Cape York meteorite
The Cape York meteorite is named for Cape York, near the location of its discovery in Savissivik, Meteorite Island, is one of the largest iron meteorites in the world. The meteorite collided with Earth thousands to millions of years ago; the iron masses were known to Inuit as Saviksoah weighing 31 metric tons. For centuries, Inuit living near the meteorites used them as a source of metal for tools and harpoons; the Inuit would work the metal using cold forging -- that is, by hammering it. The first stories of its existence reached scientific circles in 1818. Five expeditions between 1818 and 1883 failed to find the source of the iron, it was located in 1894 by the famous American Navy Arctic explorer. Peary enlisted the help of a local Inuit guide, who brought him to Saviksoah Island, just off northern Greenland's Cape York in 1894, it took Peary three years to arrange and carry out the loading of the heavy iron meteorites onto ships. This process required the building of short railroad. Peary sold the pieces for $40,000 to the American Museum of Natural History in New York City where they are still on display.
It is unknown whether the fragments were stolen. Today the 3.4 by 2.1 by 1.7 metres piece named Ahnighito is open for viewing at the American Museum of Natural History in the Arthur Ross Hall. It is the second-heaviest meteorite to have been relocated, it is so heavy that it was necessary to build its display stand so that the supports reached directly to the bedrock below the museum. In 1963, a fourth major piece of the Cape York meteorite was discovered by Vagn F. Buchwald on Agpalilik peninsula; the Agpalilik meteorite known as the Man, weighs about 20 metric tons, it is on display in the Geological Museum of the University of Copenhagen, Denmark. Other smaller pieces have been found, such as the 3 metric tons Savik I meteorite found in 1911 and the 250 kilograms Tunorput fragment found in 1984; the Cape York asteroid has been suggested by the crater discoverers to be a part of the asteroid which created the Hiawatha crater, but which split off prior to impact. Each of the most important fragments of Cape York has its own name: Ahnighito, 30,900 kilograms, 1884-1897, Meteorite Island, 76°04'N - 64°58'W Woman, 3,000 kilograms, 1897, Saveruluk, 76°09'N - 64°56'W Dog, 400 kilograms, 1897, Saveruluk, 76°09'N - 64°56'W Savik I, 3,400 kilograms, 1913, Savequarfik, 76°08'N - 64°36'W Thule, 48.6 kilograms, summer 1955, Thule, 76°32'N - 67°33'W Savik II, 7.8 kilograms, 1961, Savequarfik, 76°08'N - 64°36'W Agpalilik, 20,000 kilograms, 1963, Agpalilik, 76°09'N - 65°10'W Tunorput, 250 kilograms, 1984 It is an iron meteorite and belongs to the chemical group IIIAB.
There are abundant elongated troilite nodules. The troilite nodules contain inclusions of chromite, phosphates and copper; the rare nitride mineral carlsbergite occurs within the matrix of the metal phase. Graphite was not observed and the nitrogen isotopes are in disequilibrium. Glossary of meteoritics History of ferrous metallurgy Archaeometallurgy Inuit culture Meteoric iron Patricia A. M. Huntington. Robert E Peary and the Cape York meteorites American Museum of Natural History www.meteoritestudies.com Cape York on the Meteoritical Bulletin Database
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is hard and ductile. Pure nickel, powdered to maximize the reactive surface area, shows a significant chemical activity, but larger pieces are slow to react with air under standard conditions because an oxide layer forms on the surface and prevents further corrosion. So, pure native nickel is found in Earth's crust only in tiny amounts in ultramafic rocks, in the interiors of larger nickel–iron meteorites that were not exposed to oxygen when outside Earth's atmosphere. Meteoric nickel is found in combination with iron, a reflection of the origin of those elements as major end products of supernova nucleosynthesis. An iron–nickel mixture is thought to compose Earth's outer and inner cores. Use of nickel has been traced as far back as 3500 BCE. Nickel was first isolated and classified as a chemical element in 1751 by Axel Fredrik Cronstedt, who mistook the ore for a copper mineral, in the cobalt mines of Los, Hälsingland, Sweden.
The element's name comes from a mischievous sprite of German miner mythology, who personified the fact that copper-nickel ores resisted refinement into copper. An economically important source of nickel is the iron ore limonite, which contains 1–2% nickel. Nickel's other important ore minerals include pentlandite and a mixture of Ni-rich natural silicates known as garnierite. Major production sites include the Sudbury region in Canada, New Caledonia in the Pacific, Norilsk in Russia. Nickel is oxidized by air at room temperature and is considered corrosion-resistant, it has been used for plating iron and brass, coating chemistry equipment, manufacturing certain alloys that retain a high silvery polish, such as German silver. About 9% of world nickel production is still used for corrosion-resistant nickel plating. Nickel-plated objects sometimes provoke nickel allergy. Nickel has been used in coins, though its rising price has led to some replacement with cheaper metals in recent years. Nickel is one of four elements that are ferromagnetic at room temperature.
Alnico permanent magnets based on nickel are of intermediate strength between iron-based permanent magnets and rare-earth magnets. The metal is valuable in modern times chiefly in alloys. A further 10% is used for nickel-based and copper-based alloys, 7% for alloy steels, 3% in foundries, 9% in plating and 4% in other applications, including the fast-growing battery sector; as a compound, nickel has a number of niche chemical manufacturing uses, such as a catalyst for hydrogenation, cathodes for batteries and metal surface treatments. Nickel is an essential nutrient for some microorganisms and plants that have enzymes with nickel as an active site. Nickel is a silvery-white metal with a slight golden tinge, it is one of only four elements that are magnetic at or near room temperature, the others being iron and gadolinium. Its Curie temperature is 355 °C; the unit cell of nickel is a face-centered cube with the lattice parameter of 0.352 nm, giving an atomic radius of 0.124 nm. This crystal structure is stable to pressures of at least 70 GPa.
Nickel belongs to the transition metals. It is hard and ductile, has a high for transition metals electrical and thermal conductivity; the high compressive strength of 34 GPa, predicted for ideal crystals, is never obtained in the real bulk material due to the formation and movement of dislocations. The nickel atom has two electron configurations, 3d8 4s2 and 3d9 4s1, which are close in energy – the symbol refers to the argon-like core structure. There is some disagreement. Chemistry textbooks quote the electron configuration of nickel as 4s2 3d8, which can be written 3d8 4s2; this configuration agrees with the Madelung energy ordering rule, which predicts that 4s is filled before 3d. It is supported by the experimental fact that the lowest energy state of the nickel atom is a 3d8 4s2 energy level the 3d8 4s2 3F, J = 4 level. However, each of these two configurations splits into several energy levels due to fine structure, the two sets of energy levels overlap; the average energy of states with configuration 3d9 4s1 is lower than the average energy of states with configuration 3d8 4s2.
For this reason, the research literature on atomic calculations quotes the ground state configuration of nickel as 3d9 4s1. The isotopes of nickel range in atomic weight from 48 u to 78 u. Occurring nickel is composed of five stable isotopes. Isotopes heavier than 62Ni cannot be formed by nuclear fusion without losing energy. Nickel-62 has the highest mean nuclear binding energy per nucleon of any nuclide, at 8.7946 MeV/nucleon. Its binding energy is greater than both 56Fe and 58Fe, more abundant elements incorrectly cited as having the most tightly-bound nuclides. Although this would seem to predict nickel-62 as the most abundant heavy element in the universe, the high rate of photodisintegration of nickel in stellar interiors causes iron to be by far the most abundant. Stable isotope nickel-60 is the daughter product of the extinct radionuclide 60Fe, whi
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
Iron meteorites are meteorites that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that consists of two mineral phases: kamacite and taenite. Iron meteorites originate from cores of planetesimals; the iron found in iron meteorites was one of the earliest sources of usable iron available to humans, before the development of smelting that signaled the beginning of the Iron Age. Although they are rare compared to the stony meteorites, comprising only about 5.7% of witnessed falls, iron meteorites have been over-represented in meteorite collections. This is due to several factors: They are recognized as unusual by laymen, as opposed to stony meteorites. Modern-day searches for meteorites in deserts and Antarctica yield a much more representative sample of meteorites overall, they are much more resistant to weathering. They are much more to survive atmospheric entry, are more resistant to the resulting ablation. Hence, they are more to be found as large pieces, they can be found when buried by use of surface metal detecting equipment, due to their metallic composition.
Because they are denser than stony meteorites, iron meteorites account for 90% of the mass of all known meteorites, about 500 tons. All the largest known meteorites are of this type, including the largest—the Hoba meteorite. Iron meteorites have been linked to M-type asteroids because both have similar spectral characteristics in the visible and near-infrared. Iron meteorites are thought to be the fragments of the cores of larger ancient asteroids that have been shattered by impacts; the heat released from the radioactive decay of the short-lived nuclides 26Al and 60Fe is considered as a plausible cause for the melting and differentiation of their parent bodies in the early Solar System. Melting produced from the heat of impacts is another cause of melting and differentiation The IIE iron meteorites may be a notable exception, in that they originate from the crust of S-type asteroid 6 Hebe. Chemical and isotope analysis indicates; this implies that there were once at least this many large, asteroids in the asteroid belt – many more than today.
The overwhelming bulk of these meteorites consists of the FeNi-alloys taenite. Minor minerals, when occurring form rounded nodules of troilite or graphite, surrounded by schreibersite and cohenite. Schreibersite and troilite occur as plate shaped inclusions, which show up on cut surfaces as cm-long and mm-thick lamellae; the troilite plates are called Reichenbach lamellae. The chemical composition is dominated by the elements Fe, Ni and Co, which make up more than 95%. Ni is always present. A significant percentage of nickel can be used in the field to distinguish meteoritic irons from man-made iron products, which contain lower amounts of Ni, but it is not enough to prove meteoritic origin. Iron meteorites were used for their meteoric iron, forged into cultural objects, tools or weapons. With the advent of smelting and the beginning of the iron age the importance of iron meteorites as a resource decreased, at least in those cultures that developed those techniques; the Inuit used the Cape York meteorite for a much longer time.
Iron meteorites themselves were sometimes used unaltered as collectibles or religious symbols. Today iron meteorites are prized collectibles for academic individuals; some are tourist attractions as in the case of the Hoba meteorite. Two classifications are in use: the classic structural classification and the newer chemical classification; the older structural classification is based on the presence or absence of the Widmanstätten pattern, which can be assessed from the appearance of polished cross-sections that have been etched with acid. This is connected with the relative abundance of nickel to iron; the categories are: Hexahedrites: no Widmanstätten pattern, may present Neumann lines. They can be further divided up on the basis of the width of the kamacite lamellae from coarsest to finest. Coarsest: lamellae width > 3.3 mm Coarse: lamellae width 1.3-3.3 mm Medium: lamellae width 0.5-1.3 mm Fine: lamellae width 0.2-0.5 mm Finest: lamellae width < 0.2 mm Plessitic: a transitional structure between octahedrites and ataxites Ataxites: high nickel, no Widmanstätten pattern, rare.
A newer chemical classification scheme based on the proportions of the trace elements Ga, Ge and Ir separates the iron meteorites into classes corresponding to distinct asteroid parent bodies. This classification is based on diagrams; the different iron meteorite groups appear as data point clusters. There were four of these groups designated by the Roman numerals I, II, III, IV; when more chemical data became available these were split, e.g. Group IV was split into IVA and IVB meteorites; some groups got joined again when intermediate meteorites were discovered, e.g. IIIA and IIIB were combined into the IIIAB meteorites. In 2006 iron meteorites were classified into 13 groups: IAB IA: Medium and coarse octahedrites, 6.4-8.7% Ni, 55-100 ppm Ga, 190-520 ppm Ge, 0.6-5.5 ppm Ir, Ge-Ni correlation negative. IB: Ataxites and medium octahedrites, 8.7-25% Ni, 11-55 ppm Ga, 25-190 ppm Ge, 0.3-2 ppm Ir, Ge-Ni correlation negative. IC IIAB IIA: Hexahedrites, 5.3-5.7% Ni, 57-62 ppm Ga, 170-185 ppm Ge, 2-60ppm Ir.
IIB: Coarsest oct
Gallium is a chemical element with symbol Ga and atomic number 31. It is in group 13 of the periodic table, thus has similarities to the other metals of the group, aluminium and thallium. Gallium does not occur as a free element in nature, but as gallium compounds in trace amounts in zinc ores and in bauxite. Elemental gallium is a soft, silvery blue metal at standard temperature and pressure, a brittle solid at low temperatures, a liquid at temperatures greater than 29.76 °C. The melting point of gallium is used as a temperature reference point. Gallium alloys are used in thermometers as a non-toxic and environmentally friendly alternative to mercury, can withstand higher temperatures than mercury; the alloy galinstan has an lower melting point of −19 °C, well below the freezing point of water. Since its discovery in 1875, gallium has been used to make alloys with low melting points, it is used in semiconductors as a dopant in semiconductor substrates. Gallium is predominantly used in electronics.
Gallium arsenide, the primary chemical compound of gallium in electronics, is used in microwave circuits, high-speed switching circuits, infrared circuits. Semiconducting gallium nitride and indium gallium nitride produce blue and violet light-emitting diodes and diode lasers. Gallium is used in the production of artificial gadolinium gallium garnet for jewelry. Gallium is considered a technology-critical element. Gallium has no known natural role in biology. Gallium behaves in a similar manner to ferric salts in biological systems and has been used in some medical applications, including pharmaceuticals and radiopharmaceuticals. Elemental gallium is not found in nature, but it is obtained by smelting. Pure gallium metal has a silvery color and its solid metal fractures conchoidally like glass. Gallium liquid expands by 3.10 %. Gallium shares the higher-density liquid state with a short list of other materials that includes water, germanium, antimony and plutonium. Gallium attacks most other metals by diffusing into the metal lattice.
For example, it diffuses into the grain boundaries of aluminium-zinc alloys and steel, making them brittle. Gallium alloys with many metals, is used in small quantities in the plutonium-gallium alloy in the plutonium cores of nuclear bombs to stabilize the plutonium crystal structure; the melting point of gallium, at 302.9146 K, is just above room temperature, is the same as the average summer daytime temperatures in Earth's mid-latitudes. This melting point is one of the formal temperature reference points in the International Temperature Scale of 1990 established by the International Bureau of Weights and Measures; the triple point of gallium, 302.9166 K, is used by the US National Institute of Standards and Technology in preference to the melting point. The melting point of gallium allows it to melt in the human hand, refreeze if removed; the liquid metal has a strong tendency to supercool below its melting point/freezing point: Ga nanoparticles can be kept in the liquid state below 90 K. Seeding with a crystal helps to initiate freezing.
Gallium is one of the four non-radioactive metals that are known to be liquid at, or near, normal room temperature. Of the four, gallium is the only one, neither reactive nor toxic and can therefore be used in metal-in-glass high-temperature thermometers, it is notable for having one of the largest liquid ranges for a metal, for having a low vapor pressure at high temperatures. Gallium's boiling point, 2673 K, is more than eight times higher than its melting point on the absolute scale, the greatest ratio between melting point and boiling point of any element. Unlike mercury, liquid gallium metal wets glass and skin, along with most other materials, making it mechanically more difficult to handle though it is less toxic and requires far fewer precautions. Gallium painted onto glass is a brilliant mirror. For this reason as well as the metal contamination and freezing-expansion problems, samples of gallium metal are supplied in polyethylene packets within other containers. Gallium does not crystallize in any of the simple crystal structures.
The stable phase under normal conditions is orthorhombic with 8 atoms in the conventional unit cell. Within a unit cell, each atom has only one nearest neighbor; the remaining six unit cell neighbors are spaced 27, 30 and 39 pm farther away, they are grouped in pairs with the same distance. Many stable and metastable phases are found as function of pressure; the bonding between the two nearest neighbors is covalent. This explains the low melting point relative to the neighbor elements and indium; this structure is strikingly similar to that of iodine and forms because of interactions between the single 4p electrons of gallium atoms, further away from the nucleus than the 4s electrons and the 3d10 core. This phenomenon recurs with mercury with its "pseudo-noble-gas" 4f145d106s2 electron configuration, liquid at room temperature; the 3d10 electrons do not shield the outer electrons well from the nucleus an
The Inuit are a group of culturally similar indigenous peoples inhabiting the Arctic regions of Greenland and Alaska. The Inuit languages are part of the Eskimo–Aleut family. Inuit Sign Language is a critically endangered language isolate used in Nunavut. In Canada and the States, the term "Eskimo" was used by ethnic Europeans to describe the Inuit and Siberia's and Alaska's Yupik and Iñupiat peoples. However, "Inuit" is not accepted as a term for the Yupik, "Eskimo" is the only term that applies to Yupik, Iñupiat and Inuit. Since the late 20th century, Indigenous peoples in Canada and Greenlandic Inuit consider "Eskimo" to be a pejorative term, they more identify as "Inuit" for an autonym. In Canada, sections 25 and 35 of the Constitution Act of 1982 classified the Inuit as a distinctive group of Aboriginal Canadians who are not included under either the First Nations or the Métis; the Inuit live throughout most of Northern Canada in the territory of Nunavut, Nunavik in the northern third of Quebec and NunatuKavut in Labrador, in various parts of the Northwest Territories around the Arctic Ocean.
These areas are known in the Inuktitut language as the "Inuit Nunangat". In the United States, the Iñupiat live on the Alaska North Slope and on Little Diomede Island; the Greenlandic Inuit are descendants of ancient indigenous migrations from Canada, as these people migrated to the east through the continent. They are citizens of Denmark. Inuit are the descendants of what anthropologists call the Thule people, who emerged from western Alaska around 1000 CE, they had split from the related Aleut group about 4000 years ago and from northeastern Siberian migrants related to the Chukchi language group, still earlier, descended from the third major migration from Siberia. They spread eastwards across the Arctic, they displaced the related Dorset culture, called the Tuniit in Inuktitut, the last major Paleo-Eskimo culture. Inuit legends speak of the Tuniit as people who were taller and stronger than the Inuit. Less the legends refer to the Dorset as "dwarfs". Researchers believe that Inuit society had advantages by having adapted to using dogs as transport animals, developing larger weapons and other technologies superior to those of the Dorset culture.
By 1300, Inuit migrants had reached west Greenland. During the next century, they settled in East Greenland Faced with population pressures from the Thule and other surrounding groups, such as the Algonquian and Siouan-speaking peoples to the south, the Tuniit receded; the Tuniit were thought to have become extinct as a people by about 1400 or 1500. But, in the mid-1950s, researcher Henry B. Collins determined that, based on the ruins found at Native Point, the Sadlermiut were the last remnants of the Dorset culture, or Tuniit; the Sadlermiut population survived up until winter 1902–03, when exposure to new infectious diseases brought by contact with Europeans led to their extinction as a people. In the early 21st century, mitochondrial DNA research has supported the theory of continuity between the Tuniit and the Sadlermiut peoples, it provided evidence that a population displacement did not occur within the Aleutian Islands between the Dorset and Thule transition. In contrast to other Tuniit populations, the Aleut and Sadlermiut benefited from both geographical isolation and their ability to adopt certain Thule technologies.
In Canada and Greenland, Inuit circulated exclusively north of the "arctic tree line", the effective southern border of Inuit society. The most southern "officially recognized" Inuit community in the world is Rigolet in Nunatsiavut. South of Nunatsiavut, the descendants of the southern Labrador Inuit in NunatuKavut continued their traditional transhumant semi-nomadic way of life until the mid-1900s; the Nunatukavummuit people moved among islands and bays on a seasonal basis. They did not establish stationary communities. In other areas south of the tree line, non-Inuit indigenous cultures were well established; the culture and technology of Inuit society that served so well in the Arctic were not suited to subarctic regions, so they did not displace their southern neighbors. Inuit had trade relations with more southern cultures. Warfare was not uncommon among those Inuit groups with sufficient population density. Inuit such as the Nunamiut, who inhabited the Mackenzie River delta area engaged in warfare.
The more sparsely settled Inuit in the Central Arctic, did so less often. Their first European contact was with the Vikings who settled in Greenland and explored the eastern Canadian coast; the sagas recorded meeting skrælingar an undifferentiated label for all the indigenous peoples whom the Norse encountered, whether Tuniit, Inuit, or Beothuk. After about 1350, the climate grew colder during the period known as the Little Ice Age. During this period, Alaskan natives were able to continue their whaling activities. But, in the high Arctic, the Inuit were forced to abandon their hunting and whaling sites as bowhead whales disappeared from Canada and Greenland; these Inuit had to subsist on a much poorer diet, lost access to the essential raw materials for their tools and architecture which they had derived from whaling. The changing climate forced the Inuit to work their way south, pushing them into marginal niches along the edges of the tree line; these were areas which Native Americans had not occupied or where they were weak enough for the Inuit to live near them.
Researchers have difficulty defining when Inuit stopped this territorial
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the object enters the atmosphere, various factors such as friction and chemical interactions with the atmospheric gases cause it to heat up and radiate that energy, it becomes a meteor and forms a fireball known as a shooting star or falling star. Meteorites vary in size. For geologists, a bolide is a meteorite large enough to create an impact crater. Meteorites that are recovered after being observed as they transit the atmosphere and impact the Earth are called meteorite falls. All others are known as meteorite finds; as of August 2018, there were about 1,412 witnessed falls that have specimens in the world's collections. As of 2018, there are more than 59,200 well-documented meteorite finds. Meteorites have traditionally been divided into three broad categories: stony meteorites that are rocks composed of silicate minerals.
Modern classification schemes divide meteorites into groups according to their structure and isotopic composition and mineralogy. Meteorites smaller than 2 mm are classified as micrometeorites. Extraterrestrial meteorites are such objects that have impacted other celestial bodies, whether or not they have passed through an atmosphere, they have been found on the Mars. Meteorites are always named for the places they were found a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter; the name designated by the Meteoritical Society is used by scientists and most collectors. Most meteoroids disintegrate. Five to ten a year are observed to fall and are subsequently recovered and made known to scientists. Few meteorites are large enough to create large impact craters. Instead, they arrive at the surface at their terminal velocity and, at most, create a small pit. Large meteoroids may strike the earth with a significant fraction of their escape velocity, leaving behind a hypervelocity impact crater.
The kind of crater will depend on the size, degree of fragmentation, incoming angle of the impactor. The force of such collisions has the potential to cause widespread destruction; the most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most able to transit the atmosphere intact. Examples of craters caused by iron meteoroids include Barringer Meteor Crater, Odessa Meteor Crater, Wabar craters, Wolfe Creek crater. In contrast relatively large stony or icy bodies like small comets or asteroids, up to millions of tons, are disrupted in the atmosphere, do not make impact craters. Although such disruption events are uncommon, they can cause a considerable concussion to occur. Large stony objects, hundreds of meters in diameter or more, weighing tens of millions of tons or more, can reach the surface and cause large craters, but are rare; such events are so energetic that the impactor is destroyed, leaving no meteorites. Several phenomena are well documented during witnessed meteorite falls too small to produce hypervelocity craters.
The fireball that occurs as the meteoroid passes through the atmosphere can appear to be bright, rivaling the sun in intensity, although most are far dimmer and may not be noticed during daytime. Various colors have been reported, including yellow and red. Flashes and bursts of light can occur. Explosions and rumblings are heard during meteorite falls, which can be caused by sonic booms as well as shock waves resulting from major fragmentation events; these sounds can be heard with a radius of a hundred or more kilometers. Whistling and hissing sounds are sometimes heard, but are poorly understood. Following passage of the fireball, it is not unusual for a dust trail to linger in the atmosphere for several minutes; as meteoroids are heated during atmospheric entry, their surfaces experience ablation. They can be sculpted into various shapes during this process, sometimes resulting in shallow thumbprint-like indentations on their surfaces called regmaglypts. If the meteoroid maintains a fixed orientation for some time, without tumbling, it may develop a conical "nose cone" or "heat shield" shape.
As it decelerates the molten surface layer solidifies into a thin fusion crust, which on most meteorites is black. On stony meteorites, the heat-affected zone is at most a few mm deep. Reports vary. Meteorites from multiple falls, such as Bjurbole, Tagish Lake, Buzzard Coulee, have been found having fallen on lake and sea ice suggesting that they were not hot when they