How is foliation formed in metamorphic rocks and what can it tell you about the conditions of metamorphism?
Foliation in metamorphic rock is formed by uni- or bi-directional strain, often under high temperature and pressure conditions, that allow laminar deformation along planes of weakness between layers, mineral grains, and molecular bonds. The foliation, often sheet-like planes with altered mineral composition, can often indicate the direction of increased strain and inform regional stress and plate tectonic analysis.
The types of minerals present can indicate different levels of temperature and pressure. See various temperature/pressure diagrams for P/T conditions associate with certain mineral assemblages, as many are different depending on what minerals were present.