- Glossary of Geologic Terms - National Park Service
- A Glossary of Rock and Mineral Terminology - CA Department of Conservation
- Geoglossary - University of Canterbury, New Zealand
- Glossary of geology - Wikipedia
- Illustrated Glossary of Geology - Siim Sepp as part of his Sand Atlas website
Glossary of Terms for this Website
- allochemical metamorphism - n. A type of Metamorphism characterized by a pronounced change of the original bulk composition by mass transfer processes, either addition or subtraction of material or both processes (IUGS Metamorphic Rocks, 2011)
- allogenic - adj. Said of a substance or mineral that has been introduced into a system from the outside, for example injected material in a migmatite. (IUGS Metamorphic Rocks, 2011)
- aphanitic - adj. Said of a rock whose mineral grains are not visible with the unaided eye (< 0.1mm).
- armoured relic - n. Relic mineral(s) surrounded by a rim of alteration products and protected from further reaction by the rim. (IUGS Metamorphic Rocks, 2011)
- assemblage - n. A collection of phases, typically minerals, that occur together in a rock. The term is applied when the collection of minerals is believed to have achieved chemical equilibrium. (IUGS Metamorphic Rocks, 2011)
- atoll structure - n. A type of structure characterized by a ring or shell of one mineral around a core of another mineral or other minerals resembling an atoll. (IUGS Metamorphic Rocks, 2011)
- atom units - n. A unit of quantity that may be used to describe the chemical composition of a system in terms of chemical components. Atom units, like mole units, have meaning only if a chemical composition is specified. The atom unit value of a component is the mole unit value times the number of moles of atoms in the component. Atom percentage of a component is the percentage of the total number of atoms (or moles of atoms) in a chemical system that are counted in one component. For example, in a two-component system with equal numbers of moles of the components Fe3O4 and SiO2, 3 of the moles of atoms are in SiO2 and 7 of the moles of atoms are in Fe3O4. Therefore, the system is 30 atom percent SiO2 and 70 atom percent Fe3O4. Atoms are conserved in chemical reactions, so atom units are conservative units.
- authigenesis - n. Reactions between the constituents of a sediment or sedimentary rock, that form new minerals or result in the enlargement of existing minerals. (IUGS Metamorphic Rocks, 2011)
- axial plane foliation - n. A type of foliation developed parallel to the axial plane of a fold. (IUGS Metamorphic Rocks, 2011)
- azeotropic point - n. In chemistry, an azeotropic point on a phase diagram gives the temperature and composition of a boiling liquid where the vapor produced by boiling has the same chemical composition as the boiling liquid. The temperature will be either a maximum or a minimum in the boiling temperature. In materials science and geology, the term azeotropic point also refers to a point a phase diagram where the composition of a melting solid is identical to the chemical composition of the melt produced.
- baked rock - n. A rock slightly affected by heat at the contact with a lava flow, a dike or burning combustibles. The alteration includes bleaching of carbonaceous rocks, reddening of iron-rich rocks, elimination of water and other volatile constituents, induration effects and a small degree of fritting. (IUGS Metamorphic Rocks, 2011)
- Barrovian-type facies series - n. A type of facies series characterized by the progressive development of garnet and kyanite from lower to higher grades; andalusite, cordierite, and glaucophane are absent. The facies represent medium P/T conditions and are common in regional metamorphism. (IUGS Metamorphic Rocks, 2011)
- bathograd - n. A mappable line, based on an invariant model reaction, that separates occurrences of higher-P assemblage from occurrences of lower-P assemblage. Bathograds do not need to be isochronous on the regional scale. (IUGS Metamorphic Rocks, 2011)
- bimineralic - adj. Said of a rock in which >95% of the modal content is composed of two minerals as opposed to polymineralic, monomineralic, or trimineralic. (IUGS Metamorphic Rocks, 2011)
- binary - adj. Said of a chemical system whose composition can be described by two chemical components.
- boudin - n. A fragment of a disrupted competent bed, barrel-shaped in cross-section and separated from the next fragment either by a commonly fibrous in-filling, consisting of quartz or calcite ± other minerals, or by surrounding incompetant layers which have been pinched in. it results from the failure of the competent bed under compressional stress, whereas the incompetant layer yields by shearing and/or flowing. (IUGS Metamorphic Rocks, 2011)
- boudinage - n. Fragmentation of a competent rock layer into boudins. (IUGS Metamorphic Rocks, 2011)
- boundary metasomatism - n. A type of metasomatism that occurs at the contact between two rock types. (IUGS Metamorphic Rocks, 2011)
- bulk composition - n. The chemical composition of a defined chemical system, such as a rock or a magma. Bulk composition is generally used when the system (e.g. a rock) is made of several phases (minerals), each of which may have a chemical composition different from the chemical composition of the whole system.
- burial metamorphism - n. A type of metamorphism, mostly of regional extent, which affects rocks deeply buried under a sedimentary-volcanic pile, and is typically not associated with deformation or magmatism. The resultant rocks are partially or completely recrystallized and generally lack schistosity. It commonly involves from very low to medium metamorphic temperatures and low to medium P/T ratios. (IUGS Metamorphic Rocks, 2011)
- chaotectic - n. A point giving the temperature and chemical composition of the last liquid to remain upon equilibrium vaporization of a liquid in the presence of a saturating mineral. A binary chaotectic is found at the intersection of a steam saturation curve and a mineral saturation curve in a binary system. Cf. eutectic, peritectic
- chemical component - n. A specific chemical composition.
- cleavage - n. The property of a rock to split along a regular set of parallel or subparallel closely-spaced surfaces. (IUGS Metamorphic Rocks, 2011)
- color index (color ratio) - n. "The volume percentage or ratio of dark-colored or mafic minerals to light-colored or felsic minerals in a rock (M). In terms of the QAPF terminology the color index is denoted by the parameter M'." "The color index M' is defined as M minus any muscovite, apatite, primary carbonate, etc. as ...[these minerals] are considered to be colorless minerals for the purpose of the color index." (Le Maitre et al, 2005, p.69)
- combustion metamorphism - n. A type of metamorphism of local extent produced by the spontaneous combustion of naturally occurring substances such as bituminous rocks, coal, or oil. The very high temperatures reached during the combustion may either fuse the rocks or convert them into holocrystille aggregates of high-temperature metamorphic minerals. The residual products of coal combustion may give rise to local ash deposits (coal-fire ash). (IUGS Metamorphic Rocks, 2011)
- congruent melting - n. A type of melting of a solid phase in which the liquid produced by melting has the same chemical componsition as the solid. This type of melting is in contract with incongruent melting in which the liquid produced by melting has a different chemical componsition as the solid.
- contact areole - n. A zone around a magma body, plutonic, near surface, or volcanic, in which contact metamorphism due to that body can be recognized. The thickness ranges from centimeters to kilometers depending on the dimensions and composition of the magma body, the physio-chemical properties of the surrounding rocks and the depth of the magma emplacement. The metamorphic effects are more pronounced close to the igneous mass, i.e. the inner aureole, and tend to diminish away to the outer aureole. Many contact aureoles can be subdivided into metamorphic zones; anomalous zonal patterns may reveal complications due to metasomatizing fluids. (IUGS Metamorphic Rocks, 2011)
- contact metamorphism - n. A type of metamorphism of local extent that affects the country rocks around magma bodies emplaced in a variety of environments from volcanic to upper mantle depths, in both continental and oceanic settings. It is essentially caused by the heat transfer from the intruded magma body into the country rocks. The range of metamorphic temperatures might be wide. It may or may not be accompanied by significant deformation depending on the dynamics of the intrusion. The zone where contact metamorphism occurs is called the contact aureole, whereas the products of such metamorphism are called the contact rocks. (IUGS Metamorphic Rocks, 2011)
- Contact Metasomatism - n. A type of metasomatism that occurs at or near the contact between a magmatic body and another rock. It may occur at various stages in the magmatic evolution. (IUGS Metamorphic Rocks, 2011)
- continuous reaction - n. A metamorphic reaction that continuously achieves chemical equilibrium over a range of temperatures and pressures owing to compositional variability among the minerals, for example in the Fe/Mg value. (IUGS Metamorphic Rocks, 2011)
- corona - n. A zone or zones of minerals arranged concentrically around a core mineral. The minerals typically exhibit a radial arrangement. The zone or zones may be primary or result from secondary reactions. (IUGS Metamorphic Rocks, 2011)
- cotectic - n. A line on a ternary liquidus diagram that marks the intersection of of two saturation surfaces, giving compositions and temperatures of magmas that are saturated with two mineral phases.
- crenulation cleavage or crenulation schistosity - n. A type of spaced cleavage developed during crenulation of a preexisting foliation, and orientated parallel to the existing axial plane of the crenulation. Proposed as a non-genetic synonym for strain-slip cleavage. (IUGS Metamorphic Rocks, 2011)
- crystal - n. A solid with an atomic structure that is a 3-dimensional pattern based on an atomic scale unit cell.
- crystalloblast - n. A mineral grain or crystal that has grown under metamorphic processes as opposed to crystallization from a melt. (IUGS Metamorphic Rocks, 2011)
- dactylitic structure - n. A type of structure characterized by the symplectite intergrowth of two minerals, the one with finger-like projections into the other. (IUGS Metamorphic Rocks, 2011)
- decussate structure - n.A type of structure in a metamorphic rock, in which the constituent grains have a randomly oriented and interlocking arrangement. The structure is typical of contact-metamorphosed rocks. (IUGS Metamorphic Rocks, 2011)
- deformation lamellae / deformation bands - n. Planar defects in a crystal comprising bands of material with a slightly different refractive index (relief) from the host grain and resulting from damage to the crystal lattice or an array of submicroscopic inclusions. Includes kink bands. (IUGS Metamorphic Rocks, 2011)
- diablastic structure - n. A type of structure in a metamorphic rock in which two or more minerals are intricately intergrown. The intergrowth may take a variety of forms (interfingering, inclusions, etc.). (IUGS Metamorphic Rocks, 2011)
- diagenesis - n. All the chemical, mineral, physical and biological changes undergone by a sediment after its initial deposition, and during and after its lithification, exclusive of superficial alteration (weathering) and metamorphism. (IUGS Metamorphic Rocks, 2011)
- diffusion metasomatism - n. A type of metasomatism that takes place by the diffusion along grain boundries or through a grain boundary fluid. Diffusional metasomatic rocks form rather thinly zoned bodies (rims) along cracks, veins, and contact surfaces and the composition of minerals may vary gradually across each metamorphic zone.
- discontinuous reaction - n. A metamorphic reaction that, despite compositional variability among the minerals, is at equilibrium at a fixed pressure at a specific temperature. A paragenetic diagram will show a change in topology, such as a ‘tie-line flip’. (IUGS Metamorphic Rocks, 2011)
- disequilibrium assemblage - n. An association of minerals that are not in thermodynamic equilibrium. It is common in low-grade metamorphic rocks and in the high temperature sanidinite facies rocks, where adjustment to the existing conditions could not be reached owing to the low rate of chemical reactions. The existence of a great number of mineral phases, the presence of unstable relicts, and the mutual replacement of minerals are all indicative of disequilibrium assemblages. (IUGS Metamorphic Rocks, 2011)
- dislocation metamorphism - n. A type of metamorphism of local extent, associated with fault zones or shear zones. Grain-size reduction typically occurs in the rocks, and a range of rocks largely referred to as mylonites and cataclasites is formed. Replaces terms such as mechanical metamorphism, kinetic metamorphism, cataclastic metamorphism, dynamic metamorphism, dynamometamorphism. (IUGS Metamorphic Rocks, 2011)
- domain - n. A recognizable part of a metamorphic rock at the mesoscopic or microscopic scale that has a distinctive lithological, mineralogical or chemical composition. The term is commonly used for meta-igneous rocks and in and in particular for mafic metavolcanic rocks. (IUGS Metamorphic Rocks, 2011)
- dissolve - v. To become incorporated into a liquid. This process is the result of a chemical reaction between a solid and a liquid in which the solid becomes part of the liquid and the composition of the liquid changes due to the addition of the solid.
- eclogite facies - n. Metamorphic facies corresponding to middle to high grade and middle to high P/T values, and in bounded, in rocks of suitable composition, by the omphacite + garnet-in reactions. (IUGS Metamorphic Rocks, 2011)
- end member - n. A mineral composition that is a limit for the range of chemical compositions for that mineral. For example, NaAlSi3O8 is the most sodium rich alkali feldspar, and is called the albite end member.
- endomorphism - n. A term denoting all modifications of an igneous rock due to contact-metamorphic reactions with the country rock; the term comprises all reactions at the margins of the igneous rock including complex metamorphic and metasomatic reactions and the partial or complete assimilation of fragments of the country rock. (IUGS Metamorphic Rocks, 2011)
- endothermic - adj. Said of a chemical reaction or process that consumes heat. It is the opposide of exothermic.
- epitaxis, epitaxy or epitaxial growth - n. Recrystallization in which the lattice orientation of the new crystal has a systematic relationship to the crystal lattice of the parent crystal. (IUGS Metamorphic Rocks, 2011)
- epizone - n. A zone of low-grade metamorphic rocks characterized by illite Kübler index (KI) mean values less than 0.25°Δ2θ (CuKα). The term was originally proposed by Becke and by Grubenmann to indicate a shallow depth of metamorphism (including contact metamorphism for Grubenmann). (IUGS Metamorphic Rocks, 2011)
- equant - adj. Said of mineral grains that are equidimensional. (IUGS Metamorphic Rocks, 2011)
- equigranular - adj. Said of a rock in which mineral grains are all one size. (IUGS Metamorphic Rocks, 2011)
- euhedral - n. Said of a crystal that is wholly bounded by it own crystal faces. The term was originally used to describe igneous rocks, but is now more widely applied. The specific term for metamorphic rocks is idioblastic. Antonym: anhedral. (IUGS Metamorphic Rocks, 2011)
- eutectic point - n. A point giving the temperature and chemical composition of the first liquid formed upon equilibrium melting of a mixture of two or more stoichiometric phases (minerals). A binary eutectic is found at the intersection of two mineral saturation curves (liquidus curves) in a binary system. A ternary eutectic is found at the intersection of three mineral saturation surfaces in a ternary system. Cf. peritectic, chaotectic
- exothermic - adj. Said of a chemical reaction or process that produces heat. It is the opposide of exothermic.
- fabric - n. The relative orientation of parts of a rock mass. Hence, mega, meso, microfabric. This is commonly used to refer to the crystallographic and/or shape orientation of mineral grains or groups of grains, but can also be used on a larger scale. Preferred linear orientation of the parts is termed linear fabric, preferred planar orientation planar fabric and the lack of a preferred orientation is referred to as random fabric. (IUGS Metamorphic Rocks, 2011)
- facies series or metamorphic facies series - n. A sequence of metamorphic facies developed under a particular range of P/T values. First used by Vogt, then by Eskola, and particularly developed by Miyashiro who proposed, for regional metamorphism, a classification in three principal facies series that broadly equate to radial sectors on a P-T diagram. (IUGS Metamorphic Rocks, 2011)
- fascicular - adj. A term used to describe bundles or aggregates of acicular crystals. Hence fascicular structure, fascicular schist, in which the long axes of the crystals lie in a plane. -n. fasciculite (IUGS Metamorphic Rocks, 2011)
- fault - n. A fracture structure along which rocks have moved relative to each other. (IUGS Metamorphic Rocks, 2011)
- fault breccia - n. Medium to coarse grained cataclasite containing more than 30% visible fragments. Supersedes tectonic breccia, crush breccia, cataclasite breccia and friction breccia. (IUGS Metamorphic Rocks, 2011)
- fault gouge - n. Incohesive clay-rich, fine- to ultrafine-grained cataclasite, which may possess a schistosity and contains less than 30% visible fragments. Lithic clasts may be present. (IUGS Metamorphic Rocks, 2011)
- felsic minerals - n. A collective term for modal quartz, feldspar, and feldspathoids. Hence, felsic rock, a rock mainly consisting of felsic minerals, generally feldspars and quartz. (IUGS Metamorphic Rocks, 2011)
- flaser structure - n. A type of structure produced by dominantly mylonitic deformation and characterized by the presence of fissile zones or layers of highly sheared or finely crushed rocks separating lensoid relics of relatively unaltered rock. The resultant rock may be termed a flaser-gneiss although it is generally referred to the protolith (e.g. flaser gabbro). (IUGS Metamorphic Rocks, 2011)
- foliation - n. Any repetitively occurring or penetrative planar feature in a rock body. Examples include layering on a scale of a centimeter or less, and the referred planar orientation of inequant mineral grains or grain aggregates. The surfaces in which they are parallel are called s-surfaces. (IUGS Metamorphic Rocks, 2011)
- fractional crystallization - n. The process of separation of crystals from a magma during crystalliztion leading to a change of chemical composition of the magma and the rocks that crystallize from it.
- fracture - n. A general term for any break in a rock mass, whether or not it causes displacement. Fracture includes cracks, joints and faults. (IUGS Metamorphic Rocks, 2011)
- fracture cleavage - n. A regular set of closely spaced parallel or subparallel fractures along which the rock will preferentially split. The term was originally taken to include all types of cleavage characterized by the development of discrete parallel planes of weakness, that is, spaced cleavage and crenulated cleavage. (IUGS Metamorphic Rocks, 2011)
- geotherm - n. A mathematical function or graph of the calculated or measured variation of temperature with depth in the earth. A geotherm will depend on the crust type, thickness, and tectonic setting. Geotherms are most commonly reported for settings in the middle of tectonic plates, and are different for continental and oceanic crust.
- glass - n. A solid with a disordered arrangement of atoms, not a crystal.
- gneiss - n. A type of metamorphic rock that has a gneissose structure.
- gneissose structure - n. A type of structure characterized by a schistosity that is either poorly developed thoughout the rock or, if well developed, occurs in broadlly spaced zones, such that the rock will split on a scale of more than one centineter. Mineralogical or lithological layering is commonly present. (IUGS Metamorphic Rocks, 2011)
- gram formula weight (GFW) - n. The mass of one mole of a formula. The GFW is a sum of the atomic weights of the elements in the formula, with each element's atomic weight multiplied by the subscript of that element in the formula.
- granoblastic structure - n. A type of structure in a metamorphic rock in which the constituent grains are equidimensional (equant) and of equal size (homeoblastic) and have well-sutured or irregular boundaries. (IUGS Metamorphic Rocks, 2011)
- granofels - n. A type of metamorphic rock that has a granofelsic structure.
- granofelsic structure - n. A type of structure resulting from the absence of schistosity such that he mineral grains and aggregates of mineral grains are equant (for example quartz, feldspar, garnet and pyroxene), or if inequant have a random orientation. Mineralogical or lithological layering may be present." (IUGS Metamorphic Rocks, 2011)
- granulite facies - n. A type of metamorphic facies representing the highest grades of metamorphism; it forms at high temperatures and at medium and high P/T values, and lies between the eclogite and pyroxene-hornfels facies. It is characterized by clinopyroxene-orthopyroxene-plagioclase (olivine not stable with plagioclase or garnet) assemblages in rock of basaltic composition. (IUGS Metamorphic Rocks, 2011)
- greenschist facies - n. A type of metamorphic facies representing low grades of metamorphism. It forms at low to moderate temperatures and pressures and lies between the epidote-amphibolite and subgreenschist facies. It is characterized by actinolite-albite-epidote-chlorite assemblages in rocks of basaltic composition. (IUGS Metamorphic Rocks, 2011)
- greenstone belt - n. A regional-scale elongate zone, particularly in Precambrian shield areas, that is characterized by an abundance of basic metavolcanic rocks. (IUGS Metamorphic Rocks, 2011)
- haplogranite - n. A simplified granite made of the minerals alkali feldspar and quartz. This is a ternary system (KAlSi3O8-NaAlSi3O8-SiO2). It is a good model system for granite because granite, which is largely alkali feldspar and quartz. It's 1 bar liquidus surface is shown in this phase diagram.
- helicitic structure - n. A type of a structure in a metamorphic rock, in which the trails or bands of inclusions in porphyroblasts are characteristically curved or contorted and represent older fold structures. (IUGS Metamorphic Rocks, 2011)
- heteroblastic structure - n. A type of structure in a metamorphic rock in which the constituent grains are of two or more different sizes. Antonym: homeoblastic. (IUGS Metamorphic Rocks, 2011)
- holohyaline - adj. A textural term applied to igneous rocks that are entirely composed of glass. The terms hyaline, vitreous and glassy are broadly synonymous with holohyaline except they imply a finite crystalline component. Obsidian is a holohyaline rock. (NPS Building Stone Database)
- homeoblastic structure - n. A type of structure in a metamorphic rock in which the constituent grains are of two or more different sizes. Antonym: homeoblastic. (IUGS Metamorphic Rocks, 2011)
- hornfels - n. Hard, compact contact-metamorphosed rock of any grain size, dominantly composed of silicate + oxide minerals, with a horny aspect and a subchonchoidal to jagged fracture. (IUGS Metamorphic Rocks, 2011)
- hybridization - n. The process of ntermingling of magmas, or contamination of a magma by assimilation and digestion of the country of host rock. The latter meaning is applicable to some aspects of migmatite petrology. Hence, hybrid, the product of, or relating to, hybridization. (IUGS Metamorphic Rocks, 2011)
- hydrothermal metamorphism - n. A type of metamorphism of local extent caused by hot H2O-rich fluids. It is typically of local extent in that it may be related to a specific setting or cause (e.g. where an igneous intrusion mobilizes H2O in the surrounding rocks). However, in a setting where igneous intrusion is repetitive (e.g. in ocean floor spreading centers) the repetitive operation of circulating hot H2O fluid may give rise to regional effects as in some cases or ocean-floor metamorphism. Metasomatism is commonly associated with this type of metamorphism. (IUGS Metamorphic Rocks, 2011)
- hypidioblast - n. A crystal formed in a metamorphic rock that is only partly bounded by its own crystal faces. Hence, hypidioblastic structure, synonymous with subidioblastic structure. (IUGS Metamorphic Rocks, 2011)
- idioblast - n. A crystal formed in a metamorphic rock that is wholly bounded by its own crystal faces. Hence, idioblastic structure. (IUGS Metamorphic Rocks, 2011)
- igneous rock - n. A rock that has solidified from a magma. It may be intrusive (plutonic), solidifying within the earth, or extrusive (volcanic), solidifying on the earth's surface.
- impact metamorphism - n. A type of metamorphism of local extent caused by the passage of a shock wave due to the impact of a planetary body (projectile or impactor) on a planetary surface (target). It includes melting and vaporization of the target rock(s). (IUGS Metamorphic Rocks, 2011)
- inclusion - n. A solid or liquid phase wholly contained within a crystal. Hence inclusion trails, lines of inclusions, usually relics of a preexisting structure (for example poikiloblastic and helicitic structure). Inclusion trails in porphyroblasts may be referred to as internal foliation (Si), as opposed to the external foliation (Se) of the matrix; the geometry and relationships of the foliations may indicate the growth history of the porphyroblast. (IUGS Metamorphic Rocks, 2011)
- incongruent melting - n. A type of melting of a solid phase in which the liquid produced by melting has a different chemical componsition as the solid. This type of melting is in contract with congruent melting in which the liquid produced by melting has the same chemical componsition as the solid.
- index mineral - n. A mineral that is used to characterize a zone of a specific range of metamorphic grade in a progressive metamorphic region. The first appearance of the mineral (in passing from low to higher grades of metamorphism) marks the outer limit of the zone in question. (IUGS Metamorphic Rocks, 2011)
- inequant - adj. Said of mineral grains whose dimensions are significantly different from each other. (IUGS Metamorphic Rocks, 2011)
- isochemical metamorphism - n. A type of metamorphism in which the mineralogical and structural changes do not substantially modify the bulk rock composition. (IUGS Metamorphic Rocks, 2011)
- isograd - n. A surface across a rock sequence, represented by a line on a map, defined by the appearance or disappearance of a mineral, a specific mineral composition or a mineral association. Ideally, the changes marked by an isograd are produced as a result of a specific reaction, for example the ‘staurolite-in’ isograd defined by the reaction: garnet + chlorite + muscovite = staurolite + biotite + quartz + H2O. Isograds represent mineral reactions, not changes in rock chemical compositions. (IUGS Metamorphic Rocks, 2011)
- IUGS - acronym. IUGS is an acronym for International Union of Geologcal Sciences (IUGS website).
- joint - n. A fracture or parting surface in a rock along which there has been little to no displacement. Joints are normally planar and occur in parallel sets (joint set); more than one set may be present in a rock body. Hence jointing, the presence of joints. Joints may be infilled during or after their formation. (IUGS Metamorphic Rocks, 2011)
- lepidoblastic structure - n. A type of structure in a metamorphic rock in which the constituent grains are of equal size (homeoblastic), have a flaky or platy habit, and are arranged parallel to each other. (IUGS Metamorphic Rocks, 2011)
- leucocratic - adj. A term used to describe igneous rocks whose color index (M') is between 10 and 35; light-colored.
- leucosome - n. The lightest-colored parts of a migmatite. (IUGS Metamorphic Rocks, 2011)
- lineation - n. Any repetitively occurring or penetrative visible linear feature in a rock body. It can be straight or curved and may be defined by: the alignment of the long axes of elongate mineral grains or grain aggregates (mineral lineations), the parallelism of small scale folds (crenulation lineations), or the intersection of two foliations (intersection lineation). (IUGS Metamorphic Rocks, 2011)
- liquidus or liquidus curve - n. A curved line on an equilibrium phase diagram that gives the chemical composition of a liquid, in equilibrium with another phase, commonly a solid, as a function of temperature or pressure. The name liquidus was coined to signify that the equilibrium assemblage is entirely liquid on one side of the line. A liquidus is a type of "saturation curve".
- mafic minerals - n. A collective term for modal ferromagnesian and other non-felsic minerals. Hence, mafic rock, a rock mainly consisting of mafic minerals. (IUGS Metamorphic Rocks, 2011)
- magma - n. A molten (meaning hot) silicate liquid that may have crystals in it.
- mass fraction - n. A number between 0 and 1 giving the fraction of the total mass of some portion of a rock, mineral, system, etc. For example, the the mass of a single chemical component of a system divided by the total mass of the system is the mass fraction of that component.
- melanocratic - adj. A term used to describe igneous rocks whose color index (M') is btween 65 and 90; dark-colored. Antonym: leucocratic.
- melanosome - n. The darkest parts of a migmatite, usually with prevailing dark minerals. It occurs between two leucosomes or, if remnants of the more or less unmodified parent rock (mesosome) are still present, it is arranged in rims around these remnants. (IUGS Metamorphic Rocks, 2011)
- mesoocratic - adj. A term used to describe igneous rocks whose color index (M') is between 35 and 65; medium-colored.
- mesosome - n. The part of a migmatite that is intermediate in color between leucosome and melanosome. If present, the mesosome is mostly more or less unmodified remnant of the parent rock (protolith) of the migmatite. (IUGS Metamorphic Rocks, 2011)
- metamorphic facies - n. A set of metamorphic mineral assemblages, repeatedly associated in time and space and showing a regular relationship between mineral composition and bulk chemical composition. Different metamorphic facies appear to be related to different metamorphic conditions, in particular temperature and pressure, although other variables such as P(H2O) may also be important. The SCMR recommends the use of ten facies, namely zeolite facies, greenschist facies, epidote-amphibolite facies, amphibolite facies, pyroxene hornfels facies, sanidinite facies, granulite facies, glaucophane-schist/blueschist facies and eclogite facies. (IUGS Metamorphic Rocks, 2011)
- metamorphic geotherm - n. A line joining the maximum temperature points (Tmax) on the P-T-t curves for successive mineral zones in a prograde sequence. The line differs from a geotherm or geothermal gradient because the different Tmax points did not exist simultaneously. (IUGS Metamorphic Rocks, 2011)
- metamorphic grade - n. a term used to indicate the relative conditions of metamorphism. The SCMR recommends that metamorphic grade refer only to temperature; the whole range may be divided into five grades, that is very low, low, medium, high, and very high grade of metamorphism. (IUGS Metamorphic Rocks, 2011)
- metamorphic zone - n. A mappable part of a metamorphic complex, in which rocks of the same chemical composition show identical mineral associations. The same metamorphic grade can thus be inferred for them. These zones may be named according to the typical mineral assemblage present in a specific rock type, or by only one mineral. Zones normally occur in a series indicating prograde or retrograde metamorphism. (IUGS Metamorphic Rocks, 2011)
- metamorphism - n. The process involving changes in mineral content/composition and/or microstructure of a rock, occuring dominantly in the solid state. This process is mainly due to an adjustment of the rock to physical conditions that differ from those under which the rock originally formed and that also differ from the physical conditions normally occurring at the surface of the Earth and in the zone of diagenesis. The process may also involve changes in the bulk chemical composition of the rock. (IUGS Metamorphic Rocks, 2011)
- metasomatic rock (metasomatite) - n. A metamorphic rock whose mineral and chemical bulk compositions have been substantially changed by metasomatism. (IUGS Metamorphic Rocks, 2011)
- metasomatic zone - n. A metasomatic rock defined by a specific mineral paragenesis, commonly in a banded series of parageneses produced by a metasomatic process.
- metasomatism - n. A metamorphic process by which the chemical composition of a rock or rock portion is altered in a pervasive manner and which involves the introduction or removal of chemical components. Metasomatism commonly occurs as a result of the interaction of a rock with aqueous fluids. It may also occur on a small scale due to intergranular diffusion.
- mode - n. The volmetric proportions of the minerals in a rock, commonly reported based on the two-dimensional observations in rock thin sections.
- monomineralic - adj. Said of a rock in which >95% of the modal content is composed of one mineral, as opposed to polymineralic, bimineralic, trimineralic. (IUGS Metamorphic Rocks, 2011)
- MORB - n. MORB is an acronym for Mid-OceanRidgeBasalt. The basalts that are erupted at the mid-ocean ridges are chemically similar, but have a modest range of chemical compositions (N-MORB to E-MORB) that reflect the mantle that is melting to form them. See Sun and others (1979).
- mullion structure - n. A type of structure on the mesoscopic or macroscopic scale appearing as a parallel cluster of rods or columns and produced by the presence of closely-spaced fold hinges, cleavage-bedding intersections, fault grooving, etc. (IUGS Metamorphic Rocks, 2011)
- nematoblastic structure - n. A type of structure in a metamorphic rock in which constituent grains are of equal size (homeoblastic), have an acicular or rod-like form, and are arranged parallel to each other. Synonym: fibroblastic structure. (IUGS Metamorphic Rocks, 2011)
- neoblast - n. A mineral in a metamorphic rock that is younger than the other mineral grains in the rock. A neoblast may be of a different composition from, or the same composition as, the other grains. Antonym: palaeoblast. (IUGS Metamorphic Rocks, 2011)
- norm - n. An idealized "standard mineral composition" for an igneous rock calculated from its anhydrous chemical composition. Most geologists calculate CIPW norms, named after Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington who devised the procedure early in the 20th century. More about norms, including procedures for calculating norms, can be found in the Norm Calculation igneous topic section.
- nucleation - n. Nucleation is the process in which a new crystal or phase begins to grow through the assembly of a critical number of atoms in the appropriate form (e.g. crystal structure, gas, liquid). A nucleus of a new phase reaches a critical size when the growth of the phase leads to a reduction in Gibbs energy, overcoming the surface/interfacial energy that dominates at smaller sizes.
- ocean-floor metamorphism - n. A type of metamorphism of regional or local extent related to the steep geothermal gradient occurring near spreading centers in oceanic environments. The metamorphic recrystallization, which is mostly incomplete, commonly encompasses a wide range in temperatures. The metamorphism is associated with circulating hat aqueous fluids (with related metasomatism) and typically shows an increasing temperature of metamorphism with depth. (IUGS Metamorphic Rocks, 2011)
- oxygen units - n. A unit of quantity that may be used to describe the chemical composition of a system in terms of chemical components. Oxygen units, like mole units, have meaning only if a chemical composition is specified. The oxygen unit value of a component is the mole unit value times the number of moles of oxygen in the component. Oxygen percentage of a component is the percentage of the total number of oxygen atoms (or moles of oxygen atoms) in a chemical system that are counted in one component. For example, in a two-component system with equal numbers of moles of the components Fe3O4 and SiO2, 2 of the moles of oxygen atoms are in SiO2 and 4 of the moles of oxygen atoms are in Fe3O4. Therefore, the system is 33 oxygen percent SiO2 and 67 oxygen percent Fe3O4. Oxygen is conserved in chemical reactions, so oxygen units are conservative units. Because oxygen occupies most of the volume of silicate minerals, oxygen percent approximates volume percent.
- palaeoblast - n. A mineral grain in a metamorphic rock that is older than the other mineral grains in a rock. A palaeoblast may be of a composition different from or identical to the other grains. (IUGS Metamorphic Rocks, 2011)
- palingenesis - n. The formation of a new magma by complete or nearly complete melting of preexisting rocks. Hence palingenite, rock resulting from palingenesis. (IUGS Metamorphic Rocks, 2011)
- paragenesis - n. The characteristic association of minerals in a metamorphic rock that are considered to have developed under the same physico-chemical conditions, and thus form an equilibrium assemblage. With changing metamorphic conditions new paragenesses may develop and together they form a paragenetic sequence. Different paragenetic sequences in the same rock can be deduced from the presence of replacement features (pseudomorph, corona, etc). Thus, a mineral association in a rock may represent a disequilibrium association consisting of two or more successive paragenesis always comprising an equilibrium assemblage. The term was first intended for mineral succession in ore bodies, but it is now used for all types of mineral associations of any origin. (IUGS Metamorphic Rocks, 2011)
- peralkaline - adj. Said of a rock whose chemical composition has (K2O + Na2O) > Al2O3 in mole units. Peralkaline rocks will have acmite in the norm.
- peraluminous - adj. Said of a rock whose chemical composition has Al2O3 > (K2O + Na2O + CaO) in mole units. Peraluminous rocks will have corundum in the norm.
- peritectic - n. A point giving the temperature and chemical composition of the liquid formed by the incogruent melting of a stoichiometric solid phase (mineral). A peritectic point is found at the intersection of the saturation curve (liquidus curve) of the incgongruently melting mineral and the saturation curve for the mineral formed during the melting reaction in a binary system. Cf. eutectic, chaotectic
- phaneritic - adj. Said of a rock whose mineral grains are visible with the unaided eye (>0.1mm).
- phase - n. A physically homogeneous substance that, in priciple, could be separated mechanically from other substances. In geology it is normally a crystal, glass, liquid, or gas.
- phase diagram - n. A graph of thermodynamic variables, such as temperature, pressure, and chemical composition, that shows equilibrium relationships among phases.
- pillow structures - n. Cylindrical structures produced when lava flows into water and is chilled on the margins but continues to flow through and out the tube formed by the chilled margins. The cylindrical structure is normally less than 1-2 meters in diameter, but can be many meters in length and typically occurs with many similar, somewhat parallel structures. When viewed in cross-section, the structures are flattend on the bottom so that multiple cylindars resemble a pile of pillows.
- phenocryst - n. A crystal that is much larger than the surrounding matrix in an igneous rock.
- poikiloblast - n. A large crystal formed in a metamorphic rock (porphyroblast) and characterized by the presence of abundant small included grains. A poikiloblastic structure is synonymous with a sieve structure. (IUGS Metamorphic Rocks, 2011)
- porphyroblast - n. A large crystal formed in a metamorphic rock and characterized by the presence of abundant small grains, creating a porphyroblastic structure. (IUGS Metamorphic Rocks, 2011)
- porphyroclast - n. A large, relict crystal in a metamorphic rock set in a finer-grained matrix produced by deformation. The crystals in the matrix may have undergone recrystallization. (IUGS Metamorphic Rocks, 2011)
- polygonal structure - n. A type of structure in a metamorphic rock in which the constituent grains have straight or smoothly curved crystal faces generally meeting at triple points. The structure is typical of quartz and calcite rocks. (IUGS Metamorphic Rocks, 2011)
- polymetamorphism - n. Metamorphism resulting from more than one phase of metamorphic event; each event may be monophase or polyphase. Polymetamorphism is recognized through relics of metamorphic minerals or structures. (IUGS Metamorphic Rocks, 2011)
- polymineralic - n. Said of a rock in which >95% of the modal content is composed of two or more minerals, as opposed to monomineralic. (IUGS Metamorphic Rocks, 2011)
- polymorphic transformation - n. A change in the structure of a mineral (solid phase) without change in the chemical composition. Examples are low- and high-quartz, calcite and aragonite, andalusite and kyanite. Hence polymorphs, different structural states of a mineral capable of polymorphic transformation. Also polymorphism, the property of a mineral to undergo polymorphic transformation. (IUGS Metamorphic Rocks, 2011)
- polyphase metamorphism - n. A metamorphic event with two or more temperature and/or pressure peaks. (IUGS Metamorphic Rocks, 2011)
- porphyroclast - n. A large relic crystal in a metamorphic rock, set in a finer-grained matrix that was produced by deformation. The crystals in the matrix may have undergone recrystallization. Hence porphyroclastic structure. (IUGS Metamorphic Rocks, 2011)
- precipitate - v. Cause crystals to grow from a solution.
- preferred orientation - adj. Said of inequant mineral grains or grain aggregates, a statistically significant number of which have the same orientation. The term may also be used to describe the orientation of crystallographic axes. (IUGS Metamorphic Rocks, 2011)
- prehenite-actinolite facies - n. A metamorphic facies representing very low grades of metamorphism and characterized by the mineral association of prehnite-actinolite-epidote (± chlorite, albite, quartz, and titanite) and by the absence of pumpellyite in rocks of appropriate bulk composition (mostly metabasic rocks and their clastic derivatives). Some authors prefer to regard this mineral association as a subdivision of the subgreenschist facies. (IUGS Metamorphic Rocks, 2011)
- prehenite-pumpellyite facies - n. A metamorphic facies representing very low grades of metamorphism and characterized, in metasandstones and metavolcanic rocks of appropriate composition, by the presence of prehnite and or pumpellyite in the absence of zeolites, lawsonite, or jadeite. Quartz-albite-chlorite-prehnite and/or pumpellyite may coexist stably. The SCMR regards this mineral association as a subdivision of the greenschist facies. (IUGS Metamorphic Rocks, 2011)
- pressure shadow - n. An area of low strain in a deformed rock that was protected from the maximum compressive stress by its proximity to a rigid body, either a porphyroblast or porphyroclast. The area of low strain may preserve pre-existing structures or space may be created in which new minerals may grow. The growth of new minerals, typically quartz or calcite, in the low-strain area may resemble a beard, particularly if the minerals have fibrous habit, hence bearded structure. Synonym: strain shadow. (IUGS Metamorphic Rocks, 2011)
- pressure solution - n. A deformation process by which material under stress goes into solution and is then transported by flow or diffusion to areas of relatively low stress. A diffusive mass-transfer deformation mechanism (in which the presence of intergranular water is inferred to accelerate the rate) whereby material is transferred from grain boundaries under high interfacial normal stress to interfaces under lower normal stress. The material may simply move around a mineral grain boundary (e.g. into a pressure shadow) or be transported out of the system. Hence pressure solution striping/cleavage, where the development of various types of cleavage result in the preferential movement of material, usually quartz, from the cleavage domains. (IUGS Metamorphic Rocks, 2011)
- prograde metamorphism - n. Metamorphism giving rise to the formation of minerals that are typical of a higher grade (i.e. higher temperature) than the former phase assemblage; a sequence increase in metamorphic grade; that part of the metamorphic cycle up to the thermal maximum. Note: Miyashiro used prograde metamorphism to imply a sequential increase in grade across a region (e.g. as exhibited by Barrow’s zones). The SCMR recommends that if the terms are used with this latter more restricted meaning then this should be made clear. Antonym: retrograde, retrogressive metamorphism. (IUGS Metamorphic Rocks, 2011)
- pseudomorph - n. A mineral or aggregate of minerals in a metamorphic rock, whose shape is that of a pre-existing mineral which it/they has/have replaced. (IUGS Metamorphic Rocks, 2011)
- protolith - n. A precursor rock (igneous, sedimentary, or already metamorphosed) from which a given metamorphic rock is derived. Synonym: parent rock. (IUGS Metamorphic Rocks, 2011)
- P-T-t path - n. Changing pressure(P)-temperature(T) conditions experienced by a mineral or a rock with time(t), or a line/curve on a P-T grid showing these changes. (IUGS Metamorphic Rocks, 2011)
- pumpellyite-actinolite facies - n. A metamorphic facies representing very low grades of metamorphism and characterized by the mineral association of pumpellyite-actinolite-quartz (± chlorite, albite and epidote) and by the lack of prehnite. The SCMR regards this mineral association as a subdivision of the subgreenschist facies. (IUGS Metamorphic Rocks, 2011)
- pyrometamorphism - n. A very high grade type of contact metamorphism occurring in volcanic settings or around near-surface intrusions and characterized by mineral assemblages stable at or near atmospheric pressure and very high temperatures; critical minerals are spurrite, tilleyite, rankinite, larnite and merwinite in silica-deficient carbonate rocks; mullinte and glass in aluminous rocks, tridymite and glass in silica-oversaturated rocks. (IUGS Metamorphic Rocks, 2011)
- pyroxene-hornfels facies - n. sa (IUGS Metamorphic Rocks, 2011)
- reaction rim or reaction border - n. A peripheral zone around a mineral grain, composed of another mineral species and formed by reaction between the mineral and its surroundings. (IUGS Metamorphic Rocks, 2011)
- recrystallization - n. The process of nucleation and migration of higher angle (grain) boundaries to produce new grains. Recrystallization does not necessarily involve any change in chemical composition. (IUGS Metamorphic Rocks, 2011)
- REE - n. REE is an acronym for Rare Earth Elements, the lanthanide elements (atomic numbers 57-71). REE are trace elements of particular interest to petrologists because their distribution coefficients can be different by orders of magnitude in different minerals.
- regional metamorphism - n. A type of metamorphism that occurs over an area of wide extent, i.e. affecting a large rock volume, and is associated with large-scale tectonic processes such as ocean-floor spreading, crustal thickening related to plate collision, deep basin subsidence, etc. (IUGS Metamorphic Rocks, 2011)
- relic structure - n. Type of structure in a deformed rock characterized by the presence of remnants of the pre-existing undeformed rock. The structure usually takes the form of lenticular ‘relics’ in a finer-grained matrix. (IUGS Metamorphic Rocks, 2011)
- retrograde metamorphism - n. Metamorphism giving rise to the formation of minerals that are typical of a lower grade (i.e. lower temperature) than the former phase assemblage. Note; Miyrshiro distinguished between retrograde metamorphism which he defined as that part of a metamorphic event after the thermal maximum, and retrogressive metamorphism which he took as a second metamorphic event which takes place at a lower temperature than the first. The SCMR recommends that if the terms are used with this restricted meaning, then this should be made clear. (IUGS Metamorphic Rocks, 2011)
- ribbon structure, ribbon quartz - n. A strongly elongated single crystal or very fine-grained aggregates of quartz crystals resembling ribbons in a rock. The structure is commonly the product of intense deformation at high temperature. The crystal grains may show undulose extinction or be recrystallized. (IUGS Metamorphic Rocks, 2011)
- sanidinite facies - n. A metamorphic facies representing extreme high temperatures and low or very low pressures. It lies below the granulite facies. It is characterized by the occurrence of especially high-temperature varieties and polymorphs of minerals, for example pigeonite, K-rich labradorite, and of sanidine-rich rocks derived from pelitic rocks. (IUGS Metamorphic Rocks, 2011)
- saussuritization - n. A late magmatic, metamorphic or other alteration process by which calcic plagioclase is altered to sausserite, a tough, compact, white, greenish or greyish mineral aggregate consisting of clinozoisite, zoisite, albite, and/or epidote, with variable amounts of calcite, sericite, prehnite and calcium-aluminum silicates. (IUGS Metamorphic Rocks, 2011)
- saturated - adj. An equilibrium condition for a liquid in the presence of a crystal. For example, a magma is saturated with olivine if olivine crystals are present and chemical equilibrium has been attained. This term should always be followed by "with" and the name of the crystal. The presence of a mineral puts a constraint on the chemical composition of the magma. More broadly, the term can be used to describe any phase of variable composition in equilibrium with another phase. For example, air is saturated with water at 100% humidity.
- saturation curve - n. A line on an equilibrium phase diagram that gives the chemical composition of one phase, commonly a liquid, in equilibrium with another phase, commonly a solid, as a function of temperature or pressure. Many authors will use the term "liquidus curve" to refer to this line where the mixture is entirely liquid on one side of the line.
- S-C fabric (originally C/S fabric) - n. A composite fabric produced by the intersection of two planar fabrics (C-fabrics and S-fabric) in sheared rocks. The C-fabrics forms broadly parallel to the margins of the shear zone and the S-fabric forms oblique to the margin, the angle between the two decreasing as shearing progresses. The S-planes curve into the C-planes, the nature of the curvature reflecting the sense of shear. S-C fabrics are common in strongly foliated and mica-rich mylonites or granitoid mylonites. (IUGS Metamorphic Rocks, 2011)
- schist - n. A metamorphic rock with a well developed schistosity. For rocks rich in phyllosilicates, schist is used for coarse-grained rocks in which the phyllosilicate grains are visible without a hand lens. Names such as slate or phyllite are commonly used for fine-grained phyllosilicate-rich rocks.
- schistose structure - n. A type of structure characterized by a schistosity that is well developed, either uniformly throughout the rock that or in narrowly spaced repetitive zones such that the rock will split on a scale of 1cm or less. (IUGS Metamorphic Rocks, 2011)
- schistosity - n. Preferred orientation of inequant mineral grains or grain aggregates produced by metamorphic processes. A schistosity is said to be well developed if inequant mineral grains or grain aggregates are present in a large amount and show a high degree of preferred orientation, either throughout the rock or in narrowly spaced repetitive zones, such that the rock will split on a scale of less than one centimeter. A schistosity is said to be poorly developed if inequant mineral grains or grain aggregates are present on only small amounts or, if well developed, occur in broadly spaced zones such that the rock will split on a scale of more than one centimeter. (IUGS Metamorphic Rocks, 2011)
- schlieren - n. Streaks or minor lenticular parts of a rock that differ from the main body of the rock in the mineral content or ratio of minerals and which commonly have transitional boundaries. The term was originally used for magmatic rocks but is now also used for similar structures in migmatites, for example for patches of non-leucosome within the leucosome. (IUGS Metamorphic Rocks, 2011)
- seritization - n. A hydrothermal or other metamorphic process whereby aluminosilicate minerals are replaced by sericite (fine-grained white mica, normally muscovite and possibly paragonite). (IUGS Metamorphic Rocks, 2011)
- shock metamorphism - n. Type of metamorphism of local extent caused by a shock wave compression due to the hypervelocity impact of a solid body or due to the detonation of high energy chemical or nuclear explosives. Cf. impact metamorphism. (IUGS Metamorphic Rocks, 2011)
- sieve structure - n. A type of structure synonymous poikiloblastic structure. (IUGS Metamorphic Rocks, 2011)
- skarn - n. A metasomatic rock formed at the contact between a silicate rock (or a magmatic melt) and a carbonate rock. (IUGS Metamorphic Rocks, 2011)
- solidus or solidus curve - n. A curved line on an equilibrium phase diagram that gives the melting temperatures or pressures of solids as a function of chemical composition. The name solidus was coined to signify that the equilibrium assemblage is entirely solid on one side of the line.
- slaty cleavage - n. A type of continuous cleavage in which the individual grains are too small to be seen by the unaided eye. (IUGS Metamorphic Rocks, 2011)
- snowball structure - n. A type of structure characterized by spiral-shaped inclusion trails in a porphyroblast and thought to be indicative of the rotation of the porphyroblast during growth or of the differential rotation of the fabric relative to the porphyroblast during its growth. Cf. heletic structure. (IUGS Metamorphic Rocks, 2011)
- solvus - n. A curved P-T-X line or surface that separates the field of homogeneous solid solution from the field of limited mutual solid solution. (IUGS Metamorphic Rocks, 2011)
- spaced cleavage - n. A type of cleavage in which the cleavage planes are spaced at regular intervals and separated by zones known as microlithons. The structure is visible to the unaided eye. Spaced cleavage encompasses crenulation cleavage and disjunctive cleavage. (IUGS Metamorphic Rocks, 2011)
- spilite - n. Altered basic to intermediate volcanic or sub-volcanic rock in which the feldspar is partially or completely composed of albite and is typically accompanied by chlorite, calcite, quartz, epidote, prehnite and low-temperature hydrous crystallization products. Fettes and Desmons (2011) p. 196.
- structure - n. The arrangement of the parts of a rock mass irrespective of scale, including spatial relationships between the parts, their relative size and shape, and the internal features of the part. (IUGS Metamorphic Rocks, 2011)
- subgrain boundary - n. A relatively planar array of dislocations separating two volumes of crystalline material (subgrains) with the same composition but wth small (usually <5%) angular misorientations of their crystal lattices. Commonly formed by recovery during or after deformation. (IUGS Metamorphic Rocks, 2011)
- subhedral - n. Said of a crystal that is only partly bounded by its own crystal faces. The recommended specific terms for metamorphic rocks are hypidioblastic and subidioblastic. (IUGS Metamorphic Rocks, 2011)
- sutured boundary - n. Synonymous with hypiodioblast and hypidioblastic structure. (IUGS Metamorphic Rocks, 2011)
- symplectite, symplectitic intergrowth - n. A type of (micro)structure characterized by the intimate intergrowth of two or more different minerals, one of them commonly having vermicular habit. (IUGS Metamorphic Rocks, 2011)
- syntaxial -adj. Said of a vein-infilling in which fibres grow from the walls towards the center. Hence syntaxial vein, syntaxial growth. Antonym: antitaxial. (IUGS Metamorphic Rocks, 2011)
- tabular -adj. A descriptive term for an object with two of its dimensions significantly greater than the third. Hence tabular structure, a term applicable to a single body of a parallel set of tabular bodies; the term has been applied to minerals and to rock bodies, for example the structure produced by sets of horizontal joints. (IUGS Metamorphic Rocks, 2011)
- tephra - n. A collective term used in the pyroclastic classification for pyroclastic deposits that are predominantly unconsolidated. (IUGS Metamorphic Rocks, 2011)
- thermochron -adj. A conceptual line or surface in a rock body joining points that had the same temperature at the same time. (IUGS Metamorphic Rocks, 2011)
- tie line - n. A line on an equilibrium phase diagram that extends from the composition of one phase to the composition of another phase that is in equilibrium with the first phase.
- topotaxis, topotaxy, topotaxial growth -n. Recrystallization in which the new crystal grows with minimal change to the lattice of the parent crystal. (IUGS Metamorphic Rocks, 2011)
- trace element - n. An element that comprises less than 0.1 wt.% of a chemical system (e.g. mineral, rock, magma) is considered a trace element for that system. An element that is a trace element for one system might be a major element (greater than 0.1 wt.%) in another system.
- trimineralic -adj. Said of a rock in which ≥95% of the modal content is composed of three minerals, as opposed to polymineralic, monomineralic, bimineralic. (IUGS Metamorphic Rocks, 2011)
- ultra-high pressure metamorphism -adj. That part of the metamorphic P-T field where pressures exceed the minimum necessary for the formation of coesite. Pressure may be sufficient for the formation of diamond. (IUGS Metamorphic Rocks, 2011)
- undulose extinction -n. Poorly defined and variable extinction across a strained crystal caused by local distortion of the crystal lattice. The phenomenon may occur in a variety of minerals but is most commonly seen in quartz. (IUGS Metamorphic Rocks, 2011)
- variation diagram - n. A graph of chemical data for a suite of rocks to show possible relationships among chemical variables. Axes for the graph, normally X-Y or ternary, can be any choice of chemical variables or their combinations that may reveal or test relationships being considered. X-Y variation diagrams are called Harker Diagrams in some texts because of their early use by Alfred Harker (1909, Chap.14).
- vesicles - n. Rounded holes in igneous rocks due to the presence of gas bubbles in the magma when the magma was solidified. Generally, vesicles imply solidification in volcanic or shallow intrusive rocks. In some cases, the vesicles may be filled with quartz, calcite, zeolites, or other minerals, after the rock has solidified. Filled vesicles are called amygdules.
- xenoblast -n. A crystal formed in a metamorphic rock without developing any of its own crystal faces. Hence xenoblastic structure. (IUGS Metamorphic Rocks, 2011)
- zeolite facies -n. g (IUGS Metamorphic Rocks, 2011)