porphyry

They commonly have a porphyritic texture, i.e., larger crystals within a finer-grained groundmass, indicating two periods of crystallization.

(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 faces. This general absence…

Among the most distinctive hydrothermal deposits is a class known as porphyry copper deposits, so called because they are invariably associated with igneous intrusives that are porphyritic (meaning the rock is a mixture of coarse and fine mineral grains). Porphyry copper deposits (and…

…essentially continuous, or by a porphyritic fabric, involving more than one distinct range of grain sizes. Both of these kinds of texture are common. The relatively large crystals in a porphyritic rock ordinarily occur as separate entities, known as phenocrysts, set in a groundmass or matrix of much finer-grained crystalline…

…and therefore may grade into porphyry. Pitchstone porphyry (vitrophyre) consists of a glassy base (groundmass) enclosing abundant large crystals (phenocrysts) of such minerals as quartz, alkali feldspar, and plagioclase, as well as fewer crystals of pyroxene or hornblende. Pitchstone may reveal evidence of fluid flow by the presence of wavy…