microlite

Microlite is a term used in geology and mineralogy to describe very small crystals that form within volcanic and some plutonic rocks. The name derives from the Greek mikros, “small,” and lithos, “stone.” In practice, a microlite is generally defined as a crystal smaller than about 1 millimeter in at least one dimension, often elongated (acicular) or tabular, embedded in a finer groundmass or glass.

Formation and interpretation: Microlites form when silicate melts crystallize during rapid cooling or undercooling, such as during emplacement of magma or explosive volcanic eruptions. Their presence indicates a period of quick nucleation and limited growth, leading to textures where fine crystals interlock with glassy or very fine-grained material. The mineralogy of microlites typically mirrors the host melt and can include plagioclase, pyroxene, magnetite, or other minerals present in the rock, though exact identification requires petrographic analysis.

Mineralogical usage: In some older or broader petrographic literature, microlite has been used as a non-specific term for tiny crystals without indicating a distinct mineral species. Modern practice generally reserves the term for the texture and size relationship, while the specific mineral phases are named when identified.

Significance: Microlite textures help geologists interpret the cooling history, eruption dynamics, and crystallization processes of volcanic and rapidly cooled rocks. They can influence rock microstructure and, in obsidian or glass-rich rocks, serve as indicators of undercooling conditions.