microphenocrysts

Microphenocrysts are crystals in igneous rocks that are larger than the groundmass but smaller than typical phenocrysts, giving a porphyritic texture with a two-scale crystal population. They are usually visible only with a hand lens and commonly range from about 0.1 to 2 millimeters in size, though definitions vary. In volcanic rocks, microphenocrysts occur embedded in a finer-grained groundmass, and in plutonic rocks they may be dispersed within coarser textures.

They crystallize during the early cooling of magma in the chamber or during ascent before eruption, so they record a crystallization history distinct from the later-formed groundmass. The size range places microphenocrysts between microlites (tiny crystals formed during rapid quenching) and larger phenocrysts (visible without magnification). Mineralogy is controlled by the overall magma composition; common microphenocrysts include plagioclase and olivine in mafic rocks, and quartz or feldspar in felsic compositions; other minerals such as pyroxene and magnetite may also occur.

In petrographic study, microphenocrysts help interpret crystallization sequences, magma residence times, and differentiation processes in the magmatic system. They may be antecrysts or xenocrysts in some cases, but are generally considered crystals that grew from the melt rather than being embedded from surrounding country rock.

Microphenocrysts are a characteristic feature of porphyritic rocks and are common in many igneous rock types.