Groundmass
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The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains,
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s, or clasts are embedded. The matrix of an
igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...
consists of finer-grained, often
microscopic The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale betwe ...
, crystals in which larger crystals, called
phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...
s, are embedded. This
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 ...
texture is indicative of multi-stage cooling of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
. For example, porphyritic
andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...
will have large phenocrysts of
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
in a fine-grained matrix. Also in
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
,
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...
s are often mined from a matrix of weathered
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
-like rock (
kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...
) called "yellow ground". The matrix of
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s is finer-grained sedimentary material, such as clay or
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
, in which larger grains or clasts are embedded. It is also used to describe the rock material in which a
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
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 The London Clay Formation is a Sediment#Shores and shallow seas, marine formation (geology), geological formation of Ypresian (early Eocene Epoch, c. 54-50 million years ago) age which outcrop, crops out in the southeast of England. The London C ...
) 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". 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. The redeposited cementing material is most commonly calcareous or siliceous.
Limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
s, 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 reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...
s, 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
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
s (such as Kentish rag), a crystalline matrix of
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
often envelops the sand grains. The change of
aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...
to calcite and of calcite to dolomite, by forming new crystalline masses in the interior of the rock, usually also accelerates consolidations.
Silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
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 Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina (). ...
or of
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...
. Argillaceous materials may be compacted by mere pressure, like
graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
and other scaly minerals.


References


External links

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