Question

Texture and structure of metamorphic rocks

Answer 1

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Answer 2

Texture and structure of metamorphic rocks. The structures and textures of metamorphic rocks arise during the recrystallization in the solid state of primary sedimentary and magmatic rocks. The recrystallization occurs under the action of lithostatic pressure, temperature, and abyssal solutions (fluids), often in conditions of deformation, which leads to an ordered arrangement of the mineral grains that is characteristic of gneissic and schistose structures. The textures of metamorphic rocks, which are referred to as crystalloblastic, arise as the result of the growth of minerals (crystalloblasts) in a solid or plastic medium. Irregular grains (xenoblasts) predominate; grains with crystallographic shapes (idioblasts) are formed less frequently. A distinction is made between uniformly granular (homeoblastic) and nonuni-formly granular (heteroblastic) textures. A special case of heter-oblastic texture is seen in the porphyroblastic texture, characterized by the presence of large mineral crystals (porphyroblasts) within the fine-grained mass of the rock.

The minerals in metamorphic rocks are classified according to the shape of the grain as granoblastic or granular (quartzites, marbles), lepidoblastic or foliated, a category that is characteristic of rocks containing mineral grains with a foliated form