Microlites

Microlites are very small crystals that occur within volcanic and other igneous rocks. They are typically finer than 1 millimeter in length and are often visible only under magnification or in thin section. Microlites form during rapid cooling of silica-rich magmas near or at the surface, where crystallization is extensive but restricted by the cooling rate and viscosity of the melt. They are distinct from larger crystals called phenocrysts and from the coarser groundmass or glass.

In appearance, microlites are commonly acicular (needle-like) or tabular and can include minerals such as plagioclase, pyroxene, magnetite, and other felsic or intermediate phases. In glassy rocks like obsidian or rhyolitic glass, microlites may be suspended within the vitreous matrix and can organize into spherulitic textures around a crystallized nucleus. The precise mineralogy and morphology of microlites reflect the crystallization history and chemical composition of the original melt.

The study of microlites provides petrologists with information about cooling rates, eruption dynamics, and crystallization conditions. A higher microlite content generally indicates more rapid cooling and near-surface crystallization, whereas fewer microlites suggest slower cooling or more extensive glass formation. Microlites are thus important for interpreting the textures and formation histories of volcanic rocks, in contrast to phenocrysts, which are larger crystals that predate the eruption, and to the groundmass, which forms the surrounding matrix.