Gneisses are a type of high-grade metamorphic rock characterized by their distinct banded or foliated appearance due to the segregation of mineral grains. Formed from the alteration of pre-existing rocks like granite or sedimentary rocks under high temperature and pressure conditions, gneisses contain a variety of minerals including quartz, feldspar, and mica. Their formation process is a key example of how minerals can change and evolve based on environmental factors.
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