migmatite - a gneiss that has undergone partial melting, from a quarry in India - hand specimen

Migmatites straddle the igneous and metamorphic realms. Initially a gneiss, they have undergone partial melting as well as the metamorphic heating and pressurization that caused the gneiss to segregate into foliated bands without melting. 

Migmatites, or migmatitic gneisses, show the foliation of the parent gneiss as darker bands, with lighter melted and recrystallized silica-rich bands that resemble granite and are compositionally similar to granite. The melted and recrystallized bands, leucosomes, from the Greek leucos for white and soma for body, contrast with the darker biotite and hornblende-rich bands, melanosomes, with a higher melting point, that remained foliated and retained the gneissic texture. Melanos is Greek, for black.

This migmatite comes from a quarry in Tamil Nadu State in India, near the village of Chendarapalli in the Krishnagiri District. There, gigantic blocks are quarried, cut into slabs, polished and sold as a decorative stone. The field photos show the quarry. It is in the Eastern Ghats, an ancient range that runs parallel to the Bay of Bengal along India's east coast. The age of this migmatite is Precambrian

This specimen is polished on one side. The pencil is 5 1/2" long, for scale. 

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