First comprehensive characterization of osumilite from India (Eastern Ghats Province): Physicochemical characteristics, stability of the mineral and its breakdown products

A rare occurrence of osumilite has been reported from a metapelite from parts of the Eastern Ghats Province, India. Incidentally, this is the first report of osumilite along with its composition and textural description from any Indian rock. The presence of osumilite has been confirmed by electron microscopy (BSE images), electron microprobe analyses (and X-ray intensity maps) as well as Raman spectroscopy. In the studied sample, osumilite occurs as rare anhedral relicts (similar to 300-350 mu m) that are surrounded by a variety of extremely fine-grained symplectitic intergrowths namely cordierite + K-feldspar, cordierite + K-feldspar + orthopyroxene and cordierite + K-feldspar + orthopyroxene + quartz. The symplectites partially to completely pseudomorph the early formed osumilite grains. An isochemical pressure-temperature phase diagram has been calculated in the Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O system in order to model the breakdown of osumilite and formation of such symplectitic mineral assemblages, using the average micro bulk compositions calculated with the X MapTools software. The calculated P-T phase diagram suggests that osumilite was formed during ultra-high temperature metamorphism (at P similar to 8.5 kbar and T similar to 1110 degrees C) in an Mg-Al-rich pelitic assemblage and subsequently broke down to produce the symplectitic mineral assemblages during isobaric cooling. (C) 2019 Elsevier B.V. All rights reserved.