The L type ordinary chondrites are the second most common group of meteorites, accounting for 35% of all those catalogued, 40% of the ordinary chondrites. The ordinary chondrites are thought to have originated from three parent asteroids, with the fragments making up the H chondrite, L chondrite and LL chondrite groups respectively, their name comes from their low iron abundance, with respect to the H chondrites, which are about 20–25% iron by weight. The L chondrites have been named hypersthene chondrites or olivine hypersthene chondrites for the dominant minerals, but these terms are now obsolete. Characteristic is the fayalite content in olivine of 21 to 25 mol%. About 4–10% iron–nickel is found as a free metal, making these meteorites magnetic, but not as as the H chondrites; the most abundant minerals are hypersthene, as well as iron -- nickel and troilite. Chromite, sodium-rich feldspar and calcium phosphates occur in minor amounts. Petrologic type 6 dominates, with over 60% of the L chondrites falling into this class.
This indicates. Many of the L chondrite meteors may have their origin in the Ordovician meteor event. Compared to other chondrites, a large proportion of the L chondrites have been shocked, taken to imply that the parent body was catastrophically disrupted by a large impact; this event has been radioisotope dated to around 470±6 million years ago. The parent body/bodies for this group are not known, but plausible suggestions include 433 Eros and 8 Flora, or the Flora family as a whole. 433 Eros has been found to have a similar spectrum, while several pieces of circumstantial evidence for the Flora family exist: the Flora family is thought to have formed about 1,000 to 500 million years ago. Glossary of meteoritics The Catalogue of Meteorites
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
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix that can be similar to or different from the composition of the fragments. The word has its origins in the Italian language, in which it means either "loose gravel" or "stone made by cemented gravel". A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, tectonic breccia, igneous breccia, impact breccia, hydrothermal breccia. Sedimentary breccia is a type of clastic sedimentary rock, made of angular to subangular, randomly oriented clasts of other sedimentary rocks. A conglomerate, by contrast, is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccia and conglomerate are composed of fragments averaging greater than 2 millimetres in size; the angular shape of the fragments indicates that the material has not been transported far from its source. Sedimentary breccia consists of angular, poorly sorted, immature fragments of rocks in a finer grained groundmass which are produced by mass wasting.
It is lithified scree. Thick sequences of sedimentary breccia are formed next to fault scarps in grabens. Breccia may occur along a buried stream channel where it indicates accumulation along a juvenile or flowing stream. Sedimentary breccia may be formed by submarine debris flows. Turbidites occur as fine-grained peripheral deposits to sedimentary breccia flows. In a karst terrain, a collapse breccia may form due to collapse of rock into a sinkhole or in cave development. Fault breccia results from the grinding action of two fault blocks. Subsequent cementation of these broken fragments may occur by means of the introduction of mineral matter in groundwater. Igneous clastic rocks can be divided into two classes: Broken, fragmental rocks associated with volcanic eruptions, both of the lava and pyroclastic type. Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column; this may include rocks plucked off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge.
Lavas rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation. This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are reincorporated into the lava flow again and mixed in with the remaining liquid magma; the resulting breccia is uniform in rock chemical composition. Lavas may pick up rock fragments if flowing over unconsolidated rubble on the flanks of a volcano, these form volcanic breccias called pillow breccias. Within the volcanic conduits of explosive volcanoes the volcanic breccia environment merges into the intrusive breccia environment. There the upwelling lava tends to solidify during quiescent intervals only to be shattered by ensuing eruptions. Clastic rocks are commonly found in shallow subvolcanic intrusions such as porphyry stocks and kimberlite pipes, where they are transitional with volcanic breccias. Intrusive rocks can become brecciated in appearance by multiple stages of intrusion if fresh magma is intruded into consolidated or solidified magma.
This may be seen in many granite intrusions where aplite veins form a late-stage stockwork through earlier phases of the granite mass. When intense, the rock may appear as a chaotic breccia. Clastic rocks in mafic and ultramafic intrusions have been found and form via several processes: Consumption and melt-mingling with wall rocks, where the felsic wall rocks are softened and invaded by the hotter ultramafic intrusion. Impact breccias are thought to be diagnostic of an impact event such as an asteroid or comet striking the Earth and are found at impact craters. Impact breccia, a type of impactite, forms during the process of impact cratering when large meteorites or comets impact with the Earth or other rocky planets or asteroids. Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in the ejecta expelled beyond the crater. Impact breccia may be identified by its occurrence in or around a known impact crater, and/or an association with other products of impact cratering such as shatter cones, impact glass, shocked minerals, chemical and isotopic evidence of contamination with extraterrestrial material.
An example of an impact breccia is the Neugrund breccia, formed in the Neugrund impact. Hydrothermal breccias form at shallow crustal levels between 150 and 350 °C, when seismic or volcanic activity causes a void to open along a fault deep underground; the void draws in hot water, as pressure in the cavity drops, the water violently boils. In addition, the sudden opening of a cavity causes rock at the sides of the fault to destabilise and implode inwards, the broken rock gets caught up in a churning mixture of rock and boiling water. Rock fragments collide with each other and the sides of the void, the angular fragments become more rounded. Volatile gases are lost to the steam phase in particular carbon dioxide; as a result, the chemistry of the fluids changes an
HED meteorites are a clan of achondrite meteorites. HED stands for "howardite–eucrite–diogenite"; these achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the magmatic rocks found on Earth and for this reason they resemble terrestrial igneous rocks. HED meteorites are broadly divided into: Howardites Eucrites DiogenitesSeveral subgroups of both eucrites and diogenites have been found; the HED meteorites account for about 5% of all falls, about 60% of all achondrites. These are all thought to have originated from the crust of the asteroid Vesta, their differences being due to different geologic histories of the parent rock, their crystallization ages have been determined to be between 4.43 and 4.55 billion years from radioisotope ratios. HED meteorites are differentiated meteorites, which were created by igneous processes in the crust of their parent asteroid, it is thought that the method of transport from Vesta to Earth is as follows: An impact on Vesta ejected debris, creating small V-type asteroids.
Either the asteroidal chunks were formed from smaller debris. Some of these small asteroids formed the Vesta family; this event is thought to have happened less than 1 billion years ago. There is an enormous impact crater on Vesta covering much of the southern hemisphere, the best candidate for the site of this impact; the amount of rock, excavated there is many times more than enough to account for all known V-type asteroids. Some of the more far-flung asteroid debris ended up in the 3:1 Kirkwood gap; this is an unstable region due to strong perturbations by Jupiter, asteroids which end up here get ejected onto far different orbits on a timescale of about 100 million years. Some of these bodies are perturbed into near-Earth orbits forming the small V-type near-Earth asteroids such as e.g. 3551 Verenia, 3908 Nyx, or 4055 Magellan. Smaller impacts on these near-Earth objects dislodged rock-sized meteorites, some of which struck Earth. On the basis of cosmic ray exposure measurements, it is thought that most HED meteorites arose from several distinct impact events of this kind, spent from about 6 million to 73 million years in space before striking the Earth.
Glossary of meteoritics Meteorite articles, including discussions of HEDs, in Planetary Science Research Discoveries
Ureilite is a rare type of stony meteorite that has a unique mineralogical composition different from that of other stony meteorites. This dark grey or brownish meteorite type is named after the village Novy Urey, Mordovia Republic of Russia, where a meteorite of this type fell on 4 September 1886. Notable ureilites are the Novo Urei and the Goalpara named for the town in which it landed. On 7 October 2008, tiny asteroid 2008 TC3 entered Earth's atmosphere and exploded an estimated 37 kilometres above the Nubian Desert in Sudan. Fragments of this asteroid were found to be ureilite. Scientists have discovered amino acids in meteorite 2008 TC3 where none were expected, taking into account high temperatures reached in the explosion of about 1000 °C. A technical name for ureilite would be olivine-pigeonite achondrite. Compared to most other meteorites, ureilites tend to have a high percentage of carbon in the form of graphite and nanodiamonds; the diamonds, which are more than a few micrometres in diameter, are the result of high pressure shockwaves produced by collisions of the ureilite parent body with other asteroids.
Ureilites can be divided into two subcategories: polymict. Monomict ureilites are coarse grained with olivine more abundant than pyroxene. Polymict ureilites are a mixture of clasts of dissimilar composition. Unknown; some groups of meteorites come from a single object, but there has been no parent body found as yet for the ureilites. Prior to impacting Earth, 2008 TC3 was identified as an F-type asteroid. According to one theory, Ureilites were formed in the interior of a parent body with cumulate crystals that formed crystal layers. Support for this comes from some ureilites in which the grains are aligned in a preferred orientation. Another suggestion is that ureilites represent a residuum of unmelted material after a partial melt liquid was drawn off, yet other ideas are that they are unprocessed materials which never melted or that they are mixtures of carbonaceous chondrite and basaltic rock melts. It remains unclear whether ureilites originated on different parent bodies or in different regions of a single body.
The presence of diamonds, which can form from graphite as a result of severe shock metamorphism, hints at a violent impact history. By contrast, a 2018 study of the diamond inclusions in 2008 TC3 concluded that they could only have formed over a long period of time at high pressure, suggesting that "the ureilite parent body was a Mercury- to Mars-sized planetary embryo." Glossary of meteoritics Meteorite classification
A meteorite hunting is the search for meteorites. A person engaged in the search for meteorites is known as a meteorite hunter. Meteorite hunters may be amateurs who search on the weekends and after work, or professionals who recover meteorites for a living. Both use tools such as metal detectors or magnets to discover the meteorites. If the meteorite is of the iron or stony iron variety a magnet will pick it up from the soil surface or a metal detector will detect it through many inches of soil. Stony meteorites —which make up the large majority of meteorites that fall— may not have a high enough nickel iron content to set off a metal detector. Large and sensitive metal detectors may be used as well as ground-penetrating radar and landmine detectors. Although meteorites fall uniformly across the globe they do not remain on the surface in areas with a large amount of yearly rainfall. If a newly fallen meteorite is not recovered within a few months it is to be buried with alluvium or covered by plant growth.
Some arctic and desert regions have proven to be well-suited to preserving meteorites, can provide excellent surfaces for hunting visually. Meteorites can be valuable to scientists studying planetary science and to collectors. Individual stones may weigh mere hundreds of kilograms, their values vary based on rarity and composition, as well as the conditions in which they are found. In the United States, most state laws state that a meteorite find belongs to the landowner of the land upon which the meteorite was found; this doctrine contrasts with the once-predominant rule in state courts on the finding of treasure trove, where buried gold or silver coinage is deemed to belong to the finder. Many state courts have interpreted their laws as granting the state sole title to any meteorite recovered on state-owned lands. United States laws and enforcement of laws regarding recovery of meteorites on federally owned public lands is unsettled. With respect to large meteorites, the federal government has asserted title to all such meteorites if proven to be found on federal land, because: the meteorite is the property of the federal government, the landowner meteorites found on public lands are subject to the 1906 Antiquities Act a meteorite does not qualify as a “valuable mineral” as defined under the 1872 Mining Law, thus it is not subject to mineral claim rights that could otherwise be filed by the discoverer.
This policy derives from cases as far back as 1944, when the federal government seized the Drum Mountain Meteorite in Utah from a group of interned Japanese-American U. S. citizens. The federal government has sometimes agreed to negotiate sometimes negotiating a small finders fee for large meteorites, but has never agreed to pay anything resembling full market value of the meteorite to the discoverer. In the case of small meteorites, ownership of meteorites found on federal land is not covered in the Code of Federal Regulations, in the past hobbyists have been able to remove small quantities of rock for non-commercial use. However, in recent years the U. S. Bureau of Land Management has asserted that it owns all meteorites recovered on BLM land arguing that BLM stands in the same position as a private landowner under state law; the BLM further asserts that under the 1906 Antiquities Act, all meteorites on BLM land belong to the Smithsonian Institution. A BLM memorandum of September 10, 2012, reaffirms that meteorites found on public land belong to the Federal Government.
Permits can be acquired for systematic search for meteorites on public land undertaken for scientific, educational, or commercial purposes. Antarctic prospecting is expensive and therefore can only be carried on by well funded organizations. Half of the meteorites found in Antarctica have been recovered by ANSMET; the ANSMET program is a major source of the extraterrestrial material, available for scientific investigation. Japanese finds make up the majority of the remainder, China has begun exploration. A popular geological feature employed by Antarctic meteorite hunters is an area where a natural downsloped plain meets an uprising ridge, such as where the East Antarctic Ice Sheet, creeping to the sea at about three metres per year, meets the Transantarctic Mountains; the downslope-mountain ridge combination allows the creeping gravity-driven icesheet to start rising upwards. As it does so, the exposed snow and ice are removed by fierce winds and sublimation harvesting the embedded meteorites and leaving them to lie on the surface along the length of the mountain ridge.
The famed 1.93 kilograms Allan Hills 84001 meteorite abbreviated as ALH 84001 and believed to be from Mars, was found at Allan Hills, Antarctica in 1984. In 1996 NASA scientists announced that it might contain evidence for microscopic fossils of Martian bacteria based on the carbonate globules it contained. In the aftermath of the air burst of a meteor, a large number of small meteorites can fall to the ground at terminal velocity, such as occurred with the 2013 Chelyabinsk meteor; when that occurs local residents and schoolchildren will seek to locate and pick up the fragments due to their potential value. In the case of the Chelyabinsk meteor, many were located in snowdrifts by following a visible hole, left in the outer surface of the snow. Meteorite Men is a U. S. television series following two atypical meteorite hunters. Glossary of meteoritics
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. Thus it is a type of orthosilicate, it is a common mineral in Earth's subsurface but weathers on the surface. The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite and fayalite. Compositions of olivine are expressed as molar percentages of forsterite and fayalite. Forsterite's melting temperature is unusually high at atmospheric pressure 1,900 °C, while fayalite's is much lower. Melting temperature varies smoothly between the two endmembers. Olivine incorporates only minor amounts of elements other than oxygen, silicon and iron. Manganese and nickel are the additional elements present in highest concentrations. Olivine gives its name to the group of minerals with a related structure —which includes tephroite and kirschsteinite. Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica unit being joined by metal divalent cations with each oxygen in SiO4 bound to 3 metal ions.
It has a spinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M22+ M4+O4 instead of two trivalent and one divalent cations. Olivine gemstones are called chrysolite. Olivine rock is harder than surrounding rock and stands out as distinct ridges in the terrain; these ridges are dry with little soil. Drought resistant scots pine is one of few trees. Olivine pine forest is unique to Norway, it found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord. Olivine rock is base-rich; the habitat is endangered by road construction. Olivine is named for its olive-green color, though it may alter to a reddish color from the oxidation of iron. Translucent olivine is sometimes used as a gemstone called peridot, it is called chrysolite. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad Island in the Red Sea. Olivine occurs in both mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks.
Mg-rich olivine crystallizes from magma, rich in magnesium and low in silica. That magma crystallizes to mafic rocks such as basalt. Ultramafic rocks such as peridotite and dunite can be residues left after extraction of magmas, they are more enriched in olivine after extraction of partial melts. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, olivine is one of the Earth's most common minerals by volume; the metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content produces Mg-rich olivine, or forsterite. Fe-rich olivine is much less common, but it occurs in igneous rocks in small amounts in rare granites and rhyolites, Fe-rich olivine can exist stably with quartz and tridymite. In contrast, Mg-rich olivine does not occur stably with silica minerals, as it would react with them to form orthopyroxene. Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km within Earth; because it is thought to be the most abundant mineral in Earth’s mantle at shallower depths, the properties of olivine have a dominant influence upon the rheology of that part of Earth and hence upon the solid flow that drives plate tectonics.
Experiments have documented that olivine at high pressures can contain at least as much as about 8900 parts per million of water, that such water content drastically reduces the resistance of olivine to solid flow. Moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than is contained in Earth's oceans. Mg-rich olivine has been discovered in meteorites, on the Moon and Mars, falling into infant stars, as well as on asteroid 25143 Itokawa; such meteorites include collections of debris from the early Solar System. The spectral signature of olivine has been seen in the dust disks around young stars; the tails of comets have the spectral signature of olivine, the presence of olivine was verified in samples of a comet from the Stardust spacecraft in 2006. Comet-like olivine has been detected in the planetesimal belt around the star Beta Pictoris. Minerals in the olivine group crystallize in the orthorhombic system with isolated silicate tetrahedra, meaning that olivine is a nesosilicate.
In an alternative view, the atomic structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions. There are two distinct metal sites and only one distinct silicon site. O1, O2, M2 and Si all lie on mirror planes. O3 lies in a general position. At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about 410 km olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite and, at a