Calc-aluminous insets in olivine of the Sharps chondrite

SUMMARY. A 250 /~m olivine crystal in Sharps contains I to 5/xm insets identified by microprobe analysis as ferroan monticellite, spinel, and fassaite with average compositions: olivine (host), (Feo.44Mgl.~lCao.o0 (Sio.99Alo.ol)O4; ferroan monticellite, (Cao.sgFeo.41Mgo.66) (Sil.ooAlo.o2)O4; spinel, (Feo.~lMgo.so) (FeoZ+6Cr~.23All.69Sic.o~Tio.01)O~; and fassaite, Cao.96(FeoolMgo.~0 (Alo.lsTlo.07Feo.16)- 3+ (Sit.53Alo.47)O6. Textural and experimental data suggest early crystallization of spinel from a calcic olivine melt, exsolution of ferroan rnonticellite from the host olivine, and reaction of spinel and ferroan monticellite o form fassaite. The severe depletion of alkalies and silica in this crystal suggests it is a residue from vapour fractionation. DURING a microprobe study of chondrules in the Sharps (type H-3) meteorite, the writer encountered a single 25o/~m olivine crystal crowded with 1 to 5/zm insets of several types. Microprobe scans for Fe, Ca, and Mg indicated that the host olivine is unusually calcic and suggested that some of the insets are ferroan monticellite and others are spinel and aluminous pyroxene. Quantitative analysis of these phases was undertaken for three reasons: First, ferroan monticellite would, if established, be new to meteorites. Second, the olivin