SILICA-MERRIHUEITE ROEDDERITE-BEARING CHONDRULES AND CLASTS IN ORDINARY CHONDRITES - NEW OCCURRENCES AND POSSIBLE ORIGIN/

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 describ ed only in several chondrules and irregularly-shaped fragments in the Mezo-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 foun d only in enstatite chondrites and in the reduced, subchondritic silic ate inclusions in IAB irons (Fuchs, 1966; Rambaldi et al., 1984; Olsen , 1967). We describe silica-roedderite-bearing clasts in L/LL3.5 ALHA7 7011 and LL3.7 ALHA77278, a silica-roedderite-bearing chondrule in L3 Mezo-Madaras, and a silica-merrihueite-bearing chondrule in L/LL3.5 AL HA77115. The findings of merrihueite and roedderite in ALHA77011, ALHA 77115, ALHA77278 and Mezo-Madaras fill the compositional gap between p reviously described roedderite in enstatite chondrites and silicate in clusions in IAB irons and merrihueite in Mezo-Madaras, suggesting that there is a complete solid solution of roedderite and merrihueite in m eteorites. We infer that the silica- and merrihueite/roedderite-bearin g chondrules and clasts experienced a complex formational history incl uding: (a) fractional condensation in the solar nebula that produced S i-rich and Al-poor precursors, (b) melting of fractionated nebular sol ids resulting in the formation of silica-pyroxene chondrules, (c) in s ome cases, fragmentation in the nebula or on a parent body, (d) reacti on of silica with alkali-rich gas that formed merrihueite/roedderite o n a parent body, (e) formation of fayalitic olivine and ferrosilite-ri ch pyroxene due to reaction of silica with oxidized Fe on a parent bod y, and (f) minor thermal metamorphism, possibly generated by impacts.