Arenite (Latin: arena, "sand")[1] is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix.[2] The related adjective is arenaceous. The equivalent Greek-derived term is psammite, though this is more commonly used for metamorphosed sediments.
Arenaceous Rock
Arenites mainly form by erosion of other rocks or turbiditic re-deposition of sands. Some arenites contain a varying amount of carbonatic components and thus belong to the rock-category of carbonatic sandstones or silicatic limestones. Arenites often appear as massive or bedded medium-grained rocks with a middling- to wide-spaced preferred lamination and often develop a pronounced cleavage.
These are various shaped mass or nodules of minerals found within sedimentary rocks. Their shape may be elliptical or oval or irregular. Concretions generally consist of CaCO3, SiO2 and often possess an internal radiation or concentric structure. They are formed by the deposition of mineral matter from percolation solutions about a nucleus.
The strength of sedimentary rocks depends on the strength of its grains or sediments (constituting the rocks) and cementing material. Thus, the selection of a sedimentary rock for any building or project should be done carefully based on the strength, colour and appearance of its grains as well as the cementing material.
This unusual sedimentary rock has small vugs containing clear to whitish aragonite crystals (CaCO3 - calcium carbonate). Aragonite is a less common polymorph of calcite. Aragonite is rarely found in Ohio. The host rock is an arenaceous, ferruginous, fossiiferous limestone with dolomitized crinoid stem columnals. The dark brown material in the upper right portions of the vug is goethite (FeOÂOH - iron hydroxy-oxide).
"Thumbnail" definitions attached to some of the table entries are an interim attempt to provide simple short descriptions of the "unqualified" rock types presented within the BGS Rock Classification Scheme. Every effort has been made to ensure accuracy and consistency but minor contradictions or omissions might be present. The BGS welcomes any feedback pointing out such minor imperfections and/or providing constructive comment and suggestions for improvement of the definitions. In the first instance such feedback should be directed to Dr Tim McCormick.
Agglutinated, or arenaceous, microfossils have tests (shells) constructed from sedimentary particles bound together by organic, calcareous, siliceous, or ferruginous cement. There is only one type of stratigraphically significant agglutinated microfossil: agglutinated, or arenaceous, foraminifera.
Agglutinated foraminifera are benthic microfossils found in rocks of Cambrian through Holocene age and in most marine and brackish environments, particularly in clastic facies. They construct their tests by gluing sedimentary grains together, in contrast to the other types of foraminifera, which secrete their tests. The sedimentary particles used by these forms may include silt or sand grains, glauconite, sponge spicules, or even other foraminiferal tests. Some species are highly selective in the material used and in its arrangement.
If the mineralogy of the crystal or lithic fragments can be determined, the name of the appropriate volcanic rock can be prefixed as,"rhyolite vitric crystal tuff," or simply, "rhyolite tuff."
Shale is a laminated or fissile clastic sedimentary rock that composed of predominance of silt and clay other minerals , especially quartz and calcite. Characteristic properties of shale is breaks along thin laminae or parallel layering or bedding called fissility. It is most abundant sedimentary rock. The composition (silt and clay) of shale in a category of sedimentary rocks known as mudstone. Difference between shale to mudstone, It is fissile and laminated seen. Shale rock readily into thin pieces along the laminations.
Shales are fissile clastic sedimentary rocks formed fromtransportation, deposition and compaction of detrital materials of silt andclay. Fissility of the clay is its main distinguishing characteristic fromother sedimentary rocks. Fissility is defined as the property of a rock tosplit easily along thin closely spaced (Shales characteristically contain fine-grained silt and clayparticles (Shales may be classified as quartzose, feldspathic ormicaceous shale depending on the predominance of the minerals quartz, feldsparor mica, respectively, in the rock after appropriate XRD analysis (Pettijohn,1957).
Shales like other sedimentary rocks are cemented by someminerals or elements after deposition and compaction. The dominant type ofcementing material may be used in the classification of the shale since thismay affect the properties or performance of the shale when used as anengineering material. The common cementing materials are silica, iron oxide andcalcite or lime. Accordingly, shales may be classified as siliceous,ferruginous or calcareous (sometimes also called limy), respectively.
The sedimentaryenvironment of any sedimentary rock (including shale) is a natural geographicalentity in which sediments are accumulated and later changed to rock (Reineckand Singh, 1980). Three depositional sedimentary environments are recognized,namely, continental, transitional or marginal and marine. Each depositional environmenthas various subdivisions. Shales are generally deposited in lacustrine(continental), deltaic (transitional) and marine depositional environments andmay correspondingly be classified as such; that is, lacustrine, deltaic andmarine shales (Compton, 1977; Boggs, 1995). Lacustrine deposits arecharacterized by mixture of clay, silt and sands; inorganic carbonateprecipitates; and various fresh water invertebrate organisms includingbivalves, ostracods, gastropods, diatoms and various plant deposits. Most lakedeposits are less than 10m thick. Deltaic deposits are generally paralic(consisting of orderly sequences of shales and sandstones formed as a result ofalternating marine transgressions and regressions). They are also characterizedby shallow depth and concentration of kaolinite/illite/montmorillonite clayminerals. Deposits of marine environment are characterized by homogenous rocksequences (nonparalic), great depth, oxygen deficiency, and concentration ofillite/montmorillonite clay minerals. Shales of marine depositional environmentare generally darker in colour and richer in marine planktonic fossils thanshales deposited in lacustrine and deltaic environments.
Shales may be classified as carbonaceous or bituminous onthe basis of their organic matter content (Krumbein and Sloss, 1963). Theorganic matter content of carbonaceous and bituminous shales are generallyabove 10%. The organic matter induces black or grey colour to the shales. Theblack colour of some shales may also be due to presence of iron sulphide. Whenthe dominant organic matter content is from plant fragments such as pollengrains, stems and leaves, the shale is classified as carbonaceous, and thedepositional environment is usually continental (lacustrine) or transitional(deltaic or lagoon). When the dominant organic matter content in the shale isfrom animal fragments such as fossils, the shale is classified as bituminousand its depositional environment is usually deltaic or marine. Bothcarbonaceous and bituminous shales are important source rocks for generation ofpetroleum oil and gas depending on their amount / type of kerogen content. Kerogenis that Mud Shale when laminated
Clay Minerals are major component of shale and other similarrocks. The clay minerals represented are mostly kaolinite, montmorillonite andillite. Clay minerals of Late Tertiary mudstones are expandable smectiteswhereas in older rocks especially in mid- to early Paleozoic shales illitespredominate. The transformation of smectite to illite produces silica, sodium,calcium, magnesium, iron and water. These released elements form authigenicquartz, chert, calcite, dolomite, ankerite, hematite and albite, all trace tominor (except quartz) minerals found in shales and other mudrocks
Very important component carbonaceous material in the shale rocks. This is the organic material that usually occurring in the rocks as kerogen (a mixture of organic compounds with high molecular weight). Although kerogen does not form more than about 1% of all the shales, the vast majority of kerogen is in mudstones. Shales that are rich in organic matter (>5%) are known as black shales. Black color is given to these rocks by organic matter. Organic matter should be decomposed in normal conditions by bacteria, but high productivity, rapid deposition and burial or lack of oxygen may preserve it. Pyrite is a common sulfide mineral in black shales. Organic matter and pyrite occur together in the same rock because both need oxygen-free conditions for their formation.
Shales that are subject to heat and pressure of metamorphismalter into a hard, fissile, metamorphic rock known as slate. With continuedincrease in metamorphic grade the sequence is phyllite, then schist and finallygneiss.
The process of illitization (smectite is transformed toillite) is a major change that takes place in mudstones during the diagenesis.Illitization consumes potassium (provided usually by detrital K-feldspar) andliberates iron, magnesium and calcium, which can be used by the other formingminerals like chlorite and calcite. The temperature range of illitization isabout 50-100ÂC3. Kaolinite content also decreases with increased burial depth.Kaolinite forms in hot and humid climate. The drier temperate climate tends tofavor smectite. The reason is that lots of precipitation washes soluble ionsout of the rock, while drier climate does not accomplish this task soeffectively. Kaolinite is favored in humid climate because it contains onlyaluminum in addition to silica and water. Aluminum is highly residual while theconstituents of smectite (magnesium and calcium, in addition to aluminum andiron) get carried away more easily.
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