metamorphic rock
Introduction
Metamorphic rocks (from the Greek meta, change, and morphe, form, “change of form”)[1] are rocks formed by the modification of other pre-existing rocks inside the Earth through a process called metamorphism. Through heat and/or pressure, the transformation of rocks that undergo structural and mineralogical adjustments occurs in chemically active fluids.[2] The agents of metamorphism make it possible for igneous rocks, sedimentary rocks or other metamorphic rocks, when they are subjected to pressures ranging from less than 1000 to up to 16,000 bar "Bar (pressure unit)"), at temperatures ranging from 200 to 1000 °C,[3] and/or an active fluid, cause changes in their composition, providing new substances to them. The rock that is generated will depend on the composition and texture of the original rock, the time it was subjected to the effects of the so-called metamorphic process, as well as the agents of metamorphism itself.[1] The precursor of a metamorphic rock is called protolith.[4].
Metamorphic processes produce many changes in rocks, including an increase in density, growth of larger crystals, reorientation of mineral grains into platy or banded textures, and the transformation of low-temperature minerals into high-temperature minerals.[2] Because of this, there are many ways to conveniently classify metamorphic rocks: For example, they can be grouped into broad lithological types; Other criteria are based on texture (where pressure and temperature conditions intervene) and mineralogy, chemical classes, degree of metamorphism or on the concept of metamorphic facies. A simple and practical method consists of taking into account the type of metamorphism that gave rise to the rocks and dividing them into two main groups according to their texture, that is, foliated and non-foliated.[1].
• - Foliated texture: Foliation is the resulting mineral alignment that gives the rock a sheet or banded texture. This texture occurs under extreme conditions, the pressure causes the mineral grains of a rock to not only realign, but also recrystallize, which encourages the growth of larger crystals, which is why many rocks of this type have visible crystals in bands, other crystals of minerals such as micas, recrystallize with a preferred orientation that is essentially perpendicular to the direction of the compressive forces, which gives rise to sheets.[2] Some examples are the slate "Slate (rock)") (al broken, sheets are obtained), schist (breaks easily) and gneiss (formed by light and dark minerals).
• - They are composed of a single mineral, whose crystals are characterized by having an equidimensional shape, although they usually have foliation, it is not noticeable to the naked eye.[2] Some of them are cornea, marble (crystalline appearance that is formed by metamorphism of limestone and dolomite), quartzite (it is white but can change due to impurities), serpentinite (which when transformed gives rise to asbestos).