Kelyphite and symplectite: Toward a new development of the study of metamorphic reactions

Abstract

Recent progress in the study of kelyphites is reviewed. Petrographic characters of two major types of kelyphites are summarized and the reaction mechanism and material transfer associated with Type 1 kelyphite formation (a metamorphic reaction between garnet and olivine) is dealt with in detail. The significance of the shell form of kelyphite is emphasized and it is suggested that the occurrence of internal stress, which is theoretically believed to be generated by the reaction occurring within the kelyphite shell, plays a key role in determining the mode and the flux of material transfer across the kelyphite shell. It is shown that bulk composition of the kelyphite and the occurrence of nodular spinels at the peripheries of the kelyphite shell are best explained in terms of a model assuming a constant volume for the kelyphite shell, which is considered to be the most rigid and competent material among the constituent units of the kelyphite complex. It is emphasized that the process is essentially a mechanochemical one and a better understanding of such processes requires a theoretical development for thermodynamics for open system incorporating the stress as an intensive variable in a stress-gradient field.