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Lunar mare basalts are a product of partial melting of the lunar mantle under more reducing conditions when compared to those expected for the Earth's upper mantle. Alongside Fe, Ti can be a major redox sensitive element in lunar magmas, and it can be enriched by up to a factor of ten in lunar basaltic glasses when compared to their terrestrial counterparts. Therefore, to better constrain the oxidation state of Ti and its coordination chemistry during lunar magmatic processes, we report new X-ray absorption near edge structure (XANES) spectroscopy measurements for a wide range of minerals (pyroxene, olivine, Fe-Ti oxides) and basaltic melt compositions involved in partial melting of the lunar mantle. Experiments were conducted in 1bar gas-mixing furnaces at temperatures between 1100 and 1300 degrees C and oxygen fugacities (fO(2)) that ranged from air to two orders of magnitude below the Fe-FeO redox equilibrium. Run products were analysed via electron microprobe and XANES Ti K-edge. Typical run products had large (>100 mu m) crystals in equilibrium with quenched silicate glass. Ti K-edge XANES spectra show a clear shift in energy of the absorption edge features from oxidizing to reducing conditions and yield an average valence for Fe-Ti oxides (armalcolite and ilmenite) of 3.6, i.e., a 40% of the overall Ti is Ti3+ under fO(2) conditions relevant to lunar magmatism (IW -1.5 to -1.8). Pyroxenes and olivine have average Ti valence of 3.75 (i.e., 25% of the overall Ti is trivalent), while in silicate glasses Ti is exclusively tetravalent. Pre-edge peak intensities also indicate that the coordination number of Ti varies from an average V-fold in silicate glass to VI-fold in the Fe-Ti oxides and a mixture between IV and VI-fold coordination in the pyroxenes and olivine, with up to 82% Ti-[IV](4+) in the pyroxene. In addition, our results can help to better constrain the Ti3+/Sigma Ti of the lunar mantle phases during magmatic processes and are applied to provide first i

Light and heavy REE may occupy different sites and there may be ordering and/or defect clustering in the structure. This study uses X-ray absorption spectroscopy to determine the structural state of REE in eudialyte-group minerals (EGM) from prospective eudialyte-bearing complexes. Yttrium K-edge and Nd L3-edge spectra were collected as proxies for heavy and light REE, respectively, and compared to natural and synthetic REE-bearing standards. Extended X-ray absorption fine structure data yield best fits for Y in six-fold coordination with Y-O distances of 2.24-2.32 angstrom, and a second coordination sphere comprising Fe, Na, Ca, Si and O at radial distances of 3.6-3.8 angstrom. These findings are consistent with dominant Y3+ substitution for Ca2+ on the octahedral M1 site in all the samples studied, and exclude preferential substitution of Y3+ onto the smaller octahedral Z site or the large low-symmetry N4 site. Using lattice strain theory, relative partitioning models were constructed to predict site preferences of lanthanides not measured directly. The models predict that all REE are favoured on the Ca-dominant M1 site and that preferential partitioning of heavy over light REE increases in EGM containing significant Mn in the M1-octahedral rings (oneillite subgroup). Thus, the flat REE profiles that make EGM such attractive exploration targets are not due to preferential partitioning of light and heavy REE onto different sites. Instead, local ordering of Mn- and Ca-occupied M1 sites may influence the capacity of EGM to accommodate heavy REE., Light and heavy REE may occupy different sites and there may be ordering and/or defect clustering in the structure. This study uses X-ray absorption spectroscopy to determine the structural state of REE in eudialyte-group minerals (EGM) from prospective eudialyte-bearing complexes. Yttrium K-edge and Nd L3-edge spectra were collected as proxies for heavy and light REE, respectively, and compared to natural and synthetic REE-bearing standards. Extended X-ray absorption fine structure data yield best fits for Y in six-fold coordination with Y-O distances of 2.24-2.32 angstrom, and a second coordination sphere comprising Fe, Na, Ca, Si and O at radial distances of 3.6-3.8 angstrom. These findings are consistent with dominant Y3+ substitution for Ca2+ on the octahedral M1 site in all the samples studied, and exclude preferential substitution of Y3+ onto the smaller octahedral Z site or the large low-symmetry N4 site. Using lattice strain theory, relative partitioning models were constructed to predict site preferences of lanthanides not measured directly. The models predict that all REE are favoured on the Ca-dominant M1 site and that preferential partitioning of heavy over light REE increases in EGM containing significant Mn in the M1-octahedral rings (oneillite subgroup). Thus, the flat REE profiles that make EGM such attractive exploration targets are not due to preferential partitioning of light and heavy REE onto different sites. Instead, local ordering of Mn- and Ca-occupied M1 sites may influence the capacity of EGM to accommodate heavy REE.

Eudialyte-group minerals (EGM), the focus of commercial interest because they host significant amounts of Zr, Nb and high-value medium and heavy REE, are structurally and chemically complex alkali-zirconosilicates that crystallise in unusually volatile-rich peralkaline magmas. On average they contain c.12 wt% ZrO2, 1-2 wt% Nb2O5 and c.1-10 wt% total rare earth oxides with high proportions of the valuable medium and heavy REEs, at c.35% of total REO.Synchrotron X-ray absorption spectroscopy was used to determine how light and heavy REE are accommodated in the eudialyte-group mineral (EGM) crystal structure, using Y and Nd as proxies for light and heavy REE respectively. The eudialyte-group crystal structure can accommodate REE in various sites, light and middle-weight REE being generally inferred to occupy the Ca-dominated and/or Na sites while heavy REE may occupy the smaller octahedral Zr site. Crystalline eudialyte-group minerals from Ilimaussaq, Narssarssuk, Norra Karr, Kipawa and Lovozero yielded indistinguishable micro-XANES results for Nd and Y, indicating similar LREE/HREE site occupancy in minerals of varying compositions. It is concluded that the relatively flat REE profiles that make eudialyte an attractive target for exploration for high-value REE are not the result of distribution of REE between different sites., Eudialyte-group minerals (EGM), the focus of commercial interest because they host significant amounts of Zr, Nb and high-value medium and heavy REE, are structurally and chemically complex alkali-zirconosilicates that crystallise in unusually volatile-rich peralkaline magmas. On average they contain c.12 wt% ZrO2, 1-2 wt% Nb2O5 and c.1-10 wt% total rare earth oxides with high proportions of the valuable medium and heavy REEs, at c.35% of total REO.Synchrotron X-ray absorption spectroscopy was used to determine how light and heavy REE are accommodated in the eudialyte-group mineral (EGM) crystal structure, using Y and Nd as proxies for light and heavy REE respectively. The eudialyte-group crystal structure can accommodate REE in various sites, light and middle-weight REE being generally inferred to occupy the Ca-dominated and/or Na sites while heavy REE may occupy the smaller octahedral Zr site. Crystalline eudialyte-group minerals from Ilimaussaq, Narssarssuk, Norra Karr, Kipawa and Lovozero yielded indistinguishable micro-XANES results for Nd and Y, indicating similar LREE/HREE site occupancy in minerals of varying compositions. It is concluded that the relatively flat REE profiles that make eudialyte an attractive target for exploration for high-value REE are not the result of distribution of REE between different sites.