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Migmatite

Migmatitie

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The migmatite is a heterogeneous rock which is a hybrid of both the metamorphic and igneous rocks. It contains light layers called Leucosomes (consisting of coloured minerals like feldspar, muscovite and quartz) and darker layers called Melanosomes (consisting of dark minerals like the biotite and amphibole).

This heterogeneous rock is formed from the anatexis or incomplete melting, which is caused, when there is exposure of the precursor rock to high conditions (temperature and pressure). The light layers are formed from this partial melting, having the properties of an igneous rock (due to their crystallized form) while the dark layers have undergone metamorphosis, having no similarities with the igneous rock. This difference in colour grade is similar to that of the Gneiss, but with a few differences. The Gneiss, also having alternate dark and light layers is a fully metamorphic rock. Despite this fact, the Gneiss, if exposed to high temperatures, can melt partially and transform into a migmatite. This is the reason why it is believed that migmatites were formally gneiss and then evolved to become a hybrid rock.

The multiple nature of the migmatite occurs on different scales. It could be microscopic (usually observed in thin areas), intermediate (can be seen clearly) or particularly large, being observable from a distance. The name "migmatite" comes from an ancient Greek word meaning "mixed rock" by Jakob Johannes Sederholm in 1907.

Migmatite is extraordinary due to their alternating wavy black and white strata, making them similar to the zebra stripes. These layers undulate and most times contain ptygmatic folds. The word "ptygmatic" means "fold" and was also introduced by Jakob Johannes. It describes the folds that are formed in the thicker layers when the folded substance is more resistant to flow than the adjoining media.

Hybrid rocks like the migmatite are a point of reference for Geologists because it helps them understand how such rocks behave in their melting and transforming stages.

Additional Resources

Mica   Mineralogy   Muscovite

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