Peritektoisia

Peritektoisia is a term derived from Finnish and refers to a specific geological phenomenon. It describes a type of intrusive igneous rock formation where a magma chamber cools and solidifies in a way that creates a distinctive texture. This texture is characterized by the presence of larger, euhedral or subhedral crystals (crystals with well-defined faces) that are surrounded by a finer-grained groundmass. These larger crystals are known as phenocrysts, and the finer-grained material is the matrix.

The formation of peritektoisia textures is often attributed to a two-stage cooling process. Initially, some mineral phases may have begun to crystallize while the magma was still deep within the Earth. As this magma ascended and cooled more rapidly closer to the surface, the remaining melt solidified quickly, forming the finer-grained matrix around the pre-existing larger crystals. This contrasts with equigranular textures where all crystals are roughly the same size.

Geologists use the presence of peritektoisia textures to infer aspects of a magmatic body's emplacement history and thermal regime. The size and abundance of phenocrysts, along with the grain size of the matrix, can provide clues about the rate of cooling and the degree of magma mixing or assimilation that may have occurred. While the term itself is less common internationally than descriptive textural terms like porphyritic, it represents a specific observation within Finnish petrology.