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28-538: Peridot can be found in mafic and ultramafic rocks occurring in lava and peridotite xenoliths of the mantle . The gem occurs in silica-deficient rocks such as volcanic basalt and pallasitic meteorites . Along with diamonds , peridot is one of only two gems observed to be formed not in Earth's crust , but in the molten rock of the upper mantle. Gem-quality peridot is rare on Earth's surface due to its susceptibility to alteration during its movement from deep within

56-404: A biaxial positive birefringence of 0.037–0.038. With decreasing magnesium and increasing iron concentration, the specific gravity, color darkness and refractive indices increase, and the β index shifts toward the γ index. Increasing iron concentration ultimately forms the iron-rich end-member of the olivine solid solution series fayalite . A study of Chinese peridot gem samples determined

84-413: A fraction of these settings. Peridots can also be found in meteorites. Peridots can be differentiated by size and composition. A peridot formed as a result of volcanic activity tends to contain higher concentrations of lithium, nickel and zinc than those found in meteorites. Olivine is an abundant mineral, but gem-quality peridot is rather rare due to its chemical instability on Earth's surface. Olivine

112-498: A silica content between 45 and 55 wt% , corresponding to the silica content of basalt in the TAS classification . Such rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast, the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium . The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to

140-401: Is near coral reefs , which are a popular diving attraction for tourists. The diving industry on the island has increased as well as the tourism industry on the island in general resulting in construction of hotels and diving shelves . The beaches are often quiet and are relatively unspoilt by development. However, most tourists go to this island as a "stop-off" before going to Rocky Island in

168-575: Is a portmanteau of "magnesium" and "ferric" and was coined by Charles Whitman Cross , Joseph P. Iddings , Louis V. Pirsson , and Henry Stephens Washington in 1912. Cross' group had previously divided the major rock-forming minerals found in igneous rocks into salic minerals, such as quartz , feldspars , or feldspathoids , and femic minerals, such as olivine and pyroxene . However, micas and aluminium-rich amphiboles were excluded, while some calcium minerals containing little iron or magnesium, such as wollastonite or apatite , were included in

196-431: Is a silicate mineral or igneous rock rich in magnesium and iron . Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine , pyroxene , amphibole , and biotite . Common mafic rocks include basalt , diabase and gabbro . Mafic rocks often also contain calcium -rich varieties of plagioclase feldspar. Mafic materials can also be described as ferromagnesian . The term mafic

224-468: Is no scientific evidence for any such claims.) Peridot olivine is the birthstone for the month of August. Peridot has often been mistaken for emerald beryl and other green gems. Noted gemologist G.F. Kunz discussed the confusion between beryl and peridot in many church treasures , most notably the " Three Magi treasure" in the Dom of Cologne , Germany. Mafic A mafic mineral or rock

252-466: Is not used as a rock classification in the IUGS classification scheme. Mafic rocks are sometimes more precisely defined as igneous rocks with a high proportion of pyroxene and olivine, so that their color index (the volume fraction of dark mafic minerals) is between 50 and 90. Most mafic volcanic rocks are more precisely classified as basalts . Chemically, mafic rocks are sometimes defined as rocks with

280-550: Is part of the Elba National Park meaning the island is conserved. However, the island was closed to the public for one year due to damage to corals and the disturbance of birds who breed on the island. The island serves as a breeding ground for at least nine known species of birds. The most recent discovery was that of 150 pairs of sooty falcon ( Falco concolor ) in October, 1994. There is very little vegetation on

308-856: Is the San Carlos Apache Indian Reservation in Arizona. It is also mined at another location in Arizona , and in Arkansas , Hawaii , Nevada , and New Mexico at Kilbourne Hole , in the US; and in Australia , Brazil , China , Egypt , Kenya , Mexico , Myanmar (Burma), Norway , Pakistan , Saudi Arabia , South Africa , Sri Lanka , and Tanzania . Peridot crystals have been collected from some pallasite meteorites . The most commonly studied pallasitic peridot belongs to

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336-512: Is the largest of a group of islands in Foul Bay , Egypt . It covers an area of 4.50 square kilometres (1.74 square miles). It is not a Quaternary volcanic island, but rather is believed to be an upthrust part of upper mantle material. The nearest island is known as " Rocky Island ". The island is slightly north of the Tropic of Cancer , and its highest point is 235 metres (771 feet). Before

364-412: Is uplifted mantle , a fragment of the sub- Red Sea lithosphere . Rocks on the island are mainly lower crustal igneous rocks . The island became present above sea level after African and Asiatic continental plates converged to cause rocks in the lower crust to be uplifted. The island contains three masses of peridotite , which are rich in the gemstone peridot (olivine). The island is believed to be

392-506: Is usually found as small grains and tends to exist in a heavily weathered state, unsuitable for decorative use. Large crystals of forsterite , the variety most often used to cut peridot gems, are rare; as a result, peridot is considered to be precious . In the ancient world, the mining of peridot was called topazios then, on St. John's Island, in the Red Sea began about 300  BCE . The principal source of peridot olivine today

420-533: The International Union of Geological Sciences (IUGS) classification of igneous rocks, include some light-colored ferromagnesian minerals, such as melilite , in the mafic mineral fraction. Accessory minerals , such as zircon or apatite, may also be included in the mafic mineral fraction for purposes of precise classification. When applied to rocks, the term mafic is used primarily as a field term to describe dark-colored igneous rocks. The term

448-594: The Indonesian Jeppara meteorite, but others exist such as the Brenham, Esquel, Fukang, and Imilac meteorites. Pallasitic (extraterrestrial) peridot differs chemically from its earthbound counterpart, in that pallasitic peridot lacks nickel. Orthorhombic minerals, like peridot, have biaxial birefringence defined by three principal axes: α, β , and γ . Refractive index readings of faceted gems can range around α = 1.651, β = 1.668, and γ = 1.689, with

476-418: The atomic structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied by magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions. Oxidation of peridot does not occur at natural surface temperature and pressure but begins to occur slowly at 600 °C (870 K) with rates increasing with temperature. The oxidation of

504-455: The femic minerals. Cross and his coinvestigators later clarified that micas and aluminium amphiboles belonged to a separate category of alferric minerals. They then introduced the term mafic for ferromagnesian minerals of all types, in preference to the term femag coined by A. Johannsen in 1911, whose sound they disliked. The term mafic is still widely used for dark-colored ferromagnesian minerals. Modern classification schemes, such as

532-563: The first discovered source of peridot, which was called topazios in ancient times, hence the Greek name for the island, Topazios . Layers of spinel - lherzolites with anhydrous Al - diopside pyroxenites and hydrous Cr -diopside pyroxenites can be found too on the island. Pliny, XXXV, chap. 22 says that, according to Juba , the island "Topazus in the Red Sea", also had mines producing sandarach ( realgar ) and ochre, "but neither of them are imported to us from that place." The island

560-552: The hydro-static specific gravity to be 3.36 . The visible-light spectroscopy of the same Chinese peridot samples showed light bands between 493.0–481.0 nm, the strongest absorption at 492.0 nm. The largest cut peridot olivine is a 310- carat (62-gram) specimen in the gem collection of the Smithsonian Museum in Washington, D.C. Inclusions are common in peridot crystals but their presence depends on

588-423: The island due to the lack of soil. The corals surround the island and act as a barrier for the fish and other sea life which live among it. However, the corals recently became damaged due to increased tourist activity and the expansion of the diving industry on the island. The coral is located approximately 25 metres below the surface. Fish that can be found within the nearby waters and corals are: The island

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616-476: The location where it was found and the geological conditions that led to its crystallization. Peridot has been prized since the earliest civilizations for its claimed protective powers to drive away fears and nightmares, according to superstitions. There is a superstition that it carries the gift of "inner radiance", sharpening the mind and opening it to new levels of awareness and growth, helping one to recognize and realize one's destiny and spiritual purpose. (There

644-709: The mantle and weathering at the surface. Peridot has a chemical formula of (Mg,Fe) 2 SiO 4 . Peridot is one of the birthstones for the month of August. The origin of the name peridot is uncertain. The Oxford English Dictionary suggests an alteration of Anglo – Norman pedoretés (classical Latin pæderot -), a kind of opal , rather than the Arabic word faridat , meaning " gemstone ". The Middle English Dictionary ' s entry on peridot includes several variations: peridod , peritot , pelidod and pilidod  — other variants substitute y for letter i used here. The earliest use of

672-480: The mid 270s [BCE] Ptolemaic activity along the African coast of the Red Sea had been limited with few significant results other than the discovery of the 'topaz' deposits on the island of Gazirat Zabarjad southeast of Ras Banas . ... 2. Pliny , HN 37, 108 refers to a large topaz that was brought back to Egypt as a gift for Ptolemy I 's queen Berenice I . The island is considered geologically unique as it

700-857: The older basic rock class. Mafic lava , before cooling, has a low viscosity , in comparison with felsic lava, due to the lower silica content in mafic magma . Water and other volatiles can more easily and gradually escape from mafic lava. As a result, eruptions of volcanoes made of mafic lavas are less explosively violent than felsic-lava eruptions. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Zabargad Island Zabargad Island ( Egyptian Arabic : جزيرة الزبرجد Geziret El Zabargad , also known as St. John's Island in English)

728-413: The olivine occurs by an initial breakdown of the fayalite component, and subsequent reaction with the forsterite component, to give magnetite and orthopyroxene . Olivine , of which peridot is a type, is a common mineral in mafic and ultramafic rocks , often found in lava and in peridotite xenoliths of the mantle , which lava carries to the surface; however, gem-quality peridot occurs in only

756-469: The percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, peridot may have a medium-dark toned, pure green with no secondary yellow hue or brown mask. Lighter-colored gems are due to lower iron concentrations. The molecular structure of peridot consists of isomorphic olivine, silicate, magnesium and iron in an orthorhombic crystal system . In an alternative view,

784-628: The word in English is possibly in the 1705 register of the St. Albans Abbey : The dual entry is in Latin with the translation to English listed as peridot . It records that on his death in 1245, Bishop John bequeathed various items, including peridot gems, to the Abbey. Peridot is one of the few gemstones that occur in only one color: an olive-green . The intensity and tint of the green, however, depends on

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