PORPHYRY (3)
Reading edition
PORPHYRY is an igneous rock identified by its texture rather than its mineral content, which is variable. Distinct crystals (phenocrysts) of minerals are embedded in a matrix of fine-grained rock. The phenocrysts formed before the main mass of the rock hardened.
Any igneous rock may have a porphyritic variety, such as granite porphyry and rhyolite porphyry, although porphyries are most likely to form in association with fine-grained igneous rocks.
Porphyry is found in Illinois only in glacial drift.
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