phenocryst

crystal

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July 23, 2026

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A phenocryst is a prominent, large crystal embedded within a finer-grained matrix in a porphyritic igneous rock. These larger crystals, known as phenocrysts, typically form slowly at depth before the magma rises and cools more rapidly at the Earth's surface, resulting in a finer-grained groundmass or even glass. The size difference between phenocrysts and the groundmass indicates a change in the conditions affecting the crystallizing magma. Phenocrysts can range from euhedral (fully crystal-faced) to anhedral (lacking crystal faces), though they are often subhedral (partly faced).123

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feldspar
Crystal structure
…in pegmatite masses, or as phenocrysts within porphyries. (A porphyry is an igneous rock containing conspicuous crystals, called phenocrysts, surrounded by a matrix of finer-grained minerals or glass or both.) In most rocks, both alkali and plagioclase feldspars occur as irregularly shaped grains with only a few or no crystal…
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…in pegmatite masses, or as phenocrysts within porphyries. (A porphyry is an igneous rock containing conspicuous crystals, called phenocrysts, surrounded by a matrix of finer-grained minerals or glass or both.) In most rocks, both alkali and plagioclase feldspars occur as irregularly shaped grains with only a few or no crystal…
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igneous rock
Fabric
…as separate entities, known as phenocrysts, set in a groundmass or matrix of much finer-grained crystalline material or glass. Quite commonly in many volcanic rocks, phenocrysts are aggregated. When this is observed, the term glomeroporphyritic is used to describe the texture, and the aggregate is referred to as a glomerocryst.…
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…as separate entities, known as phenocrysts, set in a groundmass or matrix of much finer-grained crystalline material or glass. Quite commonly in many volcanic rocks, phenocrysts are aggregated. When this is observed, the term glomeroporphyritic is used to describe the texture, and the aggregate is referred to as a glomerocryst.…
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rhyolite
…well-developed, large, single crystals (phenocrysts) at the time of extrusion. The amount of microcrystalline matrix (groundmass) in the final product may then be so small as to escape detection except under the microscope; such rocks (nevadites) are easily mistaken for granite in hand specimens. In most rhyolites, however, the…
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…well-developed, large, single crystals (phenocrysts) at the time of extrusion. The amount of microcrystalline matrix (groundmass) in the final product may then be so small as to escape detection except under the microscope; such rocks (nevadites) are easily mistaken for granite in hand specimens. In most rhyolites, however, the…
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