Majorite is a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

found in the mantle of the Earth. Its chemical formula is Mg₃(MgSi)(SiO₄)₃. It is a type of

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

, distinguished from other garnets in having

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

in octahedral as well as tetrahedral

coordination Coordination may refer to: * Coordination (linguistics), a compound grammatical construction * Coordination complex, consisting of a central atom or ion and a surrounding array of bound molecules or ions ** A chemical reaction to form a coordinati ...

. Majorite was first described in 1970 from the Coorara Meteorite of

Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...

and has been reported from various other

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

s in which majorite is thought to result from an extraterrestrial high pressure shock event.Handbook of Mineralogy, Mineral Data Publishing
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/ref> Mantle-derived

xenolith A xenolith ("foreign rock") is a rock (geology), rock fragment (Country rock (geology), country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusi ...

s containing majorite have been reported from potassic

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

s on

Malaita Malaita is the primary island of Malaita Province in Solomon Islands. Malaita is the most populous island of the Solomon Islands, with a population of 161,832 as of 2021, or more than a third of the entire national population. It is also the se ...

Island on the

Ontong Java Plateau The Ontong Java Plateau (OJP) is a massive oceanic plateau located in the southwestern Pacific Ocean, north of the Solomon Islands. The OJP was formed around (Ma), with a much smaller volcanic event around 90 Ma. Two other southwestern Pacific pl ...

in the southwest Pacific Ocean.

Synthetic magnesium endmember majorite

Pure synthetic magnesium majorite (MgSiO₃) is polymorphous with enstatite, and

akimotoite Akimotoite is a rare silicate mineral in the ilmenite group of minerals, with the chemical formula . It is polymorphous with pyroxene and with bridgmanite, a natural silicate perovskite that is the most abundant mineral in Earth's silicate mantle. ...

. Majorite is a member of the garnet group. It has Mg in eight-coordination with oxygen; it also has both Mg and Si in octahedral (6) coordination; and Si in tetrahedral (4) coordination with oxygen. Unlike most garnets, which are cubic, pure MgSiO₃ majorite is

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the Cube (geometry), cube becomes a rectangular Pri ...

In the mantle

Majorite is believed to be an abundant mineral in the lower transition zone and uppermost lower mantle of the Earth at depths of . It forms complex

solid solution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

s with other Al, Fe, and Ca-bearing garnets in this region. All of the minerals of the Earth's mantle are made of oxygen as the principal

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

. It has been reported that a significant property of majorite is that under conditions of high pressure and temperature as exist in the mantle the mineral tends to absorb and store

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

. However, when the temperature and pressure decrease as would occur when the majorite is drawn up toward the surface of the Earth by

convection currents Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...

the mineral breaks down and releases the oxygen. Research has suggested that the total amount of oxygen stored in majorite in the mantle is likely quite large and may in fact contribute to keeping the Earth's surface moist and habitable.Bryner, Jeanna: ''Huge Stockpile of Oxygen Found Deep Inside Earth'', LiveScience, 1 October 2007 08:38 am E

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References

{{Meteorites Nesosilicates Meteorite minerals Cubic minerals Minerals in space group 230 Garnet group

Synthetic magnesium endmember majorite

In the mantle

See also

References