autocryst
A crystal formed from the same melt as the volcanic rock which hosts it.
autocryst: a crystal grown in place within cooling magma
An autocryst is a mineral crystal that forms directly within a body of cooling magma, solidifying from the same melt that eventually becomes the host rock itself. Unlike xenocrysts, which are foreign crystals incorporated into magma from the surrounding country rock, or antecrysts, which formed in an earlier episode of crystallization before being remobilized, an autocryst grows synchronously with its matrix during a single continuous cooling episode.
In practice, autoxrysts range widely in size and habit. Phenocrysts, the larger crystals visible to the naked eye in a hand sample, are commonly autocrystic. Smaller groundmass crystals that form during the final stages of crystallization are also autoxrystic by definition. The key distinction is not size but provenance: the crystal and the surrounding rock material cooled from a chemically related magma body, without any discontinuity in their thermal or chemical history.
Recognition and significance
Distinguishing autoxrysts from xenocrysts or antecrysts in thin section requires careful observation of crystal boundaries, inclusion patterns, and compositional zoning. An autocryst typically shows equilibrium textures with the surrounding groundmass and compositional continuity that reflects the cooling history of that specific magma batch. Xenocrysts, by contrast, often show resorption features or reaction rims where the foreign crystal adjusted to the new melt chemistry.
Autocrysts carry direct information about crystallization conditions, cooling rates, and magma composition in their final form. Petrologists use autocryst compositions and textures to reconstruct the pressure, temperature, and volatile content of the original magma. Zoning patterns within autoxrysts record changes in melt composition during crystallization and can indicate mixing events or fractional crystallization.
The term itself derives from the Greek auto, meaning self or same, plus the suffix cryst for crystal. Its currency in igneous petrology reflects the broader need to discriminate between crystals of different origins when interpreting volcanic rocks in the field and laboratory. Correct identification of autocrystic phases is essential for geochemical modeling and for understanding magma chamber dynamics in volcanic systems.