Texture (geology)
[2] 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).
[2] The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation.
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.
Microstructure analysis [4] describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding, or alteration events.
[2] Phaneritic textures are where interlocking crystals of igneous rock are visible to the unaided eye.
[2] Porphyritic texture is one in which larger pieces (phenocrysts) are embedded in a background mass made of much finer grains.
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.
The surface texture of grains may be polished, frosted, or marked by small pits and scratches.
Understanding sorting is critical to making inferences on the degree of maturity and length of transport of a sediment.
Shear textures are particularly suited to analysis by microstructural investigations, especially in mylonites and other highly disturbed and deformed rocks.
On the thin section and hand-sized specimen scale a metamorphic rock may manifest a planar penetrative fabric called a foliation or a cleavage.
S-planes or schistosity planes are parallel with the shear direction and are generally defined by micas or platy minerals.
When a rock cools too quickly the liquid freezes into a solid glass, or crystalline groundmass.
Ostwald ripening is also used to explain some porphyritic igneous textures, especially orthoclase megacrystic granites.
A crystal growing in a magma adopts a habit (see crystallography) which best reflects its environment and cooling rate.
Hence, the shape of phenocrysts can provide valuable information on cooling rate and initial magma temperature.
Spherulitic texture is the result of cooling and nucleation of material in a magma which has achieved supersaturation in the crystal component.
Axiolitic texture results from spherulitic growth along fractures in volcanic glass, often from invasion of water.
These intergrowths are typical of pegmatite and granophyre, and they have been interpreted as documenting simultaneous crystallization of the intergrown minerals in the presence of a silicate melt together with a water-rich phase.
Iron-titanium oxides are extremely important, as they carry the predominant magnetic signatures of many rocks, and so they have played a major role in our understanding of plate tectonics.
For instance, ulvospinel in igneous rocks such as basalt and gabbro commonly oxidizes during subsolidus cooling to produce regular intergrowths of magnetite and ilmenite.
