S.K. Saxena – författare

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.

Chemical petrology is essentially the physical chemistry of rocks and associated fluids, although it also borrows heavily from such other sciences as mineralogy. In terms of fundamentals it is firmly grounded in chemical thermodynamics and kinetics. In its treatment of terrestrial environments it grades imperceptably into sedimentology, geochemistry, and geophysics and in extraterrestrial environments into cosmochemistry. It is one of the most important branches of planetology and meteoritics. The unity of approach of thermodynamics and kinetics to processes in these diverse environments is stressed in this book by numerous examples which have been chosen to illuminate different aspects of the subject. Thus we have discussed in some depth such problems as the genesis of layered basic complexes, calc-alkaline batholiths, chondri tic meteorites, and the surface-atmosphere interaction of the planet Venus because these are important and because they are particularly good illustrations of the chemical petrology approach. Considerable attention also has been devoted to volcanic processes. In our treatment of metamor­ phism in particular, an attempt has been made to correlate and integrate the vast number of recent experimental, theoretical, and field studies. However, we have not attempted a comprehensive survey of all known rock types or occurrences, nor did we review all the diverse opinions and conclusions on the origins of controversial rocks. Instead we have chosen to stress interpretations we regard as following most directly from the evidence.