On the alteration of Allende chondrules and the formation of matrix

Recent SEM observations of the Allende chondrules and matrix are reported. Intergrowths of both Fe-rich olivine (with a composition near Fa50), and clinoenstatite (with a composition of about Fs2). All of the clinoenstatite was involved in associations along the outer margins, grain boundaries, and internal crack surfaces of the chondrule. Most of the chondrules also contain abundant opaque inclusions which must have been strongly altered and partially replaced before chondrule formation. It is suggested that the Fe-rich olivine formed from the clinoenstatite by a reaction of the type Fe + 1/202 + MgSiO3 to (Fe,Mg)2SiO4. It is argued that a variation of the small, internally-heated planetesimal provides the most plausible environment for the chemical reaction.