gneiss
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Gneiss. Credit: Mineral Information Institute
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A course-grained banded or foliated metamorphic
rock, with poor cleavage, in which the minerals are arranged in darker and
lighter layers. The composition of gneiss is variable, but often approximates
to that of granite. The chief components
are irregular, granular bands of quartz and
feldspar alternating with thin undulating
bands of micas and amphiboles.
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