What is a Gneiss rock?

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What is a Gneiss and how is it formed? 

A gneiss is a coarse-grained metamorphic rock with a streak foliation due to parallel alignment of minerals, usually in bands of light and dark-colored minerals. 

Gneiss’ typically form by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the mineral grains’ size and segregated them into bands, a transformation that made the rock and its minerals more stable in their metamorphic environment.

 

Sources: https://mrdata.usgs.gov/geology/state/sgmc-lith.php?text=gneiss