Finland the Republic of Finland, is a country in Northern Europe bordering the Baltic Sea, Gulf of Bothnia, Gulf of Finland, between Norway to the north, Sweden to the northwest, Russia to the east. Finland is situated in the geographical region of Fennoscandia; the capital and largest city is Helsinki. Other major cities are Espoo, Tampere and Turku. Finland's population is 5.52 million, the majority of the population is concentrated in the southern region. 88.7% of the population is Finnish and speaks Finnish, a Uralic language unrelated to the Scandinavian languages. Finland is the eighth-largest country in Europe and the most sparsely populated country in the European Union; the sovereign state is a parliamentary republic with a central government based in the capital city of Helsinki, local governments in 311 municipalities, one autonomous region, the Åland Islands. Over 1.4 million people live in the Greater Helsinki metropolitan area, which produces one third of the country's GDP. Finland was inhabited when the last ice age ended 9000 BCE.
The first settlers left behind artefacts that present characteristics shared with those found in Estonia and Norway. The earliest people were hunter-gatherers; the first pottery appeared in 5200 BCE. The arrival of the Corded Ware culture in southern coastal Finland between 3000 and 2500 BCE may have coincided with the start of agriculture; the Bronze Age and Iron Age were characterised by extensive contacts with other cultures in the Fennoscandian and Baltic regions and the sedentary farming inhabitation increased towards the end of Iron Age. At the time Finland had three main cultural areas – Southwest Finland and Karelia – as reflected in contemporary jewellery. From the late 13th century, Finland became an integral part of Sweden through the Northern Crusades and the Swedish part-colonisation of coastal Finland, a legacy reflected in the prevalence of the Swedish language and its official status. In 1809, Finland was incorporated into the Russian Empire as the autonomous Grand Duchy of Finland.
In 1906, Finland became the first European state to grant all adult citizens the right to vote, the first in the world to give all adult citizens the right to run for public office. Following the 1917 Russian Revolution, Finland declared itself independent. In 1918, the fledgling state was divided by civil war, with the Bolshevik-leaning Red Guard supported by the new Soviet Russia, fighting the White Guard, supported by the German Empire. After a brief attempt to establish a kingdom, the country became a republic. During World War II, the Soviet Union sought to occupy Finland, with Finland losing parts of Karelia, Kuusamo and some islands, but retaining their independence. Finland established an official policy of neutrality; the Finno-Soviet Treaty of 1948 gave the Soviet Union some leverage in Finnish domestic politics during the Cold War era. Finland joined the OECD in 1969, the NATO Partnership for Peace in 1994, the European Union in 1995, the Euro-Atlantic Partnership Council in 1997, the Eurozone at its inception, in 1999.
Finland was a relative latecomer to industrialisation, remaining a agrarian country until the 1950s. After World War II, the Soviet Union demanded war reparations from Finland not only in money but in material, such as ships and machinery; this forced Finland to industrialise. It developed an advanced economy while building an extensive welfare state based on the Nordic model, resulting in widespread prosperity and one of the highest per capita incomes in the world. Finland is a top performer in numerous metrics of national performance, including education, economic competitiveness, civil liberties, quality of life, human development. In 2015, Finland was ranked first in the World Human Capital and the Press Freedom Index and as the most stable country in the world during 2011–2016 in the Fragile States Index, second in the Global Gender Gap Report, it ranked first on the World Happiness Report report for 2018 and 2019. A large majority of Finns are members of the Evangelical Lutheran Church, freedom of religion is guaranteed under the Finnish Constitution.
The earliest written appearance of the name Finland is thought to be on three runestones. Two have the inscription finlonti; the third was found in Gotland. It dates back to the 13th century; the name can be assumed to be related to the tribe name Finns, mentioned at first known time AD 98. The name Suomi has uncertain origins, but a candidate for a source is the Proto-Baltic word *źemē, meaning "land". In addition to the close relatives of Finnish, this name is used in the Baltic languages Latvian and Lithuanian. Alternatively, the Indo-European word * gʰm-on "man" has been suggested; the word referred only to the province of Finland Proper, to the northern coast of Gulf of Finland, with northern regions such as Ostrobothnia still sometimes being excluded until later. Earlier theories suggested derivation from suomaa or suoniemi, but these are now considered outdated; some have suggested common etymology with saame and Häme, but that theory is uncertain
Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in small concentrations. Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; the volcanic equivalent of syenite is trachyte. The feldspar component of syenite is predominantly alkaline in character. Plagioclase feldspars may be present in small proportions, less than 10%; such feldspars are interleaved as perthitic components of the rock. When ferromagnesian minerals are present in syenite at all, they occur in the form of hornblende and clinopyroxene. Biotite is rare, because in a syenite magma the formation of feldspar consumes nearly all the aluminium, however less Al rich phyllosilicates may be included such as annite. Other common accessory minerals are apatite, titanite and opaques. Most syenites are either peralkaline with high proportions of alkali elements relative to aluminum, or peraluminous with a higher concentration of aluminum relative to alkali and earth-alkali elements.
Syenites are products of alkaline igneous activity formed in thick continental crustal areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a granitic or igneous protolith to a low degree of partial melting; this is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce plagioclase, hence a granite, adamellite or tonalite. At low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or analcime. Conversely in certain conditions, large volumes of anorthite crystals may precipitate from molten magma in a cumulate process as it cools; this leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state. Syenite is not a common rock. Regions where it occurs in significant quantities include the following.
In the Kola Peninsula of Russia two giant nepheline syenite bodies exists making up the Lovozero Massif and the Khibiny Mountains. These syenites are part of the Kola Alkaline Province. In North America syenite occurs in Montana. Regions in New England have sizable amounts, in New York syenite gneisses occur; the "great syenite dyke" extends from Hanging Rock, South Carolina through Taxahaw, South Carolina to the Brewer and Edgeworth mine in Chesterfield, South Carolina. Syenite clasts containing fluorescent sodalite were found on a beach in Michigan in 2017, their discoverer Erik Rintamaki finding that they glowed under ultraviolet light and naming them "Yooperlite". Rintamaki's discovery was made public and verified in May 2018. In Europe syenite may be found in parts of Switzerland, Norway, Portugal, in Plovdiv, Bulgaria and in Ditrău, Romania. In Africa there are syenite formations in Aswan, in Malawi in the Mulanje Mountain Forest Reserve. Syenite rock was used to make the Quay with Sphinxes.
In Australia syenite occurs as small intrusive bodies in nearly every state. In New South Wales, a large syenite intruded during the breakup of Gondwana in the Cretaceous. Instead of the usual rock syenite, some of the more important events in New England, Montana, New York, Germany, Plovdiv, Bulgaria and Romania; the Malvern Hills, on the border between the counties of Herefordshire and Worcestershire United Kingdom are formed of syenite. Paatusoq and Kangerluluk fjords in southeastern Greenland, where a bay within the latter and a headland are named after the rock; the term syenite was applied to hornblende granite like that of Syene in Egypt, from which the name is derived. Episyenite is a term used in petrology to describe the depletion of silicon dioxide in rock rich in silicon dioxide. A process that results in depletion is termed episyenitization; the term refers only to the macroscopic effect of relative depletion in a rock. Many different metamorphic processes can lead to episyenitization.
For example: chemical components in a stagnant melt can diffuse under the influence of chemical potential gradients that cause their segregation from low- SiO2 components when the melt begins to solidify a SiO2-undersaturated fluid may dissolve quartz from rock and remove it by advection, thus leaving the parent rock depleted of silica. A marginally molten rock mass may retain its unmolten silica-rich components, while the molten, silica-depleted fluid cools to form a syenite. on beginning to cool, a molten silica-rich melt might precipitate its silica-containing components, leaving the silica-depleted melt to form a syenite afterwards. List of rock types E. Wm. Heinrich. Microscopic Petrography, McGraw-Hill, 1956
A mineral is, broadly speaking, a solid chemical compound that occurs in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are excluded, but some minerals are biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings synthesize inorganic minerals that occur in rocks. In geology and mineralogy, the term "mineral" is reserved for mineral species: crystalline compounds with a well-defined chemical composition and a specific crystal structure. Minerals without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur with different crystal structures, each structure is considered different mineral species. Thus, for example and stishovite are two different minerals consisting of the same compound, silicon dioxide; the International Mineralogical Association is the world's premier standard body for the definition and nomenclature of mineral species.
As of November 2018, the IMA recognizes 5,413 official mineral species. Out of more than 5,500 proposed or traditional ones; the chemical composition of a named mineral species may vary somewhat by the inclusion of small amounts of impurities. Specific varieties of a species sometimes have official names of their own. For example, amethyst is a purple variety of the mineral species quartz; some mineral species can have variable proportions of two or more chemical elements that occupy equivalent positions in the mineral's structure. Sometimes a mineral with variable composition is split into separate species, more or less arbitrarily, forming a mineral group. Besides the essential chemical composition and crystal structure, the description of a mineral species includes its common physical properties such as habit, lustre, colour, tenacity, fracture, specific gravity, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid. Minerals are classified by key chemical constituents.
Silicate minerals comprise 90% of the Earth's crust. Other important mineral groups include the native elements, oxides, carbonates and phosphates. One definition of a mineral encompasses the following criteria: Formed by a natural process. Stable or metastable at room temperature. In the simplest sense, this means. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39 °C, water ice, solid only below 0 °C. Modern advances have included extensive study of liquid crystals, which extensively involve mineralogy. Represented by a chemical formula. Minerals are chemical compounds, as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution. For example, the olivine group is described by the variable formula 2SiO4, a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable composition, such as the sulfide mackinawite, 9S8, a ferrous sulfide, but has a significant nickel impurity, reflected in its formula.
Ordered atomic arrangement. This means crystalline. An ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form and cleavage. There have been several recent proposals to classify amorphous substances as minerals; the formal definition of a mineral approved by the IMA in 1995: "A mineral is an element or chemical compound, crystalline and, formed as a result of geological processes." Abiogenic. Biogenic substances are explicitly excluded by the IMA: "Biogenic substances are chemical compounds produced by biological processes without a geological component and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound the product can be accepted as a mineral."The first three general characteristics are less debated than the last two. Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the Strunz classification schemes.
The organic class includes a rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. According to these new r
Granite is a common type of felsic intrusive igneous rock, granular and phaneritic in texture. Granites can be predominantly white, pink, or gray 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. Speaking, granite is an igneous rock with between 20% and 60% quartz by volume, at least 35% of the total feldspar consisting of alkali feldspar, although 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 consist of feldspar, quartz and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole peppering the lighter color minerals; some individual crystals 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 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 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, its compressive strength lies above 200 MPa, its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar on the A-Q-P half of the diagram.
True granite contains both alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite; when a granitoid contains less than 10% orthoclase, it is called tonalite. A granite containing both muscovite and biotite micas is called two-mica granite. Two-mica granites are high in potassium and low in plagioclase, are S-type granites or A-type granites. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: Granite containing rock is distributed throughout the continental crust. Much of it was intruded during the Precambrian age. Outcrops of granite tend to form rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite occurs as small, less than 100 km2 stock masses and in batholiths that are associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are associated with the margins of granitic intrusions.
In some locations coarse-grained pegmatite masses occur with granite. Granite is more common in continental crust than in oceanic crust, they are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granitoids have crystallized from felsic magmas that have compositions near a eutectic point. Magmas are composed of minerals in variable abundances. Traditionally, magmatic minerals are crystallized from the melts that have separated from their parental rocks and thus are evolved because of igneous differentiation. If a granite has a cooling process, it has the potential to form larger crystals. There are peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling.
Anatectic melts are produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. The composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, titanium and sodium, enrich the melt in potassium and silicon – alkali feldspar and quartz, are two of the defining constituents of granite; this process operates regardless of the origin of parental magmas to granites, regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was; the final texture and composition of a granite are distinctive as to its parental rock. For instance, a granite, derived from partial melting of meta
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava; the magma can be crust. The melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, orogens, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are geologically important because: their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted.
In terms of modes of occurrence, igneous rocks can be either extrusive. Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock; the mineral grains in such rocks can be identified with the naked eye. Intrusive rocks can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths and dikes; when the magma solidifies within the earth's crust, it cools forming coarse textured rocks, such as granite, gabbro, or diorite. The central cores of major mountain ranges consist of intrusive igneous rocks granite; when exposed by erosion, these cores may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic rocks and are coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are medium-grained.
Hypabyssal rocks are less common than plutonic or volcanic rocks and form dikes, laccoliths, lopoliths, or phacoliths. Extrusive igneous rocks known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive solidify quicker than intrusive igneous rocks, they are formed by the cooling of molten magma on the earth's surface. The magma, brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth and fine-grained. Basalt is lava plateaus; some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example; the molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises; when magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary: 15% hotspot: 12%. Magma that erupts from a volcano behaves according to its viscosity, determined by temperature, crystal content and the amount of silica. High-temperature magma, most of, basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash and lava, may have a viscosity similar to thick, cold molasses or rubber when erupted. Felsic magma, such as rhyolite, is erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma erupt explosively, rhyolitic lava flows are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuff and ignimbrite. Fine volcanic ash is erupted and forms ash tuff deposits, which ca
A pegmatite is an igneous rock, formed underground, with interlocking crystals larger than 2.5 cm in size. Most pegmatites are found in sheets of rock near large masses of igneous rocks called batholiths; the word pegmatite derives from Homeric Greek, πήγνυμι, 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 and mica, having a similar silicic composition as granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene 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 over 5 cm in size. Individual crystals over 10 metres long have been found, many of the world's largest crystals were found within pegmatites; these include spodumene, microcline and tourmaline. 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, when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite shows gigantic perthitic texture visible to the naked eye; the product of pegmatite decomposition is euclase. The single feature, diagnostic to all pegmatites is their large size crystal components. Pegmatite bodies are of minor size compared to typical intrusive rock bodies. Pegmatite body size is on the order of magnitude of one to a few hundred meters. Compared to typical igneous rocks they are rather inhomogeneous and may show zones with different mineral assemblages. Crystal size and mineral assemblages are oriented parallel to the wall rock or concentric for pegmatite lenses; the number of crystal nuclei in pegmatites must be low and the ability of the necessary chemical components needed for crystal growth to migrate to the crystal surfaces must be enhanced to allow gigantic crystals to grow in pegmatites.
Thus, the possible growth mechanisms in a wide variety of known pegmatites may involve a combination of the following processes. The mineralogy of a pegmatite is in most cases dominated by some form of feldspar with mica and with quartz, being altogether "granitic" in character. Beyond that, pegmatite may include most minerals associated with granite and granite-associated hydrothermal systems, granite-associated mineralisation styles, for example greisens, somewhat with skarn associated mineralisation, it is however impossible to quantify the mineralogy of pegmatite in simple terms because of their varied mineralogy and difficulty in estimating the modal abundance of mineral species which are of only a trace amount. This is because of the difficulty in counting and sampling mineral grains in a rock which may have crystals from centimeters to meters across. Garnet almandine or spessartine, is a common mineral within pegmatites intruding mafic and carbonate-bearing sequences. Pegmatites associated with granitic domes within the Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red and brown almandine garnet.
Tantalum and niobium minerals are found in association with spodumene, tourmaline, cassiterite in the massive Greenbushes Pegmatite in the Yilgarn Craton of Western Australia, considered a typical metamorphic pegmatite unassociated with granite. Syenite pegmatites contain large feldspathoid crystals instead. Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Bulk samples of some 50–60 kg of rock must be crushed to obtain a meaningful and repeatable result. Hence, pegmatite is characterised by sampling the individual minerals that compose the pegmatite, comparisons are made according to mineral chemistry. Geochemically, pegmatites have major element compositions approximating "granite", when found in association with granitic plutons it is that a pegmatite dike will have a different trace element composition with greater enrichment in large-ion lithophile elements, beryllium, aluminium and lithium, thorium, cesium, et cetera. Enrichment in the unusual trace elements will result in crystallisation of unusual and rare minerals such as beryl, columbite, zinnwaldite and so forth.
In most cases, there is no particular genetic significance to the presence of rare mineralogy within a pegmatite, however it is possible to see some causative and genetic links between, tourmaline-bearing granite dikes and tourmaline-bearing
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY2O6 where X represents calcium, iron or magnesium and more zinc, manganese or lithium and Y represents ions of smaller size, such as chromium, iron, cobalt, scandium, vanadium or iron. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes, they share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that cystallize in the orthorhombic system are known as orthopyroxenes; the name pyroxene is derived from the Ancient Greek words for stranger. Pyroxenes were so named because of their presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic glass. However, they are early-forming minerals that crystallized before the lava erupted.
The upper mantle of Earth is composed of olivine and pyroxene. Pyroxene and feldspar are the major minerals in gabbro; the chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X site, the Y site, the tetrahedral T site. Cations in Y site are bound to 6 oxygens in octahedral coordination. Cations in the X site can be coordinated depending on the cation size. Twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 used names have been discarded. A typical pyroxene has silicon in the tetrahedral site and predominately ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT2O6; the names of the common calcium–iron–magnesium pyroxenes are defined in the'pyroxene quadrilateral' shown in Figure 2.
The enstatite-ferrosilite series contain up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite. Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite only crystallises in the monoclinic system. There is not complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. This leads to a miscibility gap between augite compositions. There is an arbitrary separation between the diopside-hedenbergite solid solution; the divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineral wollastonite has the formula of the hypothetical calcium end member but important structural differences mean that it is not grouped with the pyroxenes. Magnesium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure.
A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to nomenclature shown in Figure 3. The inclusion of sodium, which has a charge of +1, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In jadeite and aegirine this is added by the inclusion of a +3 cation on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron components are known as omphacite and aegirine-augite, with 80% or more of these components the pyroxene falls in the quadrilateral shown in Figure 2. Table 1 shows the wide range of other cations that can be accommodated in the pyroxene structure, indicates the sites that they occupy. In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and filling the site with the remaining aluminium and iron. Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, there are several alternative schemes: Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively.
For example, Na and Al give the jadeite composition. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site; this leads to e.g. NaFe2+0.5Ti4+0.5Si2O6. The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g. CaAlAlSiO6. In nature, more than one substitution may be found in the same mineral. Clinopyroxenes Aegirine, NaFe3+Si2O6 Augite, 2O6 Clinoenstatite, MgSiO3 Diopside, CaMgSi2O6 Esseneite, CaFe3+ Hedenbergite, CaFe2+Si2O6 Jadeite, NaSi2O6 Jervisite, Si2O6 Johannsenite, CaMn2+Si2O6 Kanoite, Mn2+Si2O6 Kosmochlor, NaCrSi2O6 Namansilite, NaMn3+Si2O6 Natalyite, NaV3+Si2O6 Omphacite, Si2O6 Petedunnite, CaSi2O6 Pigeonite, Si2O6 Spodumene, LiAl2 Orthopyroxenes Hypersthene, SiO3 Donpeacorite, MgSi2O6 Enstatite, Mg2Si2O6 Ferrosilite, Fe2Si2O6 Nchwaningite, Mn2+2SiO32•(H