The parent material that has undergone metamorphic changes is called protolith, and this material may be an igneous, sedimentary, or even another metamorphic rock. Metamorphism is also said to be a process that balances or maintains the equilibrium of the rock cycle, which is otherwise changed during the occurrence of an igneous activity. The most common characteristics or structures that are present in metamorphic rocks are foliation and lineation textures and structures. These characteristics are explained in the following paragraphs. Foliation The parallel arrangement of mineral grains forming flaky layers or planes, along which the rock, that can break easily is called foliation. This term has been derived from the Latin word folium, which means leaf. Flaky and platy minerals such as biotite, muscovite, glacophane, kyanite, chlorite, etc., form these features. Foliation is primarily seen only in zeolite, prehenite-pumpellite, greenshist, amphibolite, and blueschist facies of metamophism. Another term used to describe this feature is called slaty cleavage; it is seen mostly in low-grade metamorphic rocks like slates and phyllites. When uneven pressure acts on the parent material along with a temperature change, the original minerals are converted to flaky minerals like mica, which further get arranged in the typical parallel form due to differential pressure acting on one side of the rock. This feature occurs only during prograde metamorphism, and it is seen in regions that have undergone orogenic activity, further forming mountain ranges. Consisting of a phyllosilicate (sheet-like) structure, foliation is dominantly seen in schistose rocks and is also present in various gneisses as alternate bands and in tectonites in the form of linear rock fabric, along with lineations. Lineation These features are defined as the parallel alignment of the fabric of a rock where two foliation planes meet. Thus, in simple terms, when two slaty cleavages come together, they form these linear structures. Lineations can be present in the form of stretched and parallel mineral grains like stretched pebbles in conglomerates, crenulations, parting lineations, etc. These structures are mainly formed due to cataclastic or burial metamorphism, though the contribution of temperature-related changes might be present in a few cases. Such linear features are majorly seen in metaconglomerates, fault breccias, metasedimentary rocks, tectonites, etc. The L-tectonites are characterized by the presence of lineations, and the L-S tectonites show typical shear-deformed arrangement of foliation cleavages, as well as accompanying linear structures. In this case, all the axis lengths of minor aspects like stretched pebbles and mineral grains, if present, are measured. Lineations can be parallel as well as perpendicular to the foliated planes of schistose minerals. Unlike the previously described characteristic, linear features are rarely formed under regional metamorphism, as a temperature change would lead to recrystallization or formation of new minerals, as in the case of low-grade metamorphic rocks.

Schistose Texture The parallel arrangement of mineral grains due to a combined effect of temperature and pressure changes is called schistose texture or schistosity. It is a characteristic of foliation planes, and the rocks showing this texture are found under greenschist, amphibolite, and eclogite metamorphic facies. Gneissose Texture The alternate arrangement of schistose and granulose (appearance of aggregate of grains) bands that are mostly parallel to each other is called gneissose texture or gneissosity. Such textures are commonly seen in amphibolite, granulite, and greenschist facies of metamorphism, and they indicate medium to high metamorphic rock grade. Slaty Texture In this type, mineral grains are very fine grained, and are arranged in a particular orientation forming slaty texture or slaty cleavage. The rock has undergone low-grade metamorphism, and fine carbonaceous material often forms a major part of its composition. This texture is mainly seen in zeolite and prehnite-pumpellyite facies of metamorphism. Phyllitic Texture This type is similar to the slaty texture, but is characterized by a higher degree of metamorphism and larger mineral grains. Also, the degree of orientation and parallelism is more pronounced than that seen in slates and meta-grewackes. This texture is included under greenschist and prehnite-pumpellyite facies, and is often regarded as a transitional feature between both the categories. Granoblastic Texture This type can be simply defined as an aggregate of coarse-grained minerals, which have undergone recrystallization due to changes in temperature and pressure. This texture is primarily seen in granulite and eclogite facies of metamorphism and is mostly present in non-foliated metamorphic rocks. Migmatitic Texture This type of texture is formed due to very high grade of metamorphism, and is seen mostly in metaigneous rocks. These rocks have undergone partial melting due to very high temperatures, and the mineral layers are present in the form of bands or swirls of tight folds. Ptygmatic folds and Schlieren textures are common, and the protolith material in most cases is a gneiss rock. Hornfelsic Texture A characteristic feature of contact metamorphism, hornfelsic texture appears as a mosaic or aggregate of equidimensional mineral grains, which are situated very close to each other. This texture is formed when the protolith rock experiences very high temperature ranges with minimum pressure increase.