Geological Classification of Granite

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The rock forming the earth's crust falls into three generic group's: igneous, sedimentary and metamorphic.

Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning "changed in form," usually into a more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism.

Granites usually have been classed as igneous rocks derived from molten masses or magmas, but there is wide evidence that the origin of some granites may be attributed to regional metamorphism or preexisting rocks, rearrangement and recrystallization taking place without a liquid or molten stage.

The chief minerals which granites are formed are feldspar and quartz and smaller amounts of mica and hornblende. They are classified as fine-grained, medium-grained, or coarse-grained. Medium-grained granites are those in which the feldspar crystals average about 1/4-inch in diameter. If relatively coarse grained crystals appear in a fine-grained groundmass the rock is designated as prophyritic granite. A rock may  have the mineral constitutes of a granite but may show a banded or platy structure owing to recrystallization, folding, or other changes while the rock was in a plastic or semi-molten condition. Such metamorphic rocks are called granite gneisses.


Reference: Dimensional Stone Design Manual, The Marble Institute of America


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