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Published June 1988 | public
Journal Article

Ti^(3+) in meteoritic and synthetic hibonite


Electron spin resonance has been used to make the first direct determination of Ti^(3+) in synthetic hibonite and hibonite from inclusion SH-7 of the Murchison C2 chondrite. Ti^(3+) concentrations range from 0.02 to 0.64 wt% in synthetic blue hibonite and 0.35–0.44 wt% in hibonite from SH-7. No Ti^(3+) could be detected in orange hibonite, supporting the earlier conclusion that the orange-to-blue transition is associated with the presence of Ti^(3+). At constant temperature and oxygen fugacity, Ti^(3+)/Ti^(4+) in synthetic hibonite increases with decreasing V but is not strongly dependent on bulk Ti. At the concentration levels encountered in meteoritic hibonite, Fe and Cr contents do not have a significant effect on the amount of Ti^(3+). In both synthetic and meteoritic hibonite, Ti^(3+) occupies a 5-coordinated crystallographic site, which is consistent with the formation of doubly ionized oxygen vacancies. At low oxygen fugacities, essentially all Ti^(4+) on the five-fold Al-site has been reduced to Ti^(3+). Hibonite from SH-7 equilibrated with a gas that could have been as reducing as a gas of solar composition. This is consistent with other estimates based on mineral equilibria of high temperature oxygen fugacities in Ca-Al-rich inclusions. With the possible exception of Mo-W depletions, indicators based on bulk trace element concentrations in CAIs are inconclusive. There is considerable evidence that as CAIs cooled to lower temperatures, they experienced conditions significantly more oxidizing than those of a solar gas, perhaps in planetary environments.

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© 1988 Pergamon Press. Received 16 June 1987. Accepted 17 February 1988. Discussions with J.D. Blum, B. Fegley, P. D. Ihinger, G. J. MacPherson and G. R. Rossman are gratefully acknowledged. Reviews by J. Hertogen, A. Hofmeister, P. D. Ihinger, M. Johnson, J. Kozul, and D. A. Wark greatly improved the manuscript. A. Hashimoto is thanked for his analyses of SH-7 hibonite on the Northwestern University microprobe. This research was supported by NASA grants NAG 9-105 and NAG 9-54. Part of this work was carried out by D. Live at JPL under contract NAS-7-918 to NASA. D. Live also acknowledges an NRC senior resident research associateship. Caltech Division of Geological and Planetary Sciences. Contribution 4553. Editorial handling: J. Hertogen

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