A metamorphic rock is determined to be foliated or non-foliated primarily by the presence or absence of directed pressure or differential stress during its formation. Foliated metamorphic rocks form in environments where there is directed pressure or shear stress, causing minerals within the rock to align perpendicularly to the main stress direction, resulting in a layered or banded texture. Non-foliated metamorphic rocks, on the other hand, form in conditions where pressure is uniform (confining pressure) or very low, and their minerals do not align into layers because the stress is not directional or the mineral crystals are blocky and equidimensional. More specifically:
- Foliation occurs due to deformation under directed pressure, which elongates or aligns minerals such as mica or amphibole perpendicular to the pressure direction. New minerals may also grow aligned under these conditions, enhancing foliation.
- Non-foliated rocks often form near the surface under low pressure or from parent rocks composed of minerals like quartz and calcite that do not easily align into layers even under directed pressure.
Examples of foliated rocks include slate, phyllite, schist, and gneiss, while non-foliated metamorphic rocks include marble, quartzite, and hornfels.