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Instruments / Technical Area	Publication Details	Published
I18-Microfocus Spectroscopy	The effect of site geometry, Ti content and Ti oxidation state on the Ti K-edge XANES spectrum of synthetic hibonite P. M. Doyle , A. Berry , P. Schofield , J. F. W Mosselmans Geochimica Et Cosmochimica Acta DOI: 10.1016/j.gca.2016.05.001 Diamond Proposal Number(s): [490, 1175] Show Abstract ▼ Abstract: The Al-rich oxide hibonite (CaAl12O19) is modelled to be the second mineral to condense from a gas of solar composition and is found within calcium-aluminum-rich inclusions and the matrix of chondritic meteorites. Both Ti3+ and Ti4+ are reported in meteoritic hibonite, so hibonite has been proposed as a single mineral oxybarometer that could be used to elucidate conditions within the first 0.2 Myrs of the Solar System. Synthetic hibonites with Ti3+/(Ti3++Ti4+) (hereafter Ti3+/ΣTi) ranging between 0 and 1 were prepared as matrix-matched standards for meteoritic hibonite. The largest yield of both Ti-free and Ti-bearing hibonite at ∼1300 and ∼1400 °C was obtained by a single sinter under reducing conditions. In situ micro-beam Ti K-edge X-ray absorption near edge structure (XANES) spectra were collected from the synthetic hibonites, as well as from terrestrial hibonite. Spectral features in the post-crest region were shown to correlate with the Ti4+ content. Furthermore, Ti4+ on the M2 trigonal bipyramidal and the adjoining M4 octahedral sites appears to cause variability in the post-crest region as a function of orientation. For this suite of synthetic hibonites it was observed that the pre-edge peak region is not influenced by orientation, but is controlled by Ti3+/ΣTi, site geometry and/or Ti concentration. In particular, the pre-edge peak intensities reflect Ti coordination environment and distortion of the M4 octahedral site. Therefore, although pre-edge peak intensities have previously been used to determine Ti3+/ΣTi in meteoritic minerals, we excluded use of the pre-edge peak intensities for quantifying Ti valence states in hibonite. The energy of the absorption edge at a normalized intensity of 0.8 (E0.8) and the energy of the minimum between the pre-edge region and the absorption edge (Em1) were found to vary systematically with Ti3+/ΣTi. Ti3+/ΣTi in hibonite as a function of Em1 was modelled by a quadratic function that may be used to quantify Ti3+/ΣTi in meteoritic hibonite when the synthetic hibonite standards are crystal-chemically matched to the natural samples and are measured during the same analytical session as the meteoritic hibonites.	Jun 2016
I18-Microfocus Spectroscopy	The Ti environment in natural hibonite: XANES spectroscopy and computer modelling Antonina N Kravtsova , Alexander V Soldatov , Andrew M Walker , Andrew J Berry XAFS 16 DOI: 10.1088/1742-6596/712/1/012089 Diamond Proposal Number(s): [490, 1175] Open Access Show Abstract ▼ Abstract: The local atomic structure around Ti in Ti-bearing hibonite (CaAl12O19) was studied using X-ray absorption near-edge structure (XANES) spectroscopy and computer modelling. Structural models of the direct substitution of Al by Ti3+, Al by Ti4+ charge balanced by the coupled substitution of Mg2+ for Al, and small Ti clusters were considered. The Ti K-XANES spectra of natural hibonite with different Ti concentration were recorded. Theoretical Ti K- XANES spectra for structural models of hibonite were calculated. It was shown that the theoretical Ti K-XANES spectra for a model with Ti at the five-coordinated M2 site are in agreement with the experimental XANES spectra of hibonite with low concentrations of Ti, while the theoretical spectra for a structural model of clustered Ti are in agreement with the experimental spectra of hibonite with higher Ti contents.	May 2016
B18-Core EXAFS I18-Microfocus Spectroscopy	The oxidation state of europium in silicate melts as a function of oxygen fugacity, composition and temperature A.d. Burnham , Andrew Berry , Helen Halse , Paul Schofield , Giannantonio Cibin , Fred Mosselmans Chemical Geology 411, 248 - 259 DOI: 10.1016/j.chemgeo.2015.07.002 Diamond Proposal Number(s): [7452, 5576, 3771, 6300] Show Abstract ▼ Abstract: Europium LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses and melts equilibrated over a range of oxygen fugacities (fO2s, from -14 to +6 logarithmic units relative to the quartz-fayalite-magnetite, QFM, buffer) and temperatures (1250 – 1500 °C). Eu3+/ΣEu (where ΣEu = Eu2+ + Eu3+) values were determined from the spectra with a precision of ±0.015. Eu3+/ΣEu varies systematically with fO2 from 0 to 1 over the range studied, increases with decreasing melt polymerisation and temperature, and can be described by the empirical equation: Eu3+/ΣEu = 1/[1+10^(-0.25*logfO2 - 6410/T - 14.2Λ - 10.1)], where Λ is the optical basicity of the melt and T is the temperature in K. Eu3+/ΣEu in glasses and melts equilibrated at the same conditions are in excellent agreement for Fe-free systems. For Fe-bearing compositions the reaction Eu2+ + Fe3+= Eu3+ + Fe2+ occurs during quenching to a glass and the high temperature value of Eu3+/ΣEu is not preserved on cooling; in situ measurements are essential for determining Eu3+/ΣEu in natural melts.	Jul 2015
B18-Core EXAFS	The effect of oxygen fugacity, melt composition, temperature and pressure on the oxidation state of cerium in silicate melts Antony Burnham , Andrew Berry Chemical Geology DOI: 10.1016/j.chemgeo.2013.12.015 Diamond Proposal Number(s): [6300, 3731] Show Abstract ▼ Abstract: Ce LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from ? 10 to + 11 in logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer), temperatures (1300–1500 °C), and pressures (1 atm and 1 GPa). The oxidation state ratio of Ce, Ce4 +/? Ce (where ? Ce = Ce3 ++ Ce4 +), was determined from the spectra allowing the relationships between Ce4 +/? Ce and fO2, temperature, pressure and melt composition to be determined. Ce4 +/? Ce varied systematically with fO2 from 0 to ~ 0.8 over the range of conditions studied. Ce4 + is stabilised relative to Ce3 + by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4 +). Ce4 + in an Fe2 +-bearing melt is not preserved on cooling to a glass due to the reaction Ce4 ++ Fe2 + = Ce3 ++ Fe3 +. Ce4 +/? Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.	Dec 2013

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