DEFINITION OF GNEISS

A coarse-grained strongly foliated metamorphic

rock in which bands or layers of granular minerals such as

quartz and feldspar generally alternate with bands or layers

of flaky or elongate minerals such as mica or amphibole.

There are many varieties of gneiss, distinguished by composition

and texture. Gneiss derived from igneous rocks is generally

known as orthogneiss, whereas gneiss derived from

sedimentary rocks is known as paragneiss. However it is

often difficult to determine the origin or protolith of the

gneiss, so it is recommended that textural and compositional

terms be used to describe a gneiss, instead of terms that purport

to know the rock’s protolith. Augen gneiss has lenticular

spots, generally made of quartz and feldspar that resemble

eyes in a strongly foliated matrix. Mylonitic gneiss is strongly

foliated and shows evidence of high shear strains. Straight

gneiss has parallel layers that extend over large distances,

resembling a sedimentary layering. Gneiss is generally formed

from regional metamorphic and tectonic processes.

See also METAMORPHISM; PLATE TECTONICS.

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