The crystal chemistry of hibonite: an indicator for oxygen fugacity during Solar nebula condensation?
Meteorites represent the main source of materials which can give information on the early history of our Solar System. In particular, Calcium-Aluminium-rich inclusions (CAIs), which are usually found in carbonaceous chondrites, yielded the oldest measured ages of our Solar System. CAIs can contain records of the conditions and the events that characterised their formation, therefore they can provide essential information on the early Solar Nebula, which is believed to have been an environment so reducing to allow the occurrence of Ti(3+). For these reasons, the oxidation state of titanium in different minerals present in CAIs has attracted considerable attention over the years as it could be used to constrain the oxygen fugacity, fO2, at the time of Solar Nebula condensation. CAIs contain minerals which appear earlier in the condensation sequence of a hot gas having Solar composition. Among these minerals, Ti-rich clinopyroxenes, Ca(Mg,Al,Ti)(Si,Al)2O6, and hibonite, Ca(Al,Mg,Ti)12O19, have been used to estimate the fO2 during the Solar Nebula condensation, producing contradictory results, with pyroxenes suggesting reducing and hibonite oxidising conditions. The particular interest on hibonite is due to the fact that it is, together with corundum, the very first mineral to condensate in the Solar Nebula. Therefore, as compared to clinopyroxenes, which have formed subsequently, hibonite potentially carries information on the earliest stages of the Solar System formation. However, little is known on the crystal chemistry of hibonite, on the influence on hibonite structure of the Ti(3+)-Ti(4+) incorporation, and on the possible interactions of Ti with other cations substituting for Al such as, for example, Mg. The aim of this thesis is, therefore, to correctly determine the cation distribution of Ti in synthetic and meteoritic samples and to quantify their Ti(3+)/Ti(tot) ratios from spectroscopic data. Correlating the information on the crystal chemistry and the Ti(3+)/Ti(tot) ratios with the fO2 at which the hibonite samples formed would then allow testing whether hibonite is a sensible oxybarometer and possibly explaining the large difference in nebular oxygen fugacities obtained using different minerals. To this end, hibonites with different Ti and Mg contents have been synthesised at different oxygen fugacities and temperatures adopting two different approaches. In the first case polycrystalline hibonite was synthesised in air at 1600 °C from a mixture of oxides and then used as starting material for piston-cylinder experiments performed at 15 kbar and at temperatures ranging from 1450 to 2000 °C. In the second case a citrate-based sol-gel procedure aimed at the synthesis of end-member hibonite has been modified in order to obtain Ti-Mg-bearing sol-gel precursors. These materials were subsequently fired at different temperatures and at variable oxygen fugacities in a gas mixing furnace in order to obtain hibonite samples. Whereas all but one of the piston-cylinder runs resulted in polycrystalline samples containing small sized crystals, most of the sol-gel starting materials produced large single crystals of hibonite. Chemical analyses performed on synthetic hibonites by means of microprobe (EPMA) indicate that most of the samples have a Ti(tot)/Mg ratio close to 1, suggesting that the coupled substitution mechanism 2Al(3+) = Ti(4+) + Mg(2+) is the major incorporation mechanism for Ti and Mg in hibonite , independently of oxygen fugacity. Only for samples synthesised at very reducing conditions the substitution of Ti(3+) takes places according to the substitution Al(3+) = Ti(3+). X-ray diffraction (XRD) investigations on both polycrystalline and single crystals hibonites indicate that the unit cell volume increases linearly with Ti and Mg content, according to the empirical relationship: V = 8.2(2)·[Ti(tot,apfu) + Mg(2+,apfu)] + 586.1(2). Fourier analysis of single crystal XRD intensity datasets revealed that the Ti atoms occupy exclusively two sites in the structure of hibonite, a trigonally distorted octahedral site occurring as face-sharing pairs, M4, and a trigonal bipyramid site, M2. The refined occupancy factor on these two sites clearly indicates that Ti partitions preferentially at the M4 site where Ti is four time more abundant than at the M2 site. Analysis of bond distances and thermal displacement parameters also suggests that both Ti(4+) and Ti(3+) are present at both M4 and M2 sites. It also has been confirmed that divalent species such as Mg(2+) occupy the tetrahedral M3 site. Energy electron loss spectroscopy (EELS) in a transmission electron microscope (TEM) has been used to measure the Ti(3+)/Ti(tot) ratios on synthetic hibonite. Due to the low concentration of Ti in hibonite, which does not allow to determine with sufficient accuracy the absolute energy of the EELS peaks, it was not possible to use the EELS calibration already available in the literature which have been derived from EELS of titanium oxides. Therefore, a hibonite-specific calibration for quantifying Ti(3+) in hibonite has been developed, taking into account the information obtained from XRD. In fact, the knowledge of the exact site distribution of Ti in hibonite allowed to better understand the fine structure of the EEL spectra. Broadening of the four peaks constituting the titanium L(2,3) edge of hibonite has been correlated with the presence of Ti(3+) via (dynamic) Jahn-Teller effect. The peak broadening has been quantified using the autocorrelation method as well as introducing a new quantity defined as the "net peak height", which links the relative intensity of the different peaks to their width. The EELS calibration method has been based on the values of net peak height of two reference synthetic hibonites assumed to contain either only Ti(4+) or only Ti(3+). The measured Ti(3+) contents in different synthetic samples confirmed that the Ti(tot)/Mg ratio play a fundamental role in determining the amount of Ti(3+) present in hibonite, whereas oxygen fugacity becomes a determining factor only at very reducing conditions or for those samples for which Mg is absent. UV/Vis optical spectroscopy measurements have been performed on oriented single crystals of synthetic hibonite. The spectra of hibonites are characterized by the presence of a prominent absorption peak centred around 715 nm, except in the case of sample synthesised in air for which the absence of absorption of light confirms that they exclusively contain Ti(4+). The intensity of this absorption band at 715 nm has been found to linearly correlate with the product of the concentrations of Ti(3+) and Ti(4+) determined from the EELS measurements. This indicates that this absorption band originates from a Ti(3+)-Ti(4+) intervalence charge transfer (IVCT) transition. Such transition requires that the involved atoms occupy edge or face-sharing coordination polyhedra. This is indeed the case for Ti in hibonite which has been found, based on the structural refinements, to occupy preferentially the M4 site (i.e. a pair of face-sharing octahedra). The linear correlation observed between the absorbance and the concentrations of Ti(3+)-Ti(4+) pairs taken from EELS validates the newly developed EELS calibration and the assumptions on which it is based. The values of Ti(3+) obtained from EELS were used to construct Ti(3+)-fO2 relationships for different Ti(tot)/Mg ratios which were then used to interpret the EELS measurements performed on natural specimens. Previous to EELS analysis, the chemical compositions of the natural hibonite grains were measured with EPMA and subsequently the crystals were prepared for TEM analysis using the focused ion beam (FIB) technique. The Ti(3+) contents in natural hibonites indicate that these samples have been likely exposed during their formation and accretion in the Solar Nebula to oxygen fugacities several orders of magnitude more oxidising than expected from thermodynamic calculations. These results can be interpreted in the frame of a recently published model which proposes the occurrence of transient oxidising conditions accompanying thermal processing of nebular precursors. According to this model, at the very high temperatures at which hibonite condensated, the fO2 conditions might have been temporarily oxidising thus favouring the presence of Ti(4+). Once hibonite formed, its crystal chemistry might have played a major role in stabilising the Ti(4+) even after more reducing conditions were re-established. The present results explain the discrepancy between fO2 estimations based on clinopyroxenes and hibonites, since clinopyroxenes, which condense at lower temperature, were not exposed to transient high temperature events and therefore are more likely to have recorded the reducing conditions predicted by thermodynamic calculations.
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