Feldspar

Feldspars (KAlSi3O8NaAlSi3O8CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.[2][3]

Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.[4] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite.[5] Feldspars are also found in many types of sedimentary rocks.[6]

Feldspar group
Compositional phase diagram of the different minerals that constitute the feldspar solid solution.

Etymology

The name feldspar derives from the German Feldspat, a compound of the words Feld, "field", and Spat meaning "a rock that does not contain ore".[7] The change from Spat to -spar was influenced by the English word spar,[8] meaning a non-opaque mineral with good cleavage.[9] Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has fallen out of use.

Compositions

This group of minerals consists of tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

Solid solutions between K-feldspar and albite are called "alkali feldspar".[10] Solid solutions between albite and anorthite are called "plagioclase",[10] or more properly "plagioclase feldspar". Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Albite is considered both a plagioclase and alkali feldspar.

Alkali feldspars

Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminum, or silicon; and those where potassium is replaced by barium. The first of these include:

Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.[14] Sanidine is stable at the highest temperatures, and microcline at the lowest.[11][12] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.[15] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Barium feldspars

Barium feldspars are also considered alkali feldspars. Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. The barium feldspars are monoclinic and include the following:

Plagioclase feldspars

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae. The specific gravity in the plagioclase series increases from albite (2.62 g/cm3) to anorthite (2.72–2.75 g/cm3).

Weathering

Chemical weathering of feldspars results in the formation of clay minerals[18] such as illite and kaolinite.

Production and uses

About 20 million tonnes of feldspar were produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt).[19]

Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics (including electrical insulators, sanitaryware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.[20]

In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, and luminescence dating.

In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.[21]

Images

Feldspar-Group-170604

Specimen of rare plumbian (lead-rich) feldspar

Beryl-Schorl-Feldspar-Group-288077

Perched on crystallized, white feldspar is a dramatic, upright 4.0 cm aquamarine

Feldspar-Group-Moonstone-36971

Feldspar and moonstone, from Sonora, Mexico

Schorl-Feldspar-Group-49985

Schorl crystal on a cluster of euhedral feldspar crystals

PIA16217-MarsCuriosityRover-1stXRayView-20121017

First X-ray view of Martian soil—feldspar, pyroxenes, olivine revealed (Curiosity rover at "Rocknest", October 17, 2012).[22]

Lunar Ferroan Anorthosite (60025)

Lunar ferrous anorthosite #60025 (plagioclase feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, D.C.

See also

References

  •  This article incorporates public domain material from the United States Geological Survey document: "Feldspar and nepheline syenite" (PDF).
  1. ^ "Feldspar". Gemology Online. Retrieved 8 November 2012.
  2. ^ Anderson, Robert S.; Anderson, Suzanne P. (2010). Geomorphology: The Mechanics and Chemistry of Landscapes. Cambridge University Press. p. 187.
  3. ^ Rudnick, R. L.; Gao, S. (2003). "Composition of the Continental Crust". In Holland, H. D.; Turekian, K. K. Treatise on Geochemistry. Treatise on Geochemistry. 3. New York: Elsevier Science. pp. 1–64. Bibcode:2003TrGeo...3....1R. doi:10.1016/B0-08-043751-6/03016-4. ISBN 978-0-08-043751-4.
  4. ^ "Metamorphic Rocks." Metamorphic Rocks Information Archived 2007-07-01 at the Wayback Machine. Retrieved on July 18, 2007
  5. ^ Blatt, Harvey and Tracy, Robert J. (1996) Petrology, Freeman, 2nd ed., pp. 206–210 ISBN 0-7167-2438-3
  6. ^ "Weathering and Sedimentary Rocks." Geology. Archived 2007-07-21 at WebCite Retrieved on July 18, 2007.
  7. ^ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
  8. ^ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
  9. ^ "spar". Oxford English Dictionary. Oxford Dictionaries. Retrieved 13 January 2018.
  10. ^ a b c d e Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007.
  11. ^ a b "The Mineral Orthoclase". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  12. ^ a b "Sanidine Feldspar". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  13. ^ "Microcline Feldspar". Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
  14. ^ Klein, Cornelis and Cornelius S. Hurlbut, Jr. Handbook of Mineralogy, Wiley, pp. 446-449 (Fig. 11-95 ISBN 0-471-80580-7
  15. ^ Ralph, Jolyon and Chou, Ida. "Perthite". Perthite Profile on mindat.org. Retrieved on February 8, 2008.
  16. ^ Celsian–orthoclase series on Mindat.org.
  17. ^ Celsian–hyalophane series on Mindat.org.
  18. ^ Nelson, Stephen A. (Fall 2008). "Weathering & Clay Minerals". Professor's lecture notes (EENS 211, Mineralogy). Tulane University. Retrieved 2008-11-13.
  19. ^ Feldspar, USGS Mineral Commodity Summaries 2011
  20. ^ Apodaca, Lori E. Feldspar and nepheline syenite, USGS 2008 Minerals Yearbook
  21. ^ Nasa's Curiosity rover finds 'unusual rock'. (12 October 2012) BBC News.
  22. ^ Brown, Dwayne (October 30, 2012). "NASA Rover's First Soil Studies Help Fingerprint Martian Minerals". NASA. Retrieved October 31, 2012.

Further reading

  • Bonewitz, Ronald Louis (2005). Rock and Gem. New York: DK Publishing. ISBN 978-0-7566-3342-4.

External links

  • Media related to Feldspar at Wikimedia Commons
Albite

Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence its name from Latin albus. It is a common constituent in felsic rocks.

Albite crystallizes with triclinic pinacoidal forms. Its specific gravity is about 2.62 and it has a Mohs hardness of 6–6.5. Albite almost always exhibits crystal twinning often as minute parallel striations on the crystal face. Albite often occurs as fine parallel segregations alternating with pink microcline in perthite as a result of exolution on cooling.

There are two variants of albite, which are referred to as low albite and high albite; the latter is also known as analbite. Although both variants are triclinic, they differ in the volume of their unit cell, which is slightly larger for the high form. The high form can be produced from the low form by heating above c. 750 °C (1382 °F). Upon further heating to more than c. 1050 °C the crystal symmetry changes from triclinic to monoclinic; this variant is also known as monalbite. Albite melts at 1100-1120 °C.It occurs in granitic and pegmatite masses, in some hydrothermal vein deposits and forms part of the typical greenschist metamorphic facies for rocks of originally basaltic composition.

It was first reported in 1815 for an occurrence in Finnbo, Falun, Dalarna, Sweden.It is used as a gemstone.

Amazonite

Amazonite also known as Amazonstone is a green tectosilicate mineral, a variety of the potassium feldspar called microcline. Its chemical formula is (KAlSi3O8), which is polymorphic to Orthoclase.

Its name is taken from that of the Amazon River, from which certain green stones were formerly obtained, but it is doubtful whether green feldspar occurs in the Amazon area. Although it has been used for over two thousand years, as attested by archaeological finds in Egypt and Mesopotamia, no ancient or medieval authority mentions it. It was first described as a distinct mineral only in the 18th century.Because of its bright green color when polished, amazonite is sometimes cut and used as a cheap gemstone, although it is easily fractured, and loses its gloss due to its softness.

Andesite

For the extinct cephalopod genus, see Andesites.

Andesite ( or ) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts they are a major component of the Martian crust. The name andesite is derived from the Andes mountain range.

Arkose

Arkose ( ) is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose.

Quartz is commonly the dominant mineral component, and some mica is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts of calcite cement, which causes it to fizz slightly in dilute hydrochloric acid; sometimes the cement also contains iron oxide.

Arkose is typically grey to reddish in colour. The sand grains making up an arkose may range from fine to very coarse, but tend toward the coarser end of the scale. Fossils are rare in arkose, due to the depositional processes that form it, although bedding is frequently visible.

Arkose is generally formed from the weathering of feldspar-rich igneous or metamorphic, most commonly granitic, rocks, which are primarily composed of quartz and feldspar (called 'grus' as a sand). These sediments must be deposited rapidly and/or in a cold or arid environment such that the feldspar does not undergo significant chemical weathering and decomposition; therefore arkose is designated a texturally immature sedimentary rock. Arkose is often associated with conglomerate deposits sourced from granitic terrain and is often found above unconformities in the immediate vicinity of granite terrains.

The central Australian monolith Uluru (Ayers Rock) is composed of late Neoproterozoic/Cambrian arkose, deposited in the Amadeus Basin.

Felsic

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Granite

Granite ( ) is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar.

The term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids. The extrusive igneous rock equivalent of granite is rhyolite.

Granite is nearly always massive (i.e., lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3 (165 and 172 lb/cu ft), its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3–6·1019 Pa·s.The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F); it is strongly reduced in the presence of water, down to 650 °C at a few kBar pressure.Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present.

Granodiorite

Granodiorite ( ) is a phaneritic-textured intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. According to the QAPF diagram, granodiorite has a greater than 20% quartz by volume, and between 65% to 90% of the feldspar is plagioclase. A greater amount of plagioclase would designate the rock as tonalite.

Granodiorite is felsic to intermediate in composition. It is the intrusive igneous equivalent of the extrusive igneous dacite. It contains a large amount of sodium (Na) and calcium (Ca) rich plagioclase, potassium feldspar, quartz, and minor amounts of muscovite mica as the lighter colored mineral components. Biotite and amphiboles often in the form of hornblende are more abundant in granodiorite than in granite, giving it a more distinct two-toned or overall darker appearance. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals. Minor amounts of oxide minerals such as magnetite, ilmenite, and ulvöspinel, as well as some sulfide minerals may also be present.

Labradorite

Labradorite ((Ca, Na)(Al, Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality. (The third direction is poor.) It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

List of rock types

The following is a list of rock types recognized by geologists. There is no agreed number of specific types of rocks. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types. Additionally, different classification systems exist for each major type of rock. There are three major types of rock: igneous, sedimentary, and metamorphic. They are all identified by their texture, streak, and location, among other factors.

Microcline

Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures than orthoclase. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, white, pale-yellow, brick-red, or green; it is generally characterized by cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.

The chemical compound name is potassium aluminium silicate, and it is known as E number reference E555.

Moonstone (gemstone)

Moonstone is a sodium potassium aluminium silicate ((Na,K)AlSi3O8) of the feldspar group that displays a pearly and opalescent schiller. An alternative name is hecatolite.

Mount Marcy

Mount Marcy (Mohawk: Tewawe’éstha) is the highest point in New York, with an elevation of 5,343 feet (1,629 m). and an isolation of 130 mile. It is located in the Town of Keene in Essex County. The mountain is in the heart of the Adirondack High Peaks Region of the High Peaks Wilderness Area. Its stature and expansive views make it a popular destination for hikers, who crowd its summit in the summer months.

Lake Tear of the Clouds, at the col between Mt. Marcy and Mt. Skylight, is often cited as the highest source of the Hudson River, via Feldspar Brook and the Opalescent River, even though the main stem of the Opalescent River has as its source a higher point two miles north of Lake of the Clouds, and that stem is a mile longer than Feldspar Brook.

Orthoclase

Orthoclase, or orthoclase feldspar (endmember formula KAlSi3O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. It is a type of potassium feldspar, also known as K-feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.

Pegmatite

A pegmatite is an igneous rock, formed underground, with interlocking crystals usually larger than 2.5 cm in size (1 in). Most pegmatites are found in sheets of rock (dikes and veins) near large masses of igneous rocks called batholiths.The word pegmatite derives from Homeric Greek, πήγνυμι (pegnymi), which means “to bind together”, in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite.Most pegmatites are composed of quartz, feldspar and mica, having a similar silicic composition as granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids and other unusual minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.

Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 metres (33 ft) long have been found, and many of the world's largest crystals were found within pegmatites. These include spodumene, microcline, beryl, and tourmaline.Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye. The product of pegmatite decomposition is euclase.

Plagioclase

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Ancient Greek for "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.

Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the highlands of the Earth's moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.

Rhyodacite

Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium-rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate-colored matrix.

Rhyodacite is a high silica rock containing 20% to 60% quartz with the remaining constituents being mostly feldspar. The feldspar is a mix of alkaline feldspar and plagioclase, with plagioclase forming 35% to 65% of the mix.

Rhyodacite often exists as explosive pyroclastic volcanic deposits.

Rhyodacite lava flows occur, for example, in northwestern Ferry County (Washington), and at An Sgùrr on the island of Eigg in Scotland.

Sandstone

Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) mineral particles or rock fragments.

Most sandstone is composed of quartz or feldspar (both silicates) because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in Bowen's reaction series. Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.

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. Fine-grained aquifers, such as sandstones, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity.

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

Sunstone

Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a spangled appearance. It has been found in Southern Norway, Sweden and in various United States localities.

Teregova mine

The Teregova mine is a large open pit mine in the western Romania in Caraş-Severin County, 85 km south-east of Reşiţa and 571 km north-west of the capital, Bucharest. Teregova represents one of the largest feldspar reserves in Romania having estimated reserves of 2,200,000 tonnes of ore.

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