Nonfoliated

Nonfoliated refers to a class of metamorphic rocks that do not exhibit foliation, the planar structure formed by aligned minerals under directed pressure. In nonfoliated rocks, minerals lack a preferred orientation and typically occur in a granular, interlocking mosaic.

Formation of nonfoliated rocks generally involves metamorphism under conditions of relatively uniform stress or high temperatures that promote recrystallization without creating directional mineral alignment. This often occurs during contact metamorphism near igneous intrusions or during burial metamorphism where differential stress is limited. Fluid activity can aid mineral recrystallization, but the resulting rocks retain an essentially non-layered fabric.

Textures and examples: The characteristic texture is granoblastic or mosaic, with grains that interlock. Common examples include marble, formed from limestone or dolostone; quartzite, formed from sandstone; and hornfels, formed by contact metamorphism of various protoliths. Granofels is another nonfoliated texture seen in some rocks. Nonfoliated rocks can range from fine- to coarse-grained and may contain porphyroblasts.

Comparison with foliated rocks: Foliated metamorphic rocks, such as slate, phyllite, schist, and gneiss, show a foliation or banding produced by the alignment of platy or elongate minerals under directed pressure. Nonfoliated rocks lack this alignment, reflecting metamorphic conditions with little differential stress or a recrystallization process that randomizes mineral orientations.

Uses and significance: Marble and quartzite are widely used as dimension stones and in construction, while hornfels serves as a hard, dense rock for various local uses. The study of nonfoliated rocks helps geologists infer the-pressure-temperature history of metamorphic environments.