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.
Textures are ''penetrative
fabrics'' of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand-specimen, and often
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 a 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 rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
microstructure aims to provide information on the conditions of deposition of the
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
, 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.
Maturity
The maturity of a sediment is related not only to the sorting (mean
grain size
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...
and deviations), but also to the fragment sphericity, rounding and composition. Quartz-only sands are more mature than
arkose
Arkose () or arkosic sandstone 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.
Quartz is c ...
or
greywacke.
Fragment shape
Fragment shape gives information on the length of
sediment transport
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural system ...
. 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
Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These p ...
or other
authigenic
Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary minerals form during sedimentation by precipitation or rec ...
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
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
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
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
of minerals.
Shear textures are particularly suited to analysis by microstructural investigations, especially in
mylonites and other highly disturbed and deformed rocks.
Foliations and crenulations
On the thin section and hand specimen scale a metamorphic rock may manifest a planar 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
porphyroblasts is diagnostic of when the foliation formed, and the P-T conditions which existed at that time.
Lineations
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
mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
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
porphyroblasts
Igneous microstructure
Analysis of
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
microstructure may complement descriptions on the hand specimen and outcrop scale. This is especially vital for describing
phenocrysts and fragmental textures of
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
s, as often relationships between
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...
and phenocryst morphology are critical for analysing cooling,
fractional crystallization and emplacement.
Analysis of
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...
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 () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite w ...
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...
s and
ultramafic
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
intrusive rocks.
General principles of igneous microstructure
Igneous microstructure is a combination of cooling rate, nucleation rate, eruption (if a
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...
), 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
*
pegmatitic: very large crystals
*
phaneritic: rocks contain minerals with crystals visible to the unaided eye, commonly intrusive
*
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This ge ...
: rapid cooling, crystal nucleation and growth is stunted, forming a uniform, fine grained rock
*
porphyritic
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning a ...
: containing
phenocrysts in a fine
groundmass
The matrix or groundmass of a 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, ...
*
vesicular: contains voids caused by trapped gas while cooling
*
vitreous
Vitreous may refer to:
Materials
* Glass, an amorphous solid material
** Vitreous enamel, a material made by fusing powdered glass to a substrate by firing
* Vitreous lustre, a glassy luster or sheen on a mineral surface
Biology
* Vitreous bod ...
:
glass
Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
y or
hyaline
A hyaline substance is one with a glassy appearance. The word is derived from el, ὑάλινος, translit=hyálinos, lit=transparent, and el, ὕαλος, translit=hýalos, lit=crystal, glass, label=none.
Histopathology
Hyaline cartilage is ...
without crystals
*
pyroclastic
Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
: rock formed of fragments of crystals, phenocrysts and rock fragments of a volcanic origin
*
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
Euhedral crystals (also known as idiomorphic or automorphic crystals) are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral (also known as '' xenomorphic'' or ''allotriomorphic''): a rock with an anhedr ...
'' or ''automorphic'', if the crystallographic shape is preserved.
* ''
Subeuhedral'' or ''
Subhedral
Euhedral crystals (also known as idiomorphic or automorphic crystals) are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral (also known as '' xenomorphic'' or ''allotriomorphic''): a rock with an anhedra ...
'', if only part is preserved.
* ''
Anhedral'' or
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
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
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 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
Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics ( condensed matter physics). The wor ...
) 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
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crysta ...
magmas, particularly
komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite w ...
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
Dendrite derives from the Greek word "dendron" meaning ( "tree-like"), and may refer to:
Biology
*Dendrite, a branched projection of a neuron
* Dendrite (non-neuronal), branching projections of certain skin cells and immune cells
Physical
*Dendr ...
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
In petrology, spherulites () are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone, and rhyolite as globules about the size of millet seed or rice grain, with a ...
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, and granophyric textures are examples of intergrowths formed during magmatic crystallization. They are angular intergrowths of quartz and
alkali feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...
. When well-developed, the intergrowths may resemble ancient cuneiform writing, hence the name. These intergrowths are typical of
pegmatite
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silici ...
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
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, ...
is an intergrowth of
K-feldspar
Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
with
albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate ...
feldspar, formed by
exsolution from an
alkali feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...
of intermediate composition: the coarseness of perthitic intergrowths is related to cooling rate. Perthite is typical of many
granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...
s.
Myrmekite
Myrmekite is 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. The ...
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 is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
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 (from the la, label= Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of larg ...
. These oxides commonly have complex textures related both to exsolution and oxidation. For instance,
ulvospinel in igneous rocks such as
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
and
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...
commonly oxidizes during subsolidus cooling to produce regular intergrowths of
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...
and
ilmenite. The process can determine what magnetic record is inherited by the rock.
See also
*
*
*
*
*
*
*
References
* Vernon, Ron H., 2004, ''A Practical Guide to Rock Microstructure'', Oxford University Press, Oxford.
{{Structural geology
Petrology
Structural geology