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Feldspathic Chromite Assemblages (FCAs) are chemically distinctive assemblages rich in Na, Al, and Cr that are found in a variety of chondrites. They consist of concentrations of chromite associated mainly with feldspathic material, either feldspar or the glassy material known as maskelynite. Professor Alan Rubin (2003) has proposed that some FCAs formed by shock melting, but the origin of different types of feldspathic chromite assemblages are unclear and have not been studied with more modern techniques such as Electron Backscatter Electron Diffraction (EBSD).Here EBSD was used to study FCAs in four metamorphosed ordinary chondrites of different shock stages, including Estacado (H6, S1), Spade (H6, S4), NWA 13533 (L6, S4), and Alfianello (L6, S5). EBSD results indicate that FCAs can be grouped into three different assemblages, including Coarse Clumped Chromite (CCC), Fine Clumped Chromite (FCC), and Chromite Poor (CP) assemblages. CCCs are compact assemblages that contain chromite crystals often larger than 5Âµm in diameter size with little physical distance between adjacent chromite grains. FCCs are compact assemblages that contain numerous chromite grains less than 5Âµm in diameter that have physical space between the chromite grains. CPs are assemblages that contains chromite congregated in one area but with less chromite grains than the FCC or CCC assemblages. The four meteorites used in this study are a mix of high and low shock stage. Out of the three assemblage types, it was found that CCCs were present in both high and low shocked meteorites. FCCs were also found in both high and low shock meteorites, but had a noticeable absence in Spade. The two CP assemblages were present only in higher shocked meteorites (Spade and NWA 13533). Quantitative characteristics (grain sizes of chromite, grain number density of chromite, grain lattice preferred orientation of chromite, and grain orientation spread (or GOS)) of these assemblages and their minerals do not appear to be different between high and low shock meteorites other than differences in the grain orientation spread. These GOS characteristics appear more related to the host meteorite. We postulate that the origins of the FCCs are an end product of exsolution of a previous grain, while the CCCs origins are more likely to be aggregates of pre-existing grains or grain-clumps that have been crystallized from a melt. The CPs found in this study do not particularly fit either model. My findings support those of Rubin (2003) in terms of Estacado being annealed but support only some of the findings of Rubin and Jones (2003) for Spade, namely that Spade is a melt breccia with the plagioclase having been melted.
Maccini, Kimberly Louisa, "Characterization and Interpretation of Feldspathic Chromite Assemblages (FCAs) in Four Ordinary Chondrites: An Electron Backscatter Diffraction (EBSD) Study" (2021). Dissertations and Theses. Paper 5878.
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