Petrology

geological feature

alt=|thumb|322x322px|Purple and green pseudotachylyte veins in outcrop (Sierra Nevada Mountains, California) Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veins that form through frictional melting and subsequent quenching during earthquakes, large-scale landslides, and impacts events. Chemical composition of pseudotachylyte generally reflects the local bulk chemistry, though may skew to slightly more mafic compositions due to the preferential incorporation of hydrous and ferro-magnesian minerals (mica and amphib

knot, mass, or lump of a mineral or mineral aggregate with a contrasting composition

thumb|Sample of gold ore from the Red Lake Mine consisting of a stockwork of gold filled veinlets (sample width: 6.6 cm) In geology, a stockwork is a complex system of structurally controlled or randomly oriented veins. Stockworks are common in many ore deposit types and in greisens. They are also referred to as stringer zones.

way that brittle materials break or fracture when they do not follow any natural planes of separation

crystal not bounded by its own crystal faces

Group of alloys

annular figures or bands formed by chemical precipitation

Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Carbonado, a form of diamond, is also cryptocrystalline. Volcanic rocks, especially of the felsic type such as felsites and rhyolites, may have a cryptocrystalline groundmass as distinguished from pure obsidian (felsic) or tachylyte (mafic), which are natural rock glasses. Agate and onyx are examples of cryptocrystalline silica (chalcedony)

thumb|Porphyritic texture in a granite. This is an Intrusive rock|intrusive porphyritic rock. The white, square feldspar [[phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada, Rock Creek Canyon, California.]] thumb|A porphyritic volcanic sand grain, as seen under the petrographic microscope. The large grain in the middle is of a much different size class than the small needle-like crystals around it. Scale box in millimeters. Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, w

thumb|Microcrystalline copper(I) oxide thumb|Silver microcrystals imaged by an electron microscope

classification system for carbonate sedimentary rocks

includes rocks over 4 billion years old from the Hadean Eon

thumbnail|A dreikanter from the Wind River Basin, Wyoming, USA. Property of Western Washington University. Photo by AJ Wakefield, 2012.

Petrogenesis, also known as petrogeny, is a branch of petrology dealing with the origin and formation of rocks. While the word petrogenesis is most commonly used to refer to the processes that form igneous rocks, it can also include metamorphic and sedimentary processes, including diagenesis and metamorphic reactions.

Ferropericlase or magnesiowüstite is a magnesium/iron oxide with the chemical formula that is interpreted to be one of the main constituents of the Earth's lower mantle together with the silicate perovskite (), a magnesium/iron silicate with a perovskite structure. Ferropericlase has been found as inclusions in a few natural diamonds. An unusually high iron content in one suite of diamonds has been associated with an origin from the lowermost mantle. Discrete ultralow-velocity zones in the deepest parts of the mantle, near the Earth's core, are thought to be blobs of ferropericlase, as seismic

type of igneous rock; rock that is either siliceous, having a high content of silica (SiO2), or rock with a low pH

spatial and geometric configuration of all the elements that make up a rock

thumb|Limestone of crinoids Bioclasts are skeletal fossil fragments of once living marine or land organisms that are found in sedimentary rocks laid down in a marine environment—especially limestone varieties around the globe, some of which take on distinct textures and coloration from their predominate bioclasts—that geologists, archaeologists and paleontologists use to date a rock strata to a particular geological era.

silicate mineral with perovskite structure, including bridgmanite or davemaoite

liquid and/or gas trapped within a crystal

mineral of density greater than 2.89

mass of crystals or rock particles or soil particles

thumb|upright=1.35|The Espinaso Formation includes a wide variety of volcaniclastic materials. Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it was deposited in, or whether nonvolcanic material is mingled with the volcanic clasts. The United States Geological Survey defines volcaniclastics somewhat more narrowly, to include only rock composed of volcanic rock fragments that have been transported so

thumb|This vug in the Cascade Range is partially filled with [[quartz crystals.]]

A maceral is a component, organic in origin, of coal or oil shale. The term 'maceral' in reference to coal is analogous to the use of the term 'mineral' in reference to igneous or metamorphic rocks. Examples of macerals are inertinite, vitrinite, and liptinite.

thumb|right|250px|Alginite Alginite is a component of some types of kerogen alongside amorphous organic matter. Alginite consists of organic-walled marine microfossils, distinct from inorganic (silica)-walled microfossils that comprise diatomaceous earth.

thumb|Benmoreite lava forms the "Dragon's Teeth" landmark on Maui, Hawaii Benmoreite is a volcanic rock of intermediate composition. It is a silica-undersaturated sodium-rich variety of trachyandesite (the other kind is latite) and belongs to the alkaline suite of igneous rocks. It was named after Ben More, a mountain on the Isle of Mull, Scotland.

thumb|upright=1.3|Scanning Electron Microscope interface image of a fayalite-[[pyroxene symplectite (at right) in a Martian meteorite]] A symplectite (or symplektite) is a material texture: a micrometre-scale or submicrometre-scale intergrowth of two or more crystals. Symplectites form from the breakdown of unstable phases, and may be composed of minerals, ceramics, or metals. Fundamentally, their formation is the result of slow grain-boundary diffusion relative to interface propagation rate.

thumb|right|Myrmekite, about 2 millimetres across 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. These quartz-plagioclase intergrowths are associated with and commonly in contact with potassium feldspar. Myrmekite is formed under metasomatic conditions, usually in conjunction with tectonic deformations. It has to be clearly separated from micrographic and granophyric intergrowths, which are magmatic.

Rock deformation mechanism involving minerals dissolution under mechanical stress

rock cavity

geologic distinction in the Pacific Ocean

Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce mid-ocean ridge basalts (MORB) by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite. The term is derived from the mineral names pyroxene and olivine. However, whether pyrolite is entirely representative of the Earth's mantle remains debated.

__NOTOC__ An isograd is a concept used in the study of metamorphic rocks. The metamorphic grade of such a rock is a rough measure of the degree of metamorphism it has undergone, as characterised by the presence of certain index minerals. An isograd is a theoretical surface comprising points all at the same metamorphic grade, and thus separates metamorphic zones whose rocks contain different index minerals.

Post-perovskite (pPv) is a high-pressure phase of magnesium silicate (MgSiO3). It is composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO2), and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle.

late Mesozoic geologic complex in the California Coast Ranges

Relatively flat rock surface that is subdivided into more or less regular shapes by fractures

Region in Norway