Matrix (geology)

Last updated June 15, 2022

Orthoclase phenocrysts within a finer-grained matrix of a granite porphyry FeldsparsGranite.JPG — Orthoclase phenocrysts within a finer-grained matrix of a granite porphyry

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, called phenocrysts, are embedded. This porphyritic texture is indicative of multi-stage cooling of magma. For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. Also in South Africa, diamonds are often mined from a matrix of weathered clay-like rock (kimberlite) called "yellow ground".

The matrix of sedimentary rocks is finer-grained sedimentary material, such as clay or silt, in which larger grains or clasts are embedded. It is also used to describe the rock material in which a fossil is embedded.

Cementation

All sediments are at first in an incoherent condition (e.g. sands, clays and gravels, beds of shells), and they may remain in this state for an indefinite period. Millions of years have elapsed since some of the early Tertiary strata gathered on the ocean floor, yet they are quite friable (e.g. the London Clay) and differ little from many recent accumulations. There are few exceptions to the rule that with increasing age sedimentary rocks become more and more indurated. Generally, the older the strata the more likely it is that they will have the firm consistency generally implied in the term "rock".^[1]

The pressure of newer sediments on underlying masses is apparently one cause of this hardening, though not in itself a very powerful one. More efficiency is generally ascribed to the action of percolating water, which takes up water-soluble materials and then redeposits them in pores and cavities. This operation is probably accelerated by the increased pressure produced by superincumbent masses, and to some extent also by the rise of temperature which inevitably takes place in rocks buried to some depth beneath the surface. The rise of temperature, however, may be only a tiny increase; we know more than one instance of sedimentary deposits which have been buried beneath four or five miles of similar strata (e.g. parts of the Old Red Sandstone), and yet no perceptible difference is apparent between beds of similar composition at the top of the series and near its base. Should the deposits have been truly "baked", that is, subject to a large increase in temperature, then differences would be evident.^[1]

The redeposited cementing material is most commonly calcareous or siliceous. Limestones, which were originally a loose accumulation of shells, corals, etc., become compacted into firm rock in this manner; and the process often takes place with surprising ease, as for example, in the deeper parts of coral reefs, or even in wind-blown masses of shelly sand exposed merely to the action of rain. The cementing substance may be regularly deposited in crystalline continuity on the original grains, where these were crystalline, and even in sandstones (such as Kentish rag), a crystalline matrix of calcite often envelops the sand grains. The change of aragonite to calcite and of calcite to dolomite, by forming new crystalline masses in the interior of the rock, usually also accelerates consolidations. Silica is less easily soluble in ordinary waters, but even this ingredient of rocks is dissolved and redeposited with great frequency. Many sandstones are held together by an infinitesimal amount of colloid or cryptocrystalline silica; when freshly dug from the quarry they are soft and easily trimmed, but after exposure to the air for some time they become much harder, as their siliceous cement sets and passes into a rigid condition. Others contain fine scales of kaolin or of mica. Argillaceous materials may be compacted by mere pressure, like graphite and other scaly minerals.^[1]

Related Research Articles

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a common type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils, and these provide scientists with information on ancient environments and on the evolution of life.

Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.

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 to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Chert Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO₂). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

Volcanic rock Rock formed from lava erupted from a volcano

Volcanic rock is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Lithology Description of its physical characteristics of a rock unit

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contain a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Greywacke Hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix

Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The term "greywacke" can be confusing, since it can refer to either the immature aspect of the rock or its fine-grained (clay) component.

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

Rock microstructure includes the texture of a rock and the small-scale rock structures. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

The Folk classification, in geology, is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.

Clastic rock Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

The Mount Pleasant Caldera is a large eroded Late Devonian volcanic caldera complex, located in the northern Appalachian Mountains of southwestern New Brunswick, Canada. It is one of few noticeable pre-Cenozoic calderas, and its formation is associated to a period of crustal thinning that followed the Acadian orogeny in the northern Appalachian Mountains. It sits relatively near to the coastline.

Granite Mountain is a 7,628-foot (2,325 m) mountain located in Yavapai County, Arizona that covers roughly 12 square miles (31 km²). It was once known as Mount Gurley, for the first governor of the Arizona Territory, John A. Gurley. Its southwest face has a sheer granite cliff approximately 500 feet high that is one of the best locations for rock climbing in the state of Arizona. It is located in the Granite Mountain Wilderness, which is managed as a part of the Prescott National Forest. The mountain stands at the northern end of the Sierra Prietas, and borders Skull Valley on the west, on the northwest by the Santa Maria Mountains, and east by the Williamson Valley.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

Cementation (geology) Process of chemical precipitation bonding sedimentary grains

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together. In this way, sand becomes sandstone, and gravel becomes conglomerate or breccia. Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz, and silica phases like cristobalite, iron oxides, and clay minerals; other mineral cements also occur.

Cape Town lies at the south-western corner of the continent of Africa. It is bounded to the south and west by the Atlantic Ocean, and to the north and east by various other municipalities in the Western Cape province of South Africa.

The Vasquez Formation (Tvz) is a geologic formation cropping out at the eponymous Vasquez Rocks in southern California. The formation dates to the Late Oligocene to Early Miocene.

References

1 2 3 One or more of the preceding sentences incorporates text from a publication now in the public domain : Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica . Vol. 21 (11th ed.). Cambridge University Press. p. 332.

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[EB1911-1] 1 2 3 One or more of the preceding sentences incorporates text from a publication now in the public domain : Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica . Vol. 21 (11th ed.). Cambridge University Press. p. 332.

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