Пешчар

Пешчар (ијек. пјешчар) или пешчењак (ијек. пјешчењак) (енгл. Sandstone, франц. Grès, нем. Sandstein, рус. Песчаник) везана je седиментна стена (везани седимент) изграђена претежно од кластичних зрна величине 2–0,05 mm. Према крупноћи везаних зрна (гранула) спада у псамите. У погледу минералног састава пешчари су обично изграђени доминантно од зрна кварца, љуспица мусковита и фелдспата, а потом и циркона, апатита, магнетита, граната, турмалина. Везиво може бити калцијумкарбонатно (вапновито), лапоровито, глиновито, доломитско, силицијско, гвожђевито(лимонит), битуминозно (органског порекла) и др.

Неки пешчари су отпорни на утицај атмосферилија. Овакве стене су обично погодне као геолошки грађевински материјал – нпр. црква Светог Марка у Београду изграђена је од пешчара који у себи садржи љуспице мусковита. Због велике тврдоће (висок садржај кварца) појединачних зрна и подношења високих температура погодни су за оштрење сечива. Други народни назив за пешчар је тоциљњак, што означава камен од кога се праве тоцила.

Rock formations that are primarily composed of sandstone usually allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs.

Quartz-bearing sandstone can be changed into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.

Порекло

Sandstones are clastic in origin (as opposed to either organic, like chalk and coal, or chemical, like gypsum and jasper). The silicate sand grains from which they form are the product of physical and chemical weathering of bedrock. Weathering and erosion are most rapid in areas of high relief, such as volcanic arcs, areas of continental rifting, and orogenic belts.

Eroded sand is transported by rivers or by the wind from its source areas to depositional environments where tectonics has created accommodation space for sediments to accumulate. Forearc basins tend to accumulate sand rich in lithic grains and plagioclase. Intracontinental basins and grabens along continental margins are also common environments for deposition of sand.

As sediments continue to accumulate in the depositional environment, older sand is buried by younger sediments, and it undergoes diagenesis. This mostly consists of compaction and lithification of the sand. Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in the sands, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place.

Compaction takes place as the sand comes under increasing pressure from overlying sediments. Sediment grains move into more compact arrangements, ductile grains (such as mica grains) are deformed, and pore space is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.

Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces.

Mechanical compaction takes place primarily at depths less than 1.000 m (3.300 ft). Chemical compaction continues to depths of 2.000 m (6.600 ft), and most cementation takes place at depths of 2.000—5.000 m (6.600—16.400 ft).

Unroofing of buried sandstone is accompanied by telogenesis, the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sandstone, such as dissolution of some of the cement to produce secondary porosity.

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  Alcove in the Navajo Sandstone
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  Kokh-type tombs cut into the multicoloured sandstone of Petra
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  Sand grains of quartz with hematite coating providing an orange colour

Компоненте

Оквирна зрна

Paradise Quarry, Sydney, Australia

Grus sand and the granitoid from which it is derived

Framework grains are sand-sized (00.625-to-2 mm (24,606-to-0,079 in) diameter) detrital fragments that make up the bulk of a sandstone. Most framework grains are composed of quartz or feldspar, which are the common minerals most resistant minerals to weathering processes at the Earth's surface, as seen in the Goldich dissolution series. Framework grains can be classified into several different categories based on their mineral composition:

• Quartz framework grains are the dominant minerals in most clastic sedimentary rocks; this is because they have exceptional physical properties, such as hardness and chemical stability. These physical properties allow the quartz grains to survive multiple recycling events, while also allowing the grains to display some degree of rounding. Quartz grains evolve from plutonic rock, which are felsic in origin and also from older sandstones that have been recycled.
• Feldspathic framework grains are commonly the second most abundant mineral in sandstones. Feldspar can be divided into alkali feldspars and plagioclase feldspars, which can be distinguished under a petrographic microscope.

• Alkali feldspar range in chemical composition from KAlSi₃O₈ to NaAlSi₃O₈.

• Plagioclase feldspar range in composition from NaAlSi₃O₈ to CaAl₂Si₂O₈.

Photomicrograph of a volcanic sand grain; upper picture is plane-polarised light, bottom picture is cross-polarised light, scale box at left-centre is 0.25 millimeter. This type of grain would be a main component of a lithic sandstone.

• Lithic framework grains (also called lithic fragments or lithic clasts) are pieces of ancient source rock that have yet to weather away to individual mineral grains. Lithic fragments can be any fine-grained or coarse-grained igneous, metamorphic, or sedimentary rock, although the most common lithic fragments found in sedimentary rocks are clasts of volcanic rocks.
• Accessory minerals are all other mineral grains in a sandstone. These minerals usually make up just a small percentage of the grains in a sandstone. Common accessory minerals include micas (muscovite and biotite), olivine, pyroxene, and corundum. Many of these accessory grains are more dense than the silicates that make up the bulk of the rock. These heavy minerals are commonly resistant to weathering and can be used as an indicator of sandstone maturity through the ZTR index. Common heavy minerals include zircon, tourmaline, rutile (hence ZTR), garnet, magnetite, or other dense, resistant minerals derived from the source rock.

Матрица

Matrix is very fine material, which is present within interstitial pore space between the framework grains. The nature of the matrix within the interstitial pore space results in a twofold classification:

• Arenites are texturally clean sandstones that are free of or have very little matrix.
• Wackes are texturally dirty sandstones that have a significant amount of matrix.

Цемент

Cement is what binds the siliciclastic framework grains together. Cement is a secondary mineral that forms after deposition and during burial of the sandstone. These cementing materials may be either silicate minerals or non-silicate minerals, such as calcite.

• Silica cement can consist of either quartz or opal minerals. Quartz is the most common silicate mineral that acts as cement. In sandstone where there is silica cement present, the quartz grains are attached to cement, which creates a rim around the quartz grain called overgrowth. The overgrowth retains the same crystallographic continuity of quartz framework grain that is being cemented. Opal cement is found in sandstones that are rich in volcanogenic materials, and very rarely is in other sandstones.
• Calcite cement is the most common carbonate cement. Calcite cement is an assortment of smaller calcite crystals. The cement adheres to the framework grains, cementing the framework grains together.
• Other minerals that act as cements include: hematite, limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite minerals.

Sandstone that becomes depleted of its cement binder through weathering gradually becomes friable and unstable. This process can be somewhat reversed by the application of tetraethyl orthosilicate (Si(OC₂H₅)₄) which will deposit amorphous silicon dioxide between the sand grains. The reaction is as follows.

Si(OC₂H₅)₄ (l) + 2 H₂O (l) → SiO₂ (s) + 4 C₂H₅OH (g)

Слике

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  Пешчар са Тоцилина, између Такова и Бруснице, узорак 20х15cm
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  Пешчар са пругама од оксида гвожђа
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  Обрађен узорак пешчара
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  Опатија Арбро, пример обојености пешчара
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  Arches National Park – пешчарски крајолик
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  Стена Парус у Русији.
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  Стенска формација пешчара у Израелу
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  Савремена вила са фасадом од пешчара
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  фосилне шкољке у пешчару
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  Црвени пешчар у Доњем Антелоп Кањону, Аризона
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  Пешчар са великом количином кварца
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  Црвени пешчар пермске старости, Стара планина

Види још

• Списак минерала
• Списак стена

Литература

• Ђорђевић В., Ђорђевић П., Миловановић Д. 1991. Основи петрологије. Београд: Наука
• Folk, R.L., 1965, Petrology of sedimentary rocks PDF version. Austin: Hemphill's Bookstore. 2nd ed. 1981, ISBN 0-914696-14-9.
• Pettijohn F. J., P.E. Potter and R. Siever, 1987, Sand and sandstone, 2nd ed. Springer-Verlag. ISBN 0-387-96350-2.
• Scholle, P.A., 1978, A Color illustrated guide to constituents, textures, cements, and porosities of sandstones and associated rocks, American Association of Petroleum Geologists Memoir no. 28. ISBN 0-89181-304-7.
• Scholle, P.A., and D. Spearing, 1982, Sandstone depositional environments: clastic terrigenous sediments , American Association of Petroleum Geologists Memoir no. 31. ISBN 0-89181-307-1.
• USGS Minerals Yearbook: Stone, Dimension, Thomas P. Dolley, U.S. Dept. of the Interior, 2005 (format: PDF).
• Webb, Jonathan. Sandstone shapes 'forged by gravity' (July 2014), BBC
• Jackson, J. (1997). Glossary of Geology. Alexandria, VA: American Geological Institute ISBN 3-540-27951-2

Спољашње везе

• Човек и камен: Панон - пешчар (РТС Образовно-научни програм - Званични канал)