Description

The third volume in this series consists of eight chapters. The first three deal with kinetic aspects of compositional variations both within individual phases and across crystal boundaries. Basically, the authors use the kinetic theory and the sparsely available rate data to explain the formation of various types of zoning and the exsolution processes in silicates. Loomis rightly argues that "the kinetic inhibitions to reequilibration that preserve primary igneous crystals and high- grade metamorphic assemblages also affect the crystallization and prograde meta- morphism of these rocks." These "kinetic inhibitions" appear in the form of zoned crystals, reaction rims and disequilibrium assemblages. Their proper recognition and quantitative characterization leads to an understanding of the physico-chem- ical history of the rock. On a similar theme, I examine possible relationships between the exsolution processes in Ca-Fe-Mg pyroxenes and the cation order-disorder on nonequiva- lent crystallographic sites. A multi-technique study of exsolutions in crystals employing electron microscopy and X-ray structural refinements should contrib- ute greatly in understanding the thermal history of the rock. Many geothermometric studies result in discordant temperatures when the estimates are done using serveral coexisting pairs of minerals in a single specimen. Lasaga uses the kinetic rates of diffusion of various chemical species and explains the discordance through his "geospeedometric" approach.