Silica‐merrihueite/roedderite‐bearing chondrules and clasts in ordinary chondrites: New occurrences and possible origin

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Abstract
Merrihueite (K,Na)2(Fe, Mg)5Si12O30 (na < 0.5, fe > 0.5, where na = Na/(Na + K), fe = Fe/(Fe + Mg) in atomic ratio) is a rare mineral described only in several chondrules and irregularly‐shaped fragments in the Mezö‐Madaras L3 chondrite (Dodd et al., 1965; Wood and Holmberg, 1994). Roedderite (Na,K)2(Mg, Fe)5Si12O30 (na > 0.5, fe < 0.5) has been found only in enstatite chondrites and in the reduced, subchondritic silicate inclusions in IAB irons (Fuchs, 1966; Rambaldi et al., 1984; Olsen, 1967). We describe silica‐roedderite‐bearing clasts in L/LL3.5 ALHA77011 and LL3.7 ALHA77278, a silica‐roedderite‐bearing chondrule in L3 Mezö‐Madaras, and a silica‐merrihueite‐bearing chondrule in L/LL3.5 ALHA77115. The findings of merrihueite and roedderite in ALHA77011, ALHA77115, ALHA77278 and Mezö‐Madaras fill the compositional gap between previously described roedderite in enstatite chondrites and silicate inclusions in IAB irons and merrihueite in Mezö‐Madaras, suggesting that there is a complete solid solution of roedderite and merrihueite in meteorites. We infer that the silica‐ and merrihueite/roedderite‐bearing chondrules and clasts experienced a complex formational history including: (a) fractional condensation in the solar nebula that produced Si‐rich and Al‐poor precursors, (b) melting of fractionated nebular solids resulting in the formation of silica‐pyroxene chondrules, (c) in some cases, fragmentation in the nebula or on a parent body, (d) reaction of silica with alkali‐rich gas that formed merrihueite/roedderite on a parent body, (e) formation of fayalitic olivine and ferrosilite‐rich pyroxene due to reaction of silica with oxidized Fe on a parent body, and (f) minor thermal metamorphism, possibly generated by impacts.