Rock microstructure
Encyclopedia
Rock microstructure includes the texture
of a rock
and the small scale rock structures. 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.
Textures are penetrative fabric
s 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.
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 microscopic analogs of larger sedimentary structural features such as cross-bedding
, syn-sedimentary faults, sediment slumping, cross-stratification, etc.
or greywacke
.
. The more rounded the clasts, the more water-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.
.
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.
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 crenulation
s. The relationship of porphyroblast
s 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 mylonite
s and other highly disturbed and deformed rocks.
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 porphyroblast
s is diagnostic of when the foliation formed, and the P-T conditions which existed at that time.
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
microstructure may complement descriptions on the hand specimen and outcrop scale. This is especially vital for describing phenocryst
s and fragmental textures of tuff
s, 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
lava
s and ultramafic intrusive rocks.
), magma composition and its relationships to what minerals will nucleate, as well as physical effects of wall rocks, contamination and especially vapor.
Rocks composed entirely of euhedral crystals are termed panidiomorphic, and rocks composed entirely of subhedral crystals are termed subidiomorphic.
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.
Ostwald ripening
is also used to explain some porphyritic igneous textures, especially orthoclase
megacryst
ic granites.
) 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 information on cooling rate and initial magma temperature.
Graphic, micrographic texture
, and granophyric textures are examples of intergrowths formed during magmatic crystallization. They are angular intergrowths of quartz and alkali feldspar
. When well-developed, the intergrowths may resemble ancient cuneiform writing, hence the name. 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.
Intergrowths that form by exsolution are aids in interpreting cooling histories of rocks. Perthite
is an intergrowth of K-feldspar
with albite
feldspar, formed by exsolution from an alkali feldspar
of intermediate composition: the coarseness of perthitic intergrowths is related to cooling rate. Perthite is typical of many granite
s. 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 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 plate tectonics
. These oxides commonly have complex textures related both to exsolution and oxidation. For instance, ulvospinel
in igneous rocks such as basalt
and gabbro
commonly oxidizes during subsolidus cooling to produce regular intergrowths of magnetite
and ilmenite
. The process can determine what magnetic record is inherited by the rock.
Texture (crystalline)
In materials science, texture is the distribution of crystallographic orientations of a polycrystalline sample. A sample in which these orientations are fully random is said to have no texture. If the crystallographic orientations are not random, but have some preferred orientation, then the...
of a rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
and the small scale rock structures. 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.
Textures are penetrative fabric
Fabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
s 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
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures
Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
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 microstructures
Description of sedimentary rockSedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
microstructure aims to provide information on the conditions of deposition of the sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
, 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 cross-bedding
Cross-bedding
In geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...
, 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 arkoseArkose
Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose....
or greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
.
Fragment shape
Fragment shape gives information on the length of sediment transportSediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...
. The more rounded the clasts, the more water-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 sectionThin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
.
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 diagenesisDiagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
results in a weak bedding-plane foliation
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
. Other effects can include flattening of grains, pressure dissolution
Pressure solution
In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress...
and sub-grain deformation. Mineralogical changes may include zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
or other authigenic
Authigenic
An authigenic mineral or sedimentary rock deposit is one that was generated where it is found or observed. Authigenic sedimentary minerals form during sedimentation by precipitation or recrystallization instead of being transported from elsewhere by water or wind. Authigenic sediments are the main...
minerals forming in low-grade metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
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 rockMetamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
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
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
and overprinting of foliations causing crenulation
Crenulation
Crenulation or Crenulation cleavage is a texture formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions resulting in superimposed foliations.-Formation:...
s. The relationship of porphyroblast
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s to the foliations and to other porphyroblasts can provide information on the order of formation of metamorphic assemblages or facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
of minerals.
Shear textures are particularly suited to analysis by microstructural investigations, especially in mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
s and other highly disturbed and deformed rocks.
Foliations and crenulations
On the thin section and hand specimen scale a metamorphic rock may manifest a linear penetrative fabric called a foliationFoliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
or a cleavage
Cleavage (geology)
This article is about rock cleavage, for cleavage in minerals see Cleavage Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the...
. Several foliations may be present in a rock, giving rise to a crenulation
Crenulation
Crenulation or Crenulation cleavage is a texture formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions resulting in superimposed foliations.-Formation:...
.
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 porphyroblast
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s 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 schistosity planes are parallel with the shear direction and are generally defined by micaMica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
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 porphyroblastPorphyroblastA porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s
Igneous microstructure
Analysis of igneous rockIgneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
microstructure may complement descriptions on the hand specimen and outcrop scale. This is especially vital for describing phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s and fragmental textures of tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s, as often relationships between magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
and phenocryst morphology are critical for analysing cooling, fractional crystallization
Fractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
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
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
s and ultramafic intrusive rocks.
General principles of igneous microstructure
Igneous microstructure is a combination of cooling rate, nucleation rate, eruption (if a lavaLava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
), 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- pegmatiticPegmatiteA pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
: very large crystals - phaneriticPhaneriticPhaneritic is a term usually used to refer to igneous rock grain size. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic . This texture forms by slow cooling of magma deep underground in the plutonic environment...
: 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
- porphyriticPorphyriticPorphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
: containing phenocrystPhenocrystthumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s in a fine groundmassMatrix (geology)The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of... - vesicularVesicular textureVesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
: contains voids caused by trapped gas while cooling - vitreousGlassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
: glassGlassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
y or hyalineHyalineThe term hyaline denotes a substance with a glass-like appearance.-Histopathology:In histopathological medical usage, a hyaline substance appears glassy and pink after being stained with haematoxylin and eosin — usually it is an acellular, proteinaceous material...
without crystals - pyroclastic: rock contain fragments of crystals, phenocrysts and rock fragments
- equigranularEquigranularAn equigranular material is composed chiefly of crystals of similar orders of magnitude to one another. Basalt and gabbro commonly exhibit an equigranular texture....
: 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:- EuhedralEuhedralEuhedral crystals are those that are well-formed with sharp, easily recognised faces. Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma...
or automorphic, if the crystallographic shape is preserved. - Subeuhedral or Subhedral, if only part is preserved.
- Anhedral or xenomorphicXenomorph (geology)A xenomorph is a mineral that did not develop its otherwise typical external crystal form because of late crystallization between earlier formed crystals. Xenomorphs are typical of matrix minerals in rapidly crystallizing volcanic lavas and shallow igneous intrusions...
, 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.
Ostwald ripening
Ostwald ripening
right|thumb|300px|Basic schematic of the Ostwald ripening process Ostwald ripening is an observed phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time...
is also used to explain some porphyritic igneous textures, especially orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
megacryst
Megacryst
In geology, a megacryst is a crystal or grain that is considerably larger than the encircling matrix. They are found in igneous and metamorphic rocks.- Notes :*...
ic granites.
Phenocryst shape: implications
A crystal growing in a magma adopts a habit (see crystallographyCrystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
) 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
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
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 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.Graphic, micrographic texture
Micrographic texture
In petrology, micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of crystallization in some igneous rocks which contain high or moderately high percentages of silica...
, and granophyric textures are examples of intergrowths formed during magmatic crystallization. They are angular intergrowths of quartz and alkali feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. When well-developed, the intergrowths may resemble ancient cuneiform writing, hence the name. These intergrowths are typical of pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
and granophyre
Granophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
, 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. Perthite
Perthite
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...
is an intergrowth of K-feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
with albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
feldspar, formed by exsolution from an alkali feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
of intermediate composition: the coarseness of perthitic intergrowths is related to cooling rate. Perthite is typical of many granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
s. Myrmekite
Myrmekite
Myrmekite describes a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated...
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 silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
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 plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. These oxides commonly have complex textures related both to exsolution and oxidation. For instance, ulvospinel
Ulvöspinel
Ulvöspinel or ulvite is an iron titanium oxide mineral with formula: Fe2TiO4 or TiFe2+2O4. It forms brown to black metallic isometric crystals with a Mohs hardness of 5.5 to 6...
in igneous rocks such as basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
and gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
commonly oxidizes during subsolidus cooling to produce regular intergrowths of magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
and ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
. The process can determine what magnetic record is inherited by the rock.
See also
- List of rock textures
- List of rock types
- MetamorphismMetamorphismMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
- Structural geologyStructural geologyStructural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
- SedimentologySedimentologySedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...
- PetrologyPetrologyPetrology is the branch of geology that studies rocks, and the conditions in which rocks form....
- BoudinageBoudinagethumb|Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as a bed of sandstone, is stretched and deformed amidst less competent surroundings...