View source for Earth:Texture (geology)

In [[Earth:Geology|geology]], '''texture''' or '''rock microstructure'''<ref>{{cite book |last=Vernon |first=R. H. |year=2004 |title=A practical guide to rock microstructure |publisher={{wipe|Cambridge University Press}} |isbn=0-521-81443-X}}</ref> refers to the relationship between the materials of which a [[Earth:Rock (geology)|rock]] is composed.<ref name=TGR/> 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).<ref name=TGR>Texture & Genesis of Rocks, Introductory Geology Laboratory, Christopher DiLeonardo, Ph.D., Marek Cichanski, Ph.D., Earth & Space Sciences, De Anza College</ref> The geometric aspects and relations amongst the component particles or crystals are referred to as the ''[[Physics:Texture (crystalline)|crystallographic texture]]'' or ''preferred orientation''. Textures can be quantified in many ways.<ref>{{cite book |last=Higgins |first=M. D. |year=2006 |title=Quantitative Textural Measurements in Igneous and Metamorphic Petrology |publisher={{wipe|Cambridge University Press}} |location=Cambridge |isbn=0-521-13515-X}}</ref> The most common{{citation needed|date=April 2013}} 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 [[Earth:Fabric (geology)|fabrics]]'' of rocks; they occur throughout the entirety of the rock mass on microscopic, hand-sized specimen, and often [[Earth:Outcrop|outcrop]] scales. This is similar in many ways to [[Earth:Foliation (geology)|foliations]], except a texture does not necessarily carry structural information in terms of deformation events and orientation information. [[Earth:Structural geology|Structures]] occur on a hand-sized specimen scale and above.

Microstructure analysis <ref>{{cite journal |last1=Fu |first1=Jinlong |last2=Thomas |first2=Hywel R. |last3=Li |first3=Chenfeng |title=Tortuosity of porous media: Image analysis and physical simulation |journal=Earth-Science Reviews |date=January 2021 |volume=212 |pages=103439 |doi=10.1016/j.earscirev.2020.103439|bibcode=2021ESRv..21203439F |s2cid=229386129 |url=https://cronfa.swan.ac.uk/Record/cronfa55808/Download/55808__18817__4aeefe32b0ee4ae7993bff0531362902.pdf }}</ref> describes the textural features of the rock, and can provide information on the conditions of formation, [[Earth:Petrogenesis|petrogenesis]], and subsequent deformation, folding, or alteration events.<ref>{{Cite journal |last1=Voznesensky |first1=A. S. |last2=Kidima-Mbombi |first2=L. K. |date=2021-07-14 |title=Formation of synthetic structures and textures of rocks when simulating in COMSOL Multiphysics |url=https://mst.misis.ru/jour/article/view/271 |journal=Gornye Nauki i Tekhnologii = Mining Science and Technology (Russia) |volume=6 |issue=2 |pages=65–72 |doi=10.17073/2500-0632-2021-2-65-72 |s2cid=237797248 |issn=2500-0632|doi-access=free }}</ref>

Crystalline textures include phaneritic, [[Earth:Foliation (geology)|foliated]], and [[Earth:Porphyritic|porphyritic]].<ref name=TGR/> 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.<ref name=TGR/> Porphyritic texture is one in which larger pieces (phenocrysts) are embedded in a background mass made of much finer grains.<ref name=TGR/>

Fragmental textures include clastic, bioclastic, and [[Earth:Pyroclastic rock|pyroclastic]].<ref name=TGR/>

A preferred mineral orientation, is the texture of [[Earth:Metamorphic rock|metamorphic rock]] in which its grains have a flattened shape (inequant), and their planes tend to be oriented in the same direction.<ref name=EG>Essentials of Geology, 3rd Ed, Stephen Marshak</ref>

[[File:Mica-fish.jpg|thumb|Texture in a thin section of mylonitic quartzite from the Alps, Italy]]
[[File:Basalto tholeitico nicol paralleli.jpg|thumb|Texture in a thin section of tholeiitic basalt]]
  
==Nomenclature==
'''Rock microstructure''' includes the texture and small-scale structures of a [[Earth:Rock (geology)|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 [[Earth:Sedimentary rock|sedimentary rock]] microstructure aims to provide information on the conditions of deposition of the [[Earth:Sediment|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 microscopic analogs of larger sedimentary structural features such as [[Earth:Cross-bedding|cross-bedding]], syn-sedimentary faults, sediment slumping, cross-stratification, etc.

===Maturity===
The maturity of a sediment is related not only to the sorting (mean [[Chemistry:Particle size|grain size]] and deviations), but also to the fragment sphericity, rounding and composition. Quartz-only sands are more mature than [[Earth:Arkose|arkose]] or [[Earth:Greywacke|greywacke]].

===Fragment shape===
Fragment shape gives information on the length of [[Physics:Sediment transport|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 [[Earth:Thin section|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 [[Earth:Diagenesis|diagenesis]] results in a weak bedding-plane [[Earth:Foliation (geology)|foliation]]. Other effects can include flattening of grains, [[Earth:Pressure solution|pressure dissolution]] and sub-grain deformation. Mineralogical changes may include [[Chemistry:Zeolite|zeolite]] or other authigenic minerals forming in low-grade [[Earth:Metamorphic rock|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.<ref>{{cite web | url = http://www.tulane.edu/~sanelson/geol111/sedrx.htm | title = Sedimentary Rocks | last = Nelson | first = Stephen A. | website = Tulane University - Earth & Environmental Sciences | url-status = dead | access-date = 8 April 2021 | archive-url = https://web.archive.org/web/20120703000049/http://www.tulane.edu/~sanelson/geol111/sedrx.htm | archive-date = 3 July 2012}}</ref>  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 [[Earth:Metamorphic rock|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 [[Earth:Foliation (geology)|foliation]] and overprinting of foliations causing [[Earth:Crenulation|crenulation]]s. The relationship of [[Earth:Porphyroblast|porphyroblast]]s to the foliations and to other porphyroblasts can provide information on the order of formation of metamorphic assemblages or [[Earth:Facies|facies]] of minerals.

[[Earth:Shear (geology)|Shear]] textures are particularly suited to analysis by microstructural investigations, especially in [[Earth:Mylonite|mylonite]]s and other highly disturbed and deformed rocks.

===Foliations and crenulations===
{{main|Earth:Foliation (geology)}} 
On the thin section and hand-sized specimen scale a metamorphic rock may manifest a planar penetrative fabric called a [[Earth:Foliation (geology)|foliation]] or a [[Earth:Cleavage (geology)|cleavage]]. Several foliations may be present in a rock, giving rise to a [[Earth:Crenulation|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 [[Earth:Porphyroblast|porphyroblast]]s is diagnostic of when the foliation formed, and the P-T conditions which existed at that time.

[[File:Tectonites.png|thumb|Flinn Diagram showing degree of stretching, or lineation (L) versus flattening, or foliation (S)]]

===Lineations===
{{main|Earth:Lineation (geology)}}
Linear structures in a rock may arise from the intersection of two foliations or planar structures, such as a sedimentary bedding plane and a tectonically induced cleavage plane. The degree of lineation compared with the degree of foliation for certain strain markers in deformed rocks are commonly plotted on a Flinn diagram.

===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 ''schistosity'' planes are parallel with the shear direction and are generally defined by [[Chemistry:Mica|mica]]s 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 [[Earth:Porphyroblast|porphyroblast]]s

==Igneous microstructure==
Analysis of [[Earth:Igneous rock|igneous rock]] microstructure may complement descriptions on the hand-sized specimen and outcrop scale. This is especially vital for describing [[Earth:Phenocryst|phenocryst]]s and fragmental textures of [[Earth:Tuff|tuff]]s, as often relationships between [[Biology:Magma|magma]] and phenocryst morphology are critical for analysing cooling, fractional crystallization and emplacement.

Analysis of [[Earth:Intrusive rock|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 [[Earth:Komatiite|komatiite]] [[Earth:Lava|lava]]s and ultramafic intrusive rocks.

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

===Grain texture===
According to the texture of the grains, igneous rocks may be classified as 
* [[Earth:Pegmatite|pegmatitic]]: very large crystals
* phaneritic: rocks contain minerals with crystals visible to the unaided eye, commonly intrusive
* aphanitic: rapid cooling, crystal nucleation and growth is stunted, forming a uniform, fine grained rock
* [[Earth:Porphyritic|porphyritic]]: containing [[Earth:Phenocryst|phenocryst]]s in a fine [[Earth:Matrix (geology)|groundmass]]
* [[Earth:Vesicular texture|vesicular]]: contains voids caused by trapped gas while cooling
* [[Chemistry:Glass|vitreous]]: [[Chemistry:Glass|glass]]y or [[Biology:Hyaline|hyaline]] without crystals
* [[Earth:Pyroclastic rock|pyroclastic]]: rock formed of fragments of crystals, phenocrysts and rock fragments of a volcanic origin
* [[Earth:Equigranular|equigranular]]: rock crystals are all the same size

===Crystal shapes===
Crystal shape is also an important factor in the texture of an igneous rock. Crystals may be euhedral, subeuhedral or anhedral:
* ''Euhedral'' or ''automorphic'', if the crystallographic shape is preserved.
* ''Subeuhedral'' or ''Subhedral'', if only part is preserved.
* ''Anhedral'' or [[Earth:Xenomorph (geology)|xenomorphic]], if the crystals present no recognizable crystallographic forms.

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 crystalline 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.
[[Chemistry:Ostwald ripening|Ostwald ripening]] is also used to explain some porphyritic igneous textures, especially [[Chemistry:Orthoclase|orthoclase]] [[Earth:Megacryst|megacryst]]ic granites.

===Phenocryst shape: implications===
A crystal growing in a magma adopts a habit (see [[Physics:Crystallography|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 [[Earth:Komatiite|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 growth is favoured at crystal edges. Spinifex or dendritic texture is an example of this result. Hence, the shape of phenocrysts can provide valuable information 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 and other intergrowth textures===
Intergrowths of two or more minerals can form in a variety of ways, and interpretations of the intergrowths can be critical in understanding both magmatic and cooling histories of igneous rocks. A few of the many important textures are presented here as examples.

[[File:SKRIFTGRANITT 03 Einerkilen-Evje.jpg|thumb|Graphic granite from Norway]] 
Graphic, micrographic, and granophyric textures are examples of intergrowths formed during magmatic crystallization. They are angular intergrowths of quartz and [[Chemistry:Feldspar|alkali feldspar]]. When well-developed, the intergrowths may resemble ancient cuneiform writing, hence the name. These intergrowths are typical of [[Earth:Pegmatite|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.

Intergrowths that form by exsolution are aids in interpreting cooling histories of rocks. [[Earth:Perthite|Perthite]] is an intergrowth of [[Chemistry:Feldspar|K-feldspar]] with [[Chemistry:Albite|albite]] feldspar, formed by exsolution from an [[Chemistry:Feldspar|alkali feldspar]] of intermediate composition: the coarseness of perthitic intergrowths is related to cooling rate. Perthite is typical of many [[Chemistry:Granite|granite]]s. [[Earth:Myrmekite|Myrmekite]] is a microscopic, vermicular (worm-like) intergrowth of quartz and sodium-rich plagioclase common in granite; myrmekite may form as alkali feldspar breaks down by exsolution and [[Chemistry:Silicon|silicon]] is transported by fluids in cooling rocks.

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 [[Earth:Plate tectonics|plate tectonics]]. These oxides commonly have complex textures related both to exsolution and oxidation. For instance, ulvospinel in igneous rocks such as [[Earth:Basalt|basalt]] and gabbro commonly oxidizes during subsolidus cooling to produce regular intergrowths of [[Chemistry:Magnetite|magnetite]] and [[Chemistry:Ilmenite|ilmenite]]. The process can determine what magnetic record is inherited by the rock.

==See also==
* {{annotated link|Earth:List of rock textures}}
* {{annotated link|Earth:List of rock types}}
* {{annotated link|Earth:Metamorphism}}
* {{annotated link|Earth:Structural geology}}
* {{annotated link|Earth:Sedimentology}}
* {{annotated link|Earth:Petrology}}
* {{annotated link|Earth:Boudinage}}
* {{annotated link|Physics:Texture (crystalline)}}
* {{annotated link|Earth:Vesicular texture}}
* {{annotated link|Earth:Soil texture}}

==References==
{{Reflist}}

==Further reading==
* Vernon, Ron H., 2004, ''A Practical Guide to Rock Microstructure'', Oxford University Press, Oxford. {{ISBN|0-521-89133-7}}

[[Category:Petrology]]

{{Sourceattribution|Texture (geology)}}