D. the arrangement of mineral grains into a foliation
Explanation:
Due to the tectonic activity of the terrestrial lithosphere, igneous and sedimentary rocks formed in certain environments and under precise environmental conditions, can be subjected to new conditions (essentially pressure and temperature). Under the new conditions, and frequently under the action of tectonic stresses (linked, for example, to the formation of mountain ranges), pre-existing rocks are transformed texturally, structurally and mineralogically into a solid state, giving rise to metamorphic rocks. Its fundamental characteristic is to present blastic texture (i.e., solid state recrystallization), and in cases of deformation, anisotropic factories.
The petrographic characteristics of these rocks are especially complicated, given the solid state transformation processes that they have suffered, usually accompanied by intense deformation. Due to the gradual nature of metamorphic (and deformational) transformations, the characteristics of this type of rock also range from igneous or sedimentary to purely metamorphic.
Foliation
The term foliation (foliatus: leaf-shaped) refers to any planar (almost flat) arrangement of mineral grains or structural features of the interior of a rock. Although there is foliation in some sedimentary rocks and even in a few types of igneous rocks, it is a fundamental characteristic of rocks that have undergone regional metamorphism, that is, rocky units that have folded and deformed greatly. In metamorphic environments, foliation is caused, ultimately, by the compressive efforts that shorten the rock masses, causing the mineral grains of pre-existing rocks to develop parallel or almost parallel alignments. Examples of foliation are the parallel alignment of minerals with planar habit and / or elongated minerals; the parallel alignment of mineral particles and flattened edges; the compositional bandeado where the separation of dark and light minerals generates a laminar appearance, and the blackboard when the rocks easily separate into thin and tabular layers along parallel surfaces. These different types of foliation can be formed in many different ways, such as:
1. Rotation of elongated or planar mineral grains towards a new orientation.
2. Recrystallization of minerals to form new grains that grow in the direction of the preferred orientation.
3. Changes in shape in equidimensional grains to elongated shapes that align in a preferred orientation.
