Metamorphic rock identification emphasizes textural aspects when the mineral content is formed by mixed silicates and the mineral content when the mineral composition is predominately one mineral. Textural aspects include crystal size (very fine/fine = invisible and medium/coarse = visible) and the presence or absence of foliation. Foliation involves parallel alignment of crystals. Types of foliation include slaty (rock) cleavage and phyllitic, schistose, and gneissic foliation. Mineral contents include the mixed silicate group and the calcite, quartz, or carbon groups.