Chondritic meteorites oxidation states

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

On Earth, the Ti(m) oxidation state is unstable. However, Ti3+-bearing minerals are well-characterized in some meteorites and Moon rocks, generally coexisting with Ti4 in such phases as calcic pyroxene, ulvospinel, hibonite and ilmenite. In hibonite, CaAlI20I9, a refractory phase in carbonaceous chondrites, EPR and optical spectral data indicate that Ti3+ ions are present (Hunger and Stolper, 1986 Live et al., 1986). The trivalent Ti ions may be stabilized in the five-coordinated trigonal bipyramidal M5 site of the hibonite structure... 

We note, however, that this classification parameter does not generally reflect the oxidation state of individual chondritic components, e.g., CAIs in all chondrite groups formed under highly reducing conditions (see Chapter 1.08), and magnesian (type-I) and ferrous (type-II) chon-drules (both of which occur in the same meteorites) require formation under different redox conditions (see Chapter 1.07). In addition, the role of nebular and asteroidal processes in establishing of the oxidation states of chondrites remains controversial (e.g., Krot et aL, 2000a). 

Rubin A. E., Fegley B., and Brett R. (1988a) Oxidation state in chondrites. In Meteorites and the Early Solar System (eds. J. F. Kerridge and M. S. Matthews). University of Arizona Press, Tucson, pp. 488—511. 

As pointed out before, many of the variations in chemical composition and oxidation state observed in chondritic meteorites must have been established at an early stage, before nebular components accreted to small planetesimals. 

Ashworth, and Hutchison, 1975 [11] made electron microscopic observations of the hydrous alteration products of olivine in an achondrite and in an ordinary chondrite. Their conclusion was that the Nakhla achondrite, and possibly the Weston chondrite, contain water of extraterrestrial origin which was mobilized by mild shock deformation. Carbonaceous chondrites are believed to be unaltered material left over from the formation of the solar system. They contain substantial amounts of reduced carbon and of water in the form of hydroxyl ions. The oxidation state of iron in some carbonaceous chondrites has been determined by means of Moess-bauer spectroscopy, and it is demonstrated that there is a correlation between the oxidation state of iron and the content of water and reduced carbon in the meteorites (Roy-Poulsen et al., 1981 [284]). 

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