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What Is Gneiss? About The Metamorphic Rock

GNEISS – In this topic, we are going to know and learn about a metamorphic rock which is known as Gneiss.

GNEISS
Image from: Geology Science

It is a foliated metamorphic rock which is identified by its bands and lenses of varying composition. It forms by regional metamorphis at convergent plate boundaries.

It is derived from the Old High German gneisto, which means ‘spark’ in reference to its sheen.

It is a rock which mineral grains recrystallized under intense heat and pressure. The alteration increased the size of mineral grains and segregated them into bands which made the rock and its minerals more stable in their environment.

Geology.com states that the rock can dorm in several different ways, with the most common starting with the sedimentary rock shale. Metamorphism can transform the latter into slate, then phyllite, then schist.

During the transformation, clay particles in shale transform into micas and increase in size. Lastly, the platy micas start to recrystallize into granular minerals. The appearance of granular minerals is what marks the transition into this rock.

Granite can also be metamorphosed by intense heat and pressure into granite gneiss. Its bands are usually light in color and alternate with bands of darker-colored minerals with platy or elongate habits.

Dark minerals sometimes exhibit an orientation determined by the pressures caused by metamorphism.

READ ALSO: What Is Lapis Lazuli? About The Metamorphic Rock

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