CORONITES FROM INDIA AND THEIR BEARING ON THE ORIGIN OF CORONAS

Abstract

Detailed petrographic and mineralogic studies of some coronites from India confirm observations of previous workers on similar rocks. The first rhombic pyroxene corona is formed by replacement of olivine, and the second corona of amphibole, amphibole-spinel, and/or garnet develops by replacement of plagioclase. Certain features such as the clouding of plagioclase, pyroxenes, and olivines and the recrystallization of primary minerals ( e.g., pyroxene) and the development of garnet are commonly noted in coronites. Their significance as metamorphic phenomena and their relationship with corona formation is stressed. From a crystallochemical examination of the reacting minerals and the resulting corona minerals, the writer concludes that the first corona of rhombic pyroxene is formed from olivine by rearrangement of the independent SiO 4 tetrahedra of the nesosilicate into chains in the inosilicate rhombic pyroxene. During metamorphism this reaction is favored along the olivine-plagioclase interface where disorder and chemical potential are high and is inhibited along the olivine-pyroxene interface where disorder and chemical potential are low. The reaction is initiated by an intergranular film of water which is effective in breaking bonds in olivine. For formation of the second corona minerals, disruption of the plagioclase structure is essential. Inosilicate amphibole is the simplest structural change from plagioclase and is presumably formed in the presence of lower concentrations of water. The formation of nesosilicate garnet requires complete disruption of plagioclase and a change in the co-ordination of Al +3 , which probably occurs as a result of greater concentrations of water. Biotite, amphibole, amphibole-spinel, and garnet coronas around iron ore and spinel are also metamorphic and are formed in part by replacement of plagioclase. Regional metamorphism at comparatively high temperatures, under water-deficient conditions and with little shearing stress, gives rise to coronas; the degree of development is partly a function of time. Data and observations are given in support of these conclusions.