Phyllites are foliated metapelites of a shale or mudstone protolith that has been subjected to low-grade regional metamorphism. They are characteristic of being fine-grained with a vitreous, submetallic lustre and are primarily composed of quartz, feldspar, and micaceous minerals. Phyllite is higher grade than slate, differentiated by its silkier sheen and slightly coarser minerals, phyllite exhibits a slaty cleavage of parallel and fissile foliation perpendicular to a dominant direction of stress. This is due to the recrystallization of clay minerals into platy micas, indicated by the green markers and red arrows. Phyllites can also exhibit crenulation which is further indicative of multiple directions of stress imposed during metamorphism and is shown by the blue markers. Crenulation superimposes the foliation and cleavage planes as a wavy texture. Following the general evolution of metamorphosed shale into slate and phyllite, schist is the subsequent rock of increasing grade which exhibits similar characteristics as phyllite. However, due to higher metamorphic intensity, schists characteristically have much coarser crystals, weaker parallel foliation alignment, and are subjected to more deformational folding (see Schist sample). Metamorphism may also often subject phyllites to the growth of index minerals, such as biotite, garnet, andalusite, and staurolite, typically occurring as porphyroblasts and indicative of metamorphic grade.