In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye), and glassy (in which the particles are too small to be seen and amorphously arranged). The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways. A common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale. Textures are penetrative fabrics of rocks; they occur throughout the entirety of the rock mass on microscopic, hand-sized specimen, and often outcrop scales. This is similar in many ways to foliations, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures occur on a hand-sized specimen scale and above. Microstructure analysis describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding, or alteration events. Crystalline textures include phaneritic, foliated, and porphyritic. Phaneritic textures are where interlocking crystals of igneous rock are visible to the unaided eye. Foliated texture is where metamorphic rock is made of layers of materials. Porphyritic texture is one in which larger pieces (phenocrysts) are embedded in a background mass made of much finer grains. Fragmental textures include clastic, bioclastic, and pyroclastic. A preferred mineral orientation, is the texture of metamorphic rock in which its grains have a flattened shape (inequant), and their planes tend to be oriented in the same direction.
Nomenclature Rock microstructure includes the texture and small-scale structures of a rock. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.
Sedimentary microstructures Description of sedimentary rock microstructure aims to provide information on the conditions of deposition of the sediment, the paleoenvironment, and the provenance of the sedimentary material. Methods involve description of clast size, sorting, composition, rounding or angularity, sphericity and description of the matrix. Sedimentary microstructures, specifically, may include microscopic analogs of larger sedimentary structural features such as cross-bedding, syn-sedimentary faults, sediment slumping, cross-stratification, etc.
Maturity The maturity of a sediment is related not only to the sorting (mean grain size and deviations), but also to the fragment sphericity, rounding and composition. Quartz-only sands are more mature than arkose or greywacke.
Fragment shape Fragment shape gives information on the length of sediment transport. The more rounded the clasts, the more water or wind-worn they are. Particle shape includes form and rounding. Form indicates whether a grain is more equant (round, spherical) or platy (flat, disc-like, oblate); as well as sphericity.
Roundness Roundness refers to the degree of sharpness of the corners and edges of a grain. The surface texture of grains may be polished, frosted, or marked by small pits and scratches. This information can usually be seen best under a binocular microscope, not in a thin section.
Composition Composition of the clasts can give clues as to the derivation of a rock's sediments. For instance, volcanic fragments, fragments of cherts, well-rounded sands all imply different sources.
Matrix and cement The matrix of a sedimentary rock and the mineral cement (if any) holding it together are all diagnostic.
Diagenetic features Usually diagenesis results in a weak bedding-plane foliation. Other effects can include flattening of grains, pressure dissolution and sub-grain deformation. Mineralogical changes may include zeolite or other authigenic minerals forming in low-grade metamorphic conditions.
Sorting Sorting is used to describe the uniformity of grain sizes within a sedimentary rock. Understanding sorting is critical to making inferences on the degree of maturity and length of transport of a sediment. Sediments become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density. Sorting can be expressed mathematically by the standard deviation of the grain-size frequency curve of a sediment sample, expressed as values of φ (phi). Values range from <0.35φ (very well sorted) to >4.00φ (extremely poorly sorted).
Metamorphic microstructure The study of metamorphic rock microstructures aims to determine the timing, sequence and conditions of deformations, mineral growth and overprinting of subsequent deformation events. Metamorphic microstructures include textures formed by the development of foliation and overprinting of foliations causing crenulations. The relationship of porphyroblasts to the foliations and to other porphyroblasts can provide information on the order of formation of metamorphic assemblages or facies of minerals. Shear textures are particularly suited to analysis by microstructural investigations, especially in mylonites and other highly disturbed and deformed rocks.
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