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what determines whether a metamorphic rock is foliated or non-foliated?

A metamorphic rock is foliated when its minerals are forced to line up into layers or bands under directed pressure, and it is non‑foliated when such alignment does not develop because pressure is uniform or the minerals cannot easily align.

Key deciding factors

  • Type and direction of pressure
    • Directed (unequal) pressure or shear stress squeezes the rock more in some directions than others, causing platy or elongate minerals to rotate and recrystallize in parallel, producing foliation (e.g., slate, schist, gneiss).
* Uniform (confining) pressure, or very low pressure near the surface, does not favor any particular direction, so minerals do not line up and the rock remains non‑foliated (e.g., marble, quartzite, hornfels).
  • Mineral composition of the rock
    • Rocks rich in flat or elongate minerals such as micas and some amphiboles readily form foliation because these grains can rotate and stack like cards.
* Rocks dominated by equant, blocky minerals like quartz and calcite tend to be non‑foliated because their crystals do not easily align, even under directed pressure, leading to textures like quartzite and marble.
  • Metamorphic setting (contact vs regional)
    • Regional metamorphism (large‑scale, deep burial with strong directional stresses at plate boundaries) commonly produces foliated rocks.
* Contact metamorphism (heating around an intruding magma body, usually at relatively shallow depths with mainly confining pressure) commonly produces non‑foliated rocks such as hornfels and many marbles.
  • Degree of deformation and recrystallization
    • Strong deformation plus recrystallization under directed stress enhances foliation, increasingly segregating minerals into visible bands at higher grades.
* Where deformation is weak, or recrystallization simply grows interlocking grains without preferred orientation, texture stays massive or granular and non‑foliated.

Information gathered from public forums or data available on the internet and portrayed here.