Texture and chemical composition of pyroxenes in chondrules in carbonaceous and unequilibrated ordinary chondrites

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The texture and composition of pyroxenes in chondrules in unequilibrated ordinary chondrites (UOCs) (petrologic subtype less than 3.5) and carbonaceous chondrites (CCs) were studied with SEM and EPMA. Four kinds of pyroxene can be recognized on the basis of the mode of occurrence and chemical composition; they are protopyroxene (now inverted to twinned low-Ca clinopyroxene) (Wo<2), orthopyroxene (Wo 2-5), pigeonite (Wo 5-15), and augite (Wo<25). Subcalcic augite is rare. Pyroxene compositions in CCs differ from those in UOCs. Most of pyroxenes in CCs are classified into three groups by Fs content : less than Fs 2,Fs 3-10 and Fs>10 (typically more than 20). In contrast, the Fs content of pyroxene in UOCs ranges from 0 to 50 continuously. A TiO_2 vs. Al_2O_3 plot for augite in CCs shows two trends with different TiO_2/Al_2O_3 ratios. Augites with a high TiO_2/Al_2O_3 ratio coexist with pigeonite in CCs except for Y-790112. Y-790112 and UOCs rarely have augites with a high TiO_2/Al_2O_3 ratio, though some augites coexist with pigeonite. Minor elements in augite in chondrules show that considerable amounts of Na^+ and Fe^<2+> were probably introduced into chondrules after crystallization of pyroxenes. The oxygen fugacity at which pyroxene in CC chondrules crystallized was lower that that for UOCs. During cooling the oxygen fugacity around CC chondrules became higher than that around UOC chondrules.
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