characteristics that provide important insights ...
Pigott, J.; Wright, Kathleen; Gale, Julian; Panero, W. (2015)Interpretation of lateral variations in upper mantle seismic wave speeds requires constraints on the relationship between elasticity and water concentration at high pressure for all major mantle minerals, including the ...
Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crustWang, R.; Collins, Bill; Weinberg, R.; Li, J.; Li, Q.; He, W.; Richards, J.; Hou, Z.; Zhou, L.; Stern, R. (2016)Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous ...