anaclastic

Anaclastic is an adjective used in geology and mineralogy to describe textures and rocks produced by a process called anaclasis. Anaclasis refers to the brittle disaggregation and fragmentation of preexisting grains under tectonic stress, resulting in a mosaic of angular fragments embedded in a finer matrix. The texture emphasizes fragmentation of crystals rather than substantial new crystal growth or plastic deformation.

Typical features of anaclastic textures include angular grain boundaries, little evidence of subgrain development or recrystallization, and a prominent population of crushed or broken grains. The matrix often consists of fine, crushed material that binds the clasts together. Anaclastic rocks can resemble breccias or cataclasites, but the emphasis is on crystal fragmentation rather than on ductile flow or extensive chemical alteration.

Occurrence and formation environments for anaclastic textures are usually related to fault zones, shear zones, and other regions of intense brittle deformation in the crust. They can form under varying temperatures and pressures, generally in settings where high strain rates promote mechanical breakup of grains over recrystallization. Microstructures may show angular fragments with relatively straight boundaries and limited orientation preferredness, indicating a predominance of brittle fragmentation over large-scale grain rotation.

See also: cataclasis, cataclasite, breccia, fault zone, brittle deformation, metamorphic petrology. Although related to other forms of cataclastic deformation, anaclastic textures are distinguished by the emphasis on disaggregation of grains rather than ductile sliding or extensive recrystallization.