Secondary clay in Rajmahal basalts of India and its relation to palagonite-chlorophaeite

Abstract

SummaryThe mineralogy of the green, yellow, and brown secondary constituents occurring as amygdales, irregular infillings, and replacements in the Rajmahal basalts of eastern India and commonly described under the ill-defined names palagonite and chlorophaeite, has been revised on the basis of optical, X-ray, and thermal studies. The results indicate that these coloured constituents, which have been chiefly derived by the alteration of pyroxenes, iron ores, and primary glass, belong to the clay minerals, particularly smectite and vermiculite. This brings in question the validity of the terms palagonite and chlorophaeite which, though widely current in basaltic literature, have no definite mineralogical identity. It is suggested here that they are but forms of clay.