Rock microstructure

Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microsctructure" 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.

Textures are penetrative fabrics of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand specimen and often on an outcrop scale. 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 hand-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.

Sedimentary microstructure

Description of sedimentary rock microstructure aims to provide information on the conditions of deposition of the sediment, the paleo-environment, 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 micoscopic 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) ut to the fragment sphericity, rounding and composition. Quartz-onl 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-worn they are, etcetera. 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 a in 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. 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.

Foliations and crenultations

On the thin section and hand specimen scale a metamorphic rock may manifest a linear penetrative fabric called a foliation or a cleavage. Several foliations may be present in a rock, giving rise to a crenulation.

Identifying a foliation and its orientation is the first step in analysis of foliated metamorphic rocks. Gaining information on when the foliation formed is essential to reconstructing a P-T-t (pressure, temperature, time) path for a rock, as the relationship of a foliation to porphyroblastss is diagnostic of when the foliation formed, and the P-T conditions which existed at that time.

Ductile Shear Microstructures

Very distinctive textures form as a consequence of ductile shear. The microstructures of ductile shear zones are S-planes, C-planes and C' planes. S-planes or sissalement planes are parallel with the shear direction and are generally defined by micas or platy minerals. Define the flattened long-axis of the strain ellipse. C-planes or cissalement planes form oblique to the shear plane. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain. The C' planes are rarely observed except in ultradeformed mylonites, and form nearly perpendicular to the S-plane.

Other microstructures which can give sense of shear include

sigmoidal veins
mica fish
rotated porphyroblasts

Igneous microstructure

Analysis of igneous rock microstructure may complement descriptions on the hand specimen and outcrop scale. This is especially vital for describing phenocrysts and fragmental textures of tuffs, as often relationships between magma and phenocryst morphology are critical for analysing cooling, fractional crystallization and emplacement. Analysis of intrusive rock microstructures can provide information on source and genesis, including contamination of igneous rocks by wall rocks and identifying crystals which may have been accumulated or dropped out of the melt. This is especially critical for komatiite lavas and ultramafic intrusive rocks.

General principles of igneous microstructure
Igneous microstructure is a combination of cooling rate, nucleation rate, eruption (if a lava), magma composition and its relationships to what minerals will nucleate, as well as physical effects of wall rocks, contamination and especially vapor.

Grain size
According to the size of the grains, igneous rocks may be classified as

pegmatitic (very large grains),
phaneritic rocks contain minerals with crystals visible to the unaided eye and are commonly intrusive.
aphanitic where cooling is rapid, crystal nucleation and growth is stunted, forming a uniform, fine grained rock, such as aplite
porphyritic (containing phenocrysts in a fine groundmass)
vesicular (voids caused by trapped gas while cooling),
hyaline, vitreous or glassy
pyroclastic rocks contain fragments of crystals, phenocrysts and rock fragments

Crystal shapes
Crystal shape is also an important factor in the texture of an igneous rock. Crystals may be euhedral, subeuhedral or anhedral:

Euhedral, if the crystallographic shape is preserved.
Subeuhedral or Subhedral, if only part is preserved.
Anhedral, if the crystal presents no recognizable crystallographic direction.

Rocks composed entirely of euhedral crystals are termed panidiomorphic, and rocks composed entirely of subhedral crystals are termed subidiomorphic.

Porphyritic Structure
Porphyritic structure is caused by the nucleation of crystal sites and the growth of crystals in a liquid magma. Often a magma can only grow one mineral at a time especially if it is cooling slowly. This is why most igneous rocks have only one type of phenocryst mineral. Rhythmic cumulate layers in ultramafic intrusions are a result of uninterrupted slow cooling.
When a rock cools too quickly the liquid freezes into a solid glass, or crystallne groundmass. Often vapor loss from a magma chamber will cause a porphyritic texture.
Embayments or 'corroded' margins to phenocrysts infer that they were being resorbed by the magma and may imply addition of fresh, hotter magma. Ostwald ripening is also used to explain some porphyritic igneous textures, especially orthoclase megacrystic granites.

Phenocryst Shape: Implications
A crystal growing in a magma adopts a habit (see crystallography) which best reflects its environment and cooling rate. The usual phenocryst habit is the ones commonly observed. This may imply a 'normal' cooling rate.
Abnormal cooling rates occur in supercooled magmas, particularly komatiite lavas. Here, low nucleation rates due to superfluidity prevent nucleation until the liquid is well below the mineral growth curve. Growth then occurs at extreme rates, favoring slender, long crystals. Additionally, at crystal vertices and terminations, spikes and skeletal shapes may form because nucleation favors crystal edges. Spinifex or dendritic texture is an example of this result. Hence, the shape of phenocrysts can provide valuable imformation on cooling rate and initial magma temperature.

Spherulites
Spherulitic texture is the result of cooling and nucleation of material in a magma which has achieved supersaturation in the crystal component. Thus it is often a subsolidus process in supercooler felsic rocks. Often, two minerals will grow together in the spherulite. Axiolitic texture results from spherulitic growth along fractures in volcanic glass, often from invasion of water.

Graphic Textures
Graphic textures are formed by intergrowth of two minerals, typically quartz and feldsar or orthoclase feldspar with albite feldspar, formed by exsolution crystallization at or below the solidus. These textures are typical of subsolidus granites, but can also be formed in ultramafic minerals such as ^*]-ilmenite exsolution growths.

References

Vernon, Ron H., 2004, A Practical Guide to Rock Microstructure, Oxford University Press, Oxford. ISBN 0521891337

Petrology | Structural geology