Peridot
Peridot is gem-quality olivine, which is a silicate mineral with the formula of 2SiO4. As peridot is the variety of olivine, the formula approaches Mg2SiO4. The origin of the name peridot is uncertain, the Oxford English Dictionary suggests an alteration of Anglo–Norman pedoretés, a kind of opal, rather than the Arabic word faridat, meaning gem. The Middle English Dictionarys entry on peridot includes several variations, peritot, the earliest use in England is in the register of the St Albans Abbey, in Latin, and its translation in 1705 is possibly the first use of peridot in English. It records that on his death in 1245, Bishop John bequeathed various items including peridot to the Abbey, Peridot is one of the few gemstones that occur in only one color, an olive-green. In rare cases, peridot may occur in a medium-dark toned, peridots can be found in meteorites. Olivine in general is an abundant mineral, but gem quality peridot is rather rare. This is due to the chemical instability on the Earths surface.
Olivine is usually found as small grains, and tends to exist in a heavily weathered state, large crystals of forsterite, the variety most often used to cut peridot gems, are rare, as a result olivine is considered to be precious. Peridot crystals have been collected from some pallasite meteorites, Peridot is sometimes mistaken for emeralds and other green gems. Notable gemologist George Frederick Kunz discussed the confusion between emeralds and peridots in many church treasures, notably the Three Magi treasure in the Dom of Cologne, the largest cut peridot olivine is a 310 carat specimen in the Smithsonian Museum in Washington, D. C. Peridot olivine is the birthstone for the month of August, USGS peridot data Emporia Edu Florida State University – Peridot
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4 in the crystal system. Its name comes from Latin spina, balas ruby is an old name for a rose-tinted variety of spinel. Spinel crystallizes in the system, common crystal forms are octahedra. It has an imperfect cleavage and a conchoidal fracture. Its hardness is 8, its gravity is 3. 5–4.1. It may be colorless, but is usually shades of red, green, brown, black. There is a natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. The Samarian Spinel is the largest known spinel in the world, the transparent red spinels were called spinel-rubies or balas rubies. In the past, before the arrival of modern science, after the 18th century the word ruby was only used for the red gem variety of the mineral corundum and the word spinel came to be used. Balas is derived from Balascia, the ancient name for Badakhshan, mines in the Gorno Badakhshan region of Tajikistan was for centuries the main source for red and pink spinels.
Spinel has long been found in the gravel of Sri Lanka and in limestones of the Badakshan Province in modern-day Afghanistan and Tajikistan. Recently gem quality spinels found in the marbles of Luc Yen and Matombo, Tsavo and in the gravels of Tunduru and this is why spinel and ruby are often found together. Spinel, 2O4, is common in peridotite in the uppermost Earths mantle, Spinel, Al2O4, is a common mineral in the Ca-Al-rich inclusions in some chondritic meteorites. Synthetic spinel, accidentally produced in the middle of the 18th century, has described more recently in scientific publications in 2000 and 2004. By 2015, transparent spinel was being made in sheets and other shapes through sintering, synthetic spinel which looks like glass but has notably higher strength against pressure, can have applications in military and commercial use. Spinel group Ceylonite The Samarian Spinel, the largest known spinel in the world, part of the Iranian Crown Jewels Black Princes Ruby Deer, Howie, an Introduction to the Rock-Forming Minerals, Longman, pp. 424–433, ISBN 0-582-44210-9.
Gemstones of the World 3rd edition, Sterling, pp. 116–117, Spinel structure at the University of Wisconsin - Green Bay Spinel structure at the Institut for materials science of the University of Kiel Value of Spinel
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. Thus it is a type of nesosilicate or orthosilicate and it is a common mineral in the Earths subsurface but weathers quickly on the surface. The ratio of magnesium and iron varies between the two endmembers of the solid solution series and fayalite, compositions of olivine are commonly expressed as molar percentages of forsterite and fayalite. Forsterite has a high melting temperature at atmospheric pressure, almost 1,900 °C. The melting temperature varies smoothly between the two endmembers, as do other properties, olivine incorporates only minor amounts of elements other than oxygen, silicon and iron. Manganese and nickel commonly are the elements present in highest concentrations. Olivine gives its name to the group of minerals with a structure which includes tephroite and kirschsteinite. It has a structure similar to magnetite but uses one quadravalent. Olivine gemstones are called peridot and chrysolite, olivine is named for its typically 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, 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 mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks. Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica and that magma crystallizes to mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite and dunite can be left after extraction of magmas. Olivine and high pressure structural variants constitute over 50% of the Earths upper mantle, the metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content produces Mg-rich olivine, or forsterite. In contrast, Mg-rich olivine does not occur stably with silica minerals, Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km within Earth. Mg-rich olivine has discovered in meteorites, on the Moon and Mars, falling into infant stars.
Such meteorites include chondrites, 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 often have the signature of olivine
Ophiolite
An ophiolite is a section of the Earths oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Ophio is Greek for snake, and lite means stone from the Greek lithos, the term was little-used in other areas until the late 1950s to early 1960s, with the recognition that this assemblage provided an analog for oceanic crust and the process of seafloor spreading. Moores and Vine concluded that the dike complex at Troodos could form only by a process similar to the seafloor spreading proposed by Vine. Thus, it widely accepted that ophiolites represent oceanic crust that had been emplaced on land. This insight was one of the pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory. The stratigraphic sequence observed in ophiolites corresponds to the processes at mid-oceanic ridges, muds. Extrusive sequence, basaltic pillow lavas show magma/seawater contact, sheeted dike complex, parallel dikes that fed lavas above.
High level intrusives, isotropic gabbro, indicative of fractionated magma chamber, layered gabbro, resulting from settling out of minerals from a magma chamber. Cumulate peridotite, dunite-rich layers of minerals that settled out from a magma chamber, the Penrose field conference on ophiolites in 1972 redefined the term ophiolite to include only the igneous rocks listed above, excluding the sediments formed independently of the crust they sit on. Oceanic crust has a layered velocity structure that implies a layered rock series similar to that listed above, in detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating oceanic crust and ophiolites is that the thick layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. Seismic sounding of mid-ocean ridges has revealed only a few magma chambers beneath ridges, a few deep drill holes into oceanic crust have intercepted gabbro, but it is not layered like ophiolite gabbro.
For example, plagioclase and olivine in the dikes and lavas will alter to albite, chlorite. Thus there is reason to believe that ophiolites are indeed oceanic mantle and crust and these chemical differences extend to a range of trace elements as well. The crystallization order of feldspar and pyroxene in the gabbros is reversed, there is increasing evidence that most ophiolites are generated when subduction begins and thus represent fragments of fore-arc lithosphere. This led to introduction of the term supra-subduction zone ophiolite in the 1980s to acknowledge that some ophiolites are more related to island arcs than ocean ridges. A fore-arc setting for most ophiolites solves the problem of how oceanic lithosphere can be emplaced on top of continental crust. Most ophiolites can be divided one of two groups and Cordilleran
Garnet
Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, the different species are pyrope, spessartine, grossular and andradite. The garnets make up two solid solution series, pyrope-almandine-spessartine and uvarovite-grossular-andradite, the word garnet comes from the 14th‑century Middle English word gernet, meaning dark red. It is derived from the Latin granatus, from granum, Garnet species are found in many colors including red, yellow, purple, blue, black and colorless, with reddish shades most common. Garnet species light transmission properties can range from the gemstone-quality transparent specimens to the varieties used for industrial purposes as abrasives. The minerals luster is categorized as vitreous or resinous, garnets are nesosilicates having the general formula X3Y23. The X site is occupied by divalent cations 2+ and the Y site by trivalent cations 3+ in an octahedral/tetrahedral framework with 4− occupying the tetrahedra.
Garnets are most often found in the crystal habit, but are commonly found in the trapezohedron habit. They crystallize in the system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this group shows a range of hardness on the Mohs scale of about 6.5 to 7.5. The harder species like almandine are often used for abrasive purposes, for gem identification purposes, a pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones commonly used in the jewelry trade. Almandine, Fe3Al23 Pyrope, Mg3Al23 Spessartine, Mn3Al23 Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle, the term carbuncle is derived from the Latin meaning live coal or burning charcoal. The name Almandine is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times, almandine is an iron-aluminium garnet with the formula Fe3Al23, the deep red transparent stones are often called precious garnet and are used as gemstones.
Almandine occurs in metamorphic rocks like mica schists, associated with such as staurolite, andalusite. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle, Pyrope is red in color and chemically an aluminium silicate with the formula Mg3Al23, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from red to black. A variety of pyrope from Macon County, North Carolina is a shade and has been called rhodolite
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latters development and solidification. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement, a xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes, although the term xenolith is most commonly associated with igneous inclusions, a broad definition could include rock fragments which have become encased in sedimentary rock. Xenoliths are sometimes found in recovered meteorites and xenocrysts provide important information about the composition of the otherwise inaccessible mantle. Basalts, kimberlites and lamprophyres, which have their source in the mantle, often contain fragments. Xenoliths of dunite and spinel lherzolite in basaltic lava flows are one example, kimberlites contain, in addition to diamond xenocrysts, fragments of lherzolites of varying composition.
The aluminium-bearing minerals of these fragments provide clues to the depth of origin, calcic plagioclase is stable to a depth of 25 km. Between 25 km and about 60 km, spinel is the stable aluminium phase, at depths greater than about 60 km, dense garnet becomes the aluminium-bearing mineral. Some kimberlites contain xenoliths of eclogite, which is considered to be the high-pressure metamorphic product of basaltic oceanic crust, the large-scale inclusion of foreign rock strata at the margins of an igneous intrusion is called a roof pendant. Blatt and Robert J. Tracy
Diopside
Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and it forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical of the pyroxene series. It has a Mohs hardness of six, a Vickers hardness of 7.7 GPa at a load of 0.98 N, and a specific gravity of 3.25 to 3.55. It is transparent to translucent with indices of refraction of nα=1. 663–1.699, nβ=1. 671–1.705, the optic angle is 58° to 63°. Diopside is found in igneous rocks, and diopside-rich augite is common in mafic rocks, such as olivine basalt. Diopside is found in a variety of rocks, such as in contact metamorphosed skarns developed from high silica dolomites. It is an important mineral in the Earths mantle and is common in peridotite xenoliths erupted in kimberlite, some vermiculite deposits, most notably those in Libby, are contaminated with chrysotile that formed from diopside.
At relatively high temperatures, there is a miscibility gap between diopside and pigeonite, and at lower temperatures, between diopside and orthopyroxene. Chrome diopside is a constituent of peridotite xenoliths, and dispersed grains are found near kimberlite pipes. Occurrences are reported in Canada, South Africa, Brazil, much chromian diopside from the Green River Basin localities and several of the State Line Kimberlites have been gem in character. Gemstone quality diopside is found in two forms, the black star diopside and the chrome diopside, at 5. 5–6.5 on the Mohs scale, chrome diopside is relatively soft to scratch. Violane is a variety of diopside, violet to light blue in color. Diopside derives its name from the Greek dis, and òpsè, Diopside was discovered and first described about 1800, by Brazilian naturalist Jose Bonifacio de Andrada e Silva. Diopside based ceramics and glass-ceramics have potential applications in various technological areas, a diopside based glass-ceramic named silceram was produced by scientists from Imperial College, UK during the 1980s from blast furnace slag and other waste products.
The as produced glass-ceramic is a structural material. Similarly, diopside based ceramics and glass-ceramics have potential applications in the field of biomaterials, nuclear waste immobilization, S. Carter, C. B. Ponton, R. D. Rawlings, P. S. Rogers, chemistry, elastic properties and internal-friction of silceram glass-ceramics, T. Nonami, S. Tsutsumi, Study of diopside ceramics for biomaterials, Journal of Materials Science, Materials in Medicine 10 475-479
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with amounts of other minerals such as pyroxene, magnetite. Dunite is the olivine-rich end-member of the group of mantle-derived rocks. Dunite and other rocks are considered the major constituents of the Earths mantle above a depth of about 400 kilometers. It is found in alpine peridotite massifs that represent slivers of sub-continental mantle exposed during collisional orogeny, Dunite typically undergoes retrograde metamorphism in near-surface environments and is altered to serpentinite and soapstone. Dunite may form by the accumulation of crystals on the floor of large basaltic or picritic magma chambers. These cumulate dunites typically occur in layers in layered intrusions, associated with cumulate layers of wehrlite, olivine pyroxenite, harzburgite. Small layered intrusions may be of any age, for example, the Triassic Palisades Sill in New York.
The largest layered mafic intrusions are tens of kilometers in size and almost all are Proterozoic in age, e. g. the Stillwater igneous complex, the Muskox intrusion, and the Great Dyke. Cumulate dunite may be found in complexes, associated with layers of wehrlite, pyroxenite. Dunite was named by the German geologist, Ferdinand von Hochstetter in 1859 after Dun Mountain near Nelson, Dun Mountain was given its name because of the dun colour of the underlying ultramafic rocks. This color results from surface weathering that oxidizes the iron in olivine in temperate climates, a massive exposure of dunite in the United States can be found as Twin Sisters Mountain, near Mount Baker in the northern Cascade Range of Washington. In southern British Columbia, Canada dunite rocks form the core of an ultramafic rock complex located near the community of Tulameen. The rocks are enriched in platinum group metals and magnetite. Dunite could be used to sequester CO2 and help mitigate climate change via accelerated chemical rock weathering.
This would involve the mining of dunite rocks in quarries followed by crushing and grinding as to create fine ground rock that would react with the carbon dioxide. The resulting products are magnesite and silica which could be commercialized. Mg 2 SiO4 +2 CO2 ⟶2 MgCO3 + SiO2 Dunite Blatt and Robert J. Tracy,1996, Petrology, 2nd ed. W. H. Freeman, ISBN 0-7167-2438-3
Volcanic pipe
Volcanic pipes are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They are considered to be a type of diatreme, volcanic pipes are composed of a deep, narrow cone of solidified magma, and are usually largely composed of one of two characteristic rock types — kimberlite or lamproite. These rocks reflect the composition of the volcanoes deep magma sources and they are well known as the primary source of diamonds, and are mined for this purpose. Volcanic pipes form as the result of violent eruptions of deep-origin volcanoes, as the body of magma rises toward the surface, the volatile compounds transform to gaseous phase as pressure is reduced with decreasing depth. This sudden expansion propels the magma upward at rapid speeds, resulting in a shallow supersonic eruption, a useful analogy to this process is the uncorking of a shaken bottle of Champagne. Over time, the ring may erode back into the bowl. Kimberlite pipes are the source of most of the worlds diamond production, and contain other precious gemstones and semi-precious stones, such as garnets, spinels.
This broad cone is filled with volcanic ash and materials. Finally, the magma is pushed upward, filling the cone. The result is a martini-glass shaped deposit of material which appears mostly flat from the surface. Udachnaya pipe, a mine in Yakutia, Russia Elliott County Kimberlite Lake Ellen Kimberlite American Museum of Natural History. United States Geological Survey, Special Interest Publication
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration, the mineral alteration is particularly important at the sea floor at tectonic plate boundaries. Serpentinization is a geological low-temperature metamorphic process involving heat and water in which low-silica mafic and ultramafic rocks are oxidized and hydrolyzed with water into serpentinite. Peridotite, including dunite, at and near the seafloor and in mountain belts is converted to serpentine, brucite and other minerals — some rare, such as awaruite, and even native iron. In the process large amounts of water are absorbed into the increasing the volume. The density changes from 3.3 to 2.7 g/cm3 with a concurrent volume increase on the order of 30-40%, the reaction is highly exothermic and rock temperatures can be raised by about 260 °C, providing an energy source for formation of non-volcanic hydrothermal vents. The magnetite-forming chemical reactions produce hydrogen gas under anaerobic conditions prevailing deep in the mantle and sulfates are subsequently reduced by hydrogen and form methane and hydrogen sulfide.
The hydrogen and hydrogen sulfide provide energy sources for deep sea chemotroph microorganisms, serpentinite is formed from olivine via several reactions, some of which are complementary. Olivine is a solution between the magnesium-endmember forsterite and the iron-endmember fayalite. Serpentinite reaction 1a and 1b, exchange silica between forsterite and fayalite to form serpentine group minerals and magnetite, Reaction 1c describes the hydration of olivine with water only to yield serpentine and Mg2. Talc and magnesian chlorite are possible products, together with the serpentine minerals antigorite, the final mineralogy depends both on rock and fluid compositions and pressure. Antigorite forms in reactions at temperatures that can exceed 600 °C during metamorphism and chrysotile can form at low temperatures very near the Earths surface. In the presence of carbon dioxide, serpentinitization may form either magnesite or generate methane and it is thought that some hydrocarbon gases may be produced by serpentinite reactions within the oceanic crust.
Or, in balanced form, Reaction 2a is favored if the serpentinite is Mg-poor or if there isnt enough carbon dioxide to promote talc formation, Reaction 2b is favored in highly magnesian compositions and low partial pressure of carbon dioxide. If an olivine composition contains sufficient fayalite, olivine plus water can completely metamorphose to serpentine, serpentinitization of a mass of peridotite usually destroys all previous textural evidence because the serpentine minerals are weak and behave in a very ductile fashion. Serpentine is the product of the reaction between water and fayalites ferrous ions, the process is effected by bacteria under anaerobic conditions. The two unoxidised ferrous ions still available in the three units of fayalite finally combine with the four ferric cations and oxide anions to form two formula units of magnetite. Serpentinization has been proposed as an alternative source for the observed methane traces
Plagioclase
Plagioclase is a series of tectosilicate minerals within the feldspar group. Rather than referring to a mineral with a specific chemical composition, plagioclase is a continuous solid solution series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel in 1826, the series ranges from albite to anorthite endmembers, where sodium and calcium atoms can substitute for each other in the minerals crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or record-groove effect, plagioclase is a major constituent mineral in the Earths crust, and is consequently an important diagnostic tool in petrology for identifying the composition and evolution of igneous rocks. Plagioclase is a constituent of rock in the highlands of the Earths moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars, the extinction angle is an optical characteristic and varies with the albite fraction.
There are several named plagioclase feldspars that fall between albite and anorthite in the series, the following table shows their compositions in terms of constituent anorthite and albite percentages. Anorthite was named by Gustav Rose in 1823 from the Ancient Greek meaning oblique, anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites. Albite is named from the Latin albus, in reference to its pure white color. It is a common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline. The intermediate members of the group are very similar to each other. Bytownite, named after the name for Ottawa, Canada, is a rare mineral occasionally found in more basic rocks. Labradorite is the characteristic feldspar of the basic rock types such as diorite, andesite. Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal and it is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase.
A variety of known as spectrolite is found in Finland. Andesine is a mineral of rocks such as diorite which contain a moderate amount of silica. Oligoclase is common in granite, syenite and gneiss and it is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, sunstone is mainly oligoclase with flakes of hematite
Wehrlite
Wehrlite is an ultramafic and ultrabasic rock that is a mixture of olivine and clinopyroxene. It is a subdivision of the peridotites, the nomenclature allows up to a few percent of orthopyroxene. Accessory minerals include ilmenite, magnetite and an aluminium phase, wehrlites occur as mantle xenoliths and in ophiolites. Another occurrence is as cumulate in gabbro and norite layered intrusions, some meteorites can be classified as wehrlites. Wehrlite is named after Alois Wehrle and he was born 1791 in Kroměříž, Czech Republic and was a professor at the Ungarische Bergakademie in Banská Štiavnica, Slovakia
Media related to Peridotite at Wikimedia Commons
