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
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are found where tectonic plates are diverging or converging, most are found underwater. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can form where there is stretching and thinning of the crust's plates, e.g. in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanism away from plate boundaries has been explained as mantle plumes; these so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.
Volcanoes are not created where two tectonic plates slide past one another. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere. Volcanic winters have caused catastrophic famines; the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is sometimes spelled vulcanology. At the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock; because the crust is thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion and the partial melting of the mantle, causing volcanism and creating new oceanic crust.
Most divergent plate boundaries are at the bottom of the oceans. Black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed. Subduction zones are places where two plates an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges, under the continental plate, forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma; this magma tends to be viscous because of its high silica content, so it does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are the volcanoes in the Pacific Ring of Fire. Hotspots are volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting.
Because tectonic plates move across them, each volcano becomes dormant and is re-formed as the plate advances over the postulated plume. The Hawaiian Islands are said to have been formed in such a manner; this theory, has been doubted. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; the features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material and gases can develop anywhere on the landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea. Other types of volcano include cryovolcanoes on some moons of Jupiter and Neptune. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is a vent of an igneous volcano.
Volcanic fissure vents are linear fractures through which lava emerges. Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, they do not explode catastrophically. Since low-viscosity magma is low in silica, shield volcanoes are more common in oceanic than continental settings; the Hawaiian volcanic chain is a series of shield cones, they are common in Iceland, as well. Lava domes are built by slow eruptions of viscous lava, they are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount Saint Helen
Tuscany is a region in central Italy with an area of about 23,000 square kilometres and a population of about 3.8 million inhabitants. The regional capital is Florence. Tuscany is known for its landscapes, artistic legacy, its influence on high culture, it is regarded as the birthplace of the Italian Renaissance and has been home to many figures influential in the history of art and science, contains well-known museums such as the Uffizi and the Pitti Palace. Tuscany produces wines, including Chianti, Vino Nobile di Montepulciano, Morellino di Scansano and Brunello di Montalcino. Having a strong linguistic and cultural identity, it is sometimes considered "a nation within a nation". Tuscany is a popular destination in Italy, the main tourist spots are Florence, Lucca, Versilia and Chianti; the village of Castiglione della Pescaia is the most visited seaside destination in the region, with seaside tourism accounting for 40% of tourist arrivals. Additionally, Lucca, the Chianti region and Val d'Orcia are internationally renowned and popular spots among travellers.
Seven Tuscan localities have been designated World Heritage Sites: the historic centre of Florence. Tuscany has over 120 protected nature reserves, making Tuscany and its capital Florence popular tourist destinations that attract millions of tourists every year. In 2012, the city of Florence was the world's 89th most visited city, with over 1.834 million arrivals. Triangular in shape, Tuscany borders the regions of Liguria to the northwest, Emilia-Romagna to the north, Marche to the northeast, Umbria to the east and Lazio to the southeast; the comune of Badia Tedalda, in the Tuscan Province of Arezzo, has an exclave named Ca' Raffaello within Emilia-Romagna. Tuscany has a western coastline on the Ligurian Sea and the Tyrrhenian Sea, among, the Tuscan Archipelago, of which the largest island is Elba. Tuscany has an area of 22,993 square kilometres. Surrounded and crossed by major mountain chains, with few plains, the region has a relief, dominated by hilly country used for agriculture. Hills make up nearly two-thirds of the region's total area, covering 15,292 square kilometres, mountains, a further 25%, or 5,770 square kilometres.
Plains occupy 8.4% of the total area—1,930 square kilometres —mostly around the valley of the Arno. Many of Tuscany's largest cities lie on the banks of the Arno, including the capital Florence and Pisa; the climate is mild in the coastal areas, is harsher and rainy in the interior, with considerable fluctuations in temperature between winter and summer, giving the region a soil-building active freeze-thaw cycle, in part accounting for the region's once having served as a key breadbasket of ancient Rome. The pre-Etruscan history of the area in the late Bronze and Iron Ages parallels that of the early Greeks; the Tuscan area was inhabited by peoples of the so-called Apennine culture in the late second millennium BC who had trading relationships with the Minoan and Mycenaean civilizations in the Aegean Sea. Following this, the Villanovan culture saw Tuscany, the rest of Etruria, taken over by chiefdoms. City-states developed in the late Villanovan before "Orientalization" occurred and the Etruscan civilization rose.
The Etruscans created the first major civilization in this region, large enough to establish a transport infrastructure, to implement agriculture and mining and to produce vibrant art. The Etruscans lived in the area of Etruria well into prehistory; the civilization grew to fill the area between the Arno and Tiber from the eighth century BCE, reaching its peak during the seventh and sixth centuries B. C. succumbing to the Romans by the first century BCE. Throughout their existence, they lost territory to Magna Graecia and Celts. Despite being seen as distinct in its manners and customs by contemporary Greeks, the cultures of Greece, Rome, influenced the civilization to a great extent. One reason for its eventual demise was this increasing absorption by surrounding cultures, including the adoption of the Etruscan upper class by the Romans. Soon after absorbing Etruria, Rome established the cities of Lucca, Pisa and Florence, endowed the area with new technologies and development, ensured peace.
These developments included extensions of existing roads, introduction of aqueducts and sewers, the construction of many buildings, both public and private. However, many of these structures have been destroyed by erosion due to weather; the Roman civilization in the West of the Roman Republic and Roman Empire collapsed in the fifth century, the region fell to barbarians migrating through the Empire from Eastern Europe and Central Asia of the Goths was re-conquered by the revived Eastern Roman Empire under the strong Emperor Justinian. In the years following 572, the Lombards arrived and designated Lucca the capital of their subsequent Tuscia. Pilgrims travelling along the Via Francigena between Rome and France brought wealth and development during the medieval period; the food and shelter required by the
A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are divided into coastal lagoons and atoll lagoons, they have been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world. Lagoons are shallow elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature; some authorities include fresh water bodies in the definition of "lagoon", while others explicitly restrict "lagoon" to bodies of water with some degree of salinity. The distinction between "lagoon" and "estuary" varies between authorities. Richard A. Davis Jr. restricts "lagoon" to bodies of water with little or no fresh water inflow, little or no tidal flow, calls any bay that receives a regular flow of fresh water an "estuary". Davis does state that the terms "lagoon" and "estuary" are "often loosely applied in scientific literature."
Timothy M. Kusky characterizes lagoons as being elongated parallel to the coast, while estuaries are drowned river valleys, elongated perpendicular to the coast; when used within the context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonymous with the term "back reef" or "backreef", more used by coral reef scientists to refer to the same area. Coastal lagoons are classified as inland bodies of water. Many lagoons do not include "lagoon" in their common names. Albemarle and Pamlico sounds in North Carolina, Great South Bay between Long Island and the barrier beaches of Fire Island in New York, Isle of Wight Bay, which separates Ocean City, Maryland from the rest of Worcester County, Banana River in Florida, Lake Illawarra in New South Wales, Montrose Basin in Scotland, Broad Water in Wales have all been classified as lagoons, despite their names. In England, The Fleet at Chesil Beach has been described as a lagoon. In Latin America, the term laguna in Spanish, which lagoon translates to, may be used for a small fresh water lake in a similar way a creek is considered a small river.
However, sometimes it is popularly used to describe a full-sized lake, such as Laguna Catemaco in Mexico, the third largest lake by area in the country. The brackish water lagoon may be thus explicitly identified as a "coastal lagoon". In Portuguese the same usage is found: lagoa may be a body of shallow sea water, or a small freshwater lake not linked to the sea. Lagoon is derived from the Italian laguna, which refers to the waters around Venice, the Lagoon of Venice. Laguna is attested in English by at least 1612, had been Anglicized to "lagune" by 1673. In 1697 William Dampier referred to a "Lake of Salt water" on the coast of Mexico. Captain James Cook described an island "of Oval form with a Lagoon in the middle" in 1769. Atoll lagoons form as coral reefs grow upwards while the islands that the reefs surround subside, until only the reefs remain above sea level. Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons contain some deep portions. Coastal lagoons form along sloping coasts where barrier islands or reefs can develop off-shore, the sea-level is rising relative to the land along the shore.
Coastal lagoons do not form along steep or rocky coasts, or if the range of tides is more than 4 metres. Due to the gentle slope of the coast, coastal lagoons are shallow, they are sensitive to changes in sea level due to global warming. A relative drop in sea level may leave a lagoon dry, while a rise in sea level may let the sea breach or destroy barrier islands, leave reefs too deep under water to protect the lagoon. Coastal lagoons are young and dynamic, may be short-lived in geological terms. Coastal lagoons are common. In the United States, lagoons are found along more than 75 percent of the Gulf coasts. Coastal lagoons are connected to the open ocean by inlets between barrier islands; the number and size of the inlets, precipitation and inflow of fresh water all affect the nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no inflow of fresh water, high evaporation rates, such as Lake St. Lucia, in South Africa, may become saline. Lagoons with no connection to the open ocean and significant inflow of fresh water, such as the Lake Worth Lagoon in Florida in the middle of the 19th century, may be fresh.
On the other hand, lagoons with many wide inlets, such as the Wadden Sea, have strong tidal currents and mixing. Coastal lagoons tend to accumulate sediments from inflowing rivers, from runoff from the shores of the lagoon, from sediment carried into the lagoon through inlets by the tide. Large quantities of sediment may be be deposited in a lagoon when storm waves overwash barrier islands. Mangroves and marsh plants can facilitate the accumulation of sediment in a lagoon. Benthic organisms may destabilize sediments. River-mouth lagoons on mixed sand and gravel beaches form at the river-coast interface where a braided, although sometimes meandering, river interacts with a coastal environment, affected by longshore drift; the lagoons which form on the MSG coastlines are common on the east coast of the South Island of New Zealand and have long been referred to as hapua by the Māori. This classification differentiates hapua from similar lagoons located on the N
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations; the surface along which the lattice points are shared in twinned crystals is called a composition surface or twin plane. Crystallographers classify twinned crystals by a number of twin laws; these twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification. Twinning is an important mechanism for permanent shape changes in a crystal. Twinning can be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern. Twin laws are either defined by the direction of the twin axes. If the twin law can be defined by a simple planar composition surface, the twin plane is always parallel to a possible crystal face and never parallel to an existing plane of symmetry. If the twin law is a rotation axis, the composition surface will be irregular, the twin axis will be perpendicular to a lattice plane, but will never be an even-fold rotation axis of the existing symmetry.
For example twinning cannot occur on a new 2 fold axis, parallel to an existing 4-fold axis. In the isometric system, the most common types of twins are the Spinel Law, where the twin axis is perpendicular to an octahedral face, the Iron Cross, the interpenetration of two pyritohedrons a subtype of dodecahedron. In the hexagonal system, calcite shows the contact. Quartz shows the Brazil Law, Dauphiné Law which are penetration twins caused by transformation and Japanese Law, caused by accidents during growth. In the tetragonal system, cyclical contact twins are the most observed type of twin, such as in rutile titanium dioxide and cassiterite tin oxide. In the orthorhombic system, crystals twin on planes parallel to the prism face, where the most common is a twin which produces cyclical twins, such as in aragonite and cerussite. In the monoclinic system, twin occur most on the planes and by the Manebach Law, Carlsbad Law, Braveno Law in orthoclase, the Swallow Tail Twins in gypsum. In the triclinic system, the most twinned crystals are the feldspar minerals plagioclase and microcline.
These minerals show the Pericline Laws. Simple twinned crystals may be contact twins or penetration twins. Contact twins share a single composition surface appearing as mirror images across the boundary. Plagioclase, quartz and spinel exhibit contact twinning. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. An example is metazeunerite. In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner. Orthoclase, staurolite and fluorite show penetration twinning. If several twin crystal parts are aligned by the same twin law they are referred to as multiple or repeated twins. If these multiple twins are aligned in parallel they are called polysynthetic twins; when the multiple twins are not parallel they are cyclic twins. Albite and pyrite show polysynthetic twinning. Spaced polysynthetic twinning is observed as striations or fine parallel lines on the crystal face.
Rutile, aragonite and chrysoberyl exhibit cyclic twinning in a radiating pattern. But in general, based on the relationship between the twin axis and twin plane, there are 3 types of twinning: 1-parallel twinning, when the twin axis and compositional plane lie parallel to each other, 2-normal twining, when the twin plane and compositional plane lie and 3-complex twining, a combination of parallel twinning and normal twinning on one compositional plane. There are three modes of formation of twinned crystals. Growth twins are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion. Annealing or transformation twins are the result of a change in crystal system during cooling as one form becomes unstable and the crystal structure must re-organize or transform into another more stable form. Deformation or gliding twins are the result of stress on the crystal. If a metal with face-centered cubic structure, like Al, Cu, Ag, Au, etc. is subjected to stress, it will experience twinning.
The formation and migration of twin boundaries is responsible for ductility and malleability of fcc metals. Deformation twinning is a common result of regional metamorphism. Crystal twinning is used as an indicator of force direction in mountain building processes in orogeny research. Crystals that grow adjacent to each other may be aligned to resemble twinning; this parallel growth reduces system energy and is not twinning. Twinning can occur by cooperative displacement of atoms along the face of the twin boundary; this displacement of a large quantity of atoms requires significant energy to perform. Therefore, the theoretical stress required to form a twin is quite high, it is believed that twinning is associated with dislocation motion on a coordinated scale, in contrast to slip, caused by independent glide at several locations in the crystal. Twinning and slip are competitive mechanisms for crystal deformation; each mechanism is dominant under certain conditions. In fcc metals, slip is always dominant because the stres
Mohs scale of mineral hardness
The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative; the method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. 77 AD. While facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting. Despite its lack of precision, the Mohs scale is relevant for field geologists, who use the scale to identify minerals using scratch kits; the Mohs scale hardness of minerals can be found in reference sheets. Mohs hardness is useful in milling, it allows assessment of.
The scale is used at electronic manufacturers for testing the resilience of flat panel display components. The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly; the samples of matter used by Mohs are all different minerals. Minerals are chemically pure solids found in nature. Rocks are made up of one or more minerals; as the hardest known occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5. "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye. Materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number.
While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number. The Mohs scale is a purely ordinal scale. For example, corundum is twice as hard as topaz; the table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples. On the Mohs scale, a streak plate has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale; the table below incorporates additional substances that may fall between levels: Comparison between hardness and hardness: Mohs hardness of elements is taken from G. V. Samsonov in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA, 1968. Cordua, William S. "The Hardness of Minerals and Rocks". Lapidary Digest, c. 1990
A fumarole is an opening in a planet's crust which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrogen chloride, hydrogen sulfide. The steam forms when superheated water condenses as its pressure drops when it emerges from the ground; the name solfatara is given to fumaroles. Fumaroles may occur along long fissure, or in chaotic clusters or fields, they occur on the surface of lava or pyroclastic flows. A fumarole field is an area of thermal springs and gas vents where shallow magma or hot igneous rocks release gases or interact with groundwater; when they occur in freezing environments, fumaroles may cause fumarolic ice towers. Fumaroles may persist for centuries if located above a persistent heat source; the Valley of Ten Thousand Smokes, for example, was formed during the 1912 eruption of Novarupta in Alaska. Thousands of fumaroles occurred in the cooling ash from the eruption, but over time most of them have become extinct. An estimated four thousand fumaroles exist within the boundaries of Yellowstone National Park in the United States.
In April 2006 fumarole emissions killed three ski-patrol workers east of Chair 3 at Mammoth Mountain Ski Area in California. The workers were overpowered by toxic fumes. Another example is an array of fumaroles in the Valley of Desolation in Morne Trois Pitons National Park in Dominica. Fumaroles emitting sulfurous vapors form surface deposits of sulfur-rich minerals. Boiling Lake Cold seep Hydrothermal vent Mazuku Mofetta Mudpot Mud volcano Sulfur Mining on Gunung Welirang Volcano Chisholm, Hugh, ed.. "Fumarole". Encyclopædia Britannica. 11. Cambridge University Press. Pp. 300–301