View source for Majorite

{{Short description|Garnet mineral}}
{{Use dmy dates|date=March 2022}}
{{Infobox mineral
| name        = Majorite
| boxwidth    =
| boxbgcolor  =
| image       = Majorite.jpg
| imagesize   =
| alt         =
| caption     =
| category    = [[Nesosilicates]]
| formula     = Mg<sub>3</sub>(MgSi)(SiO<sub>4</sub>)<sub>3</sub>
| IMAsymbol   = Maj<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight   =
| strunz      = 9.AD.25
| system      = [[Cubic crystal system|Cubic]]
| class       = Hexoctahedral (m{{overline|3}}m) <br/>[[H-M symbol]]: (4/m {{overline|3}} 2/m)
| symmetry    = ''Ia''{{overline|3}}''d'' or tetragonal ''I''4<sub>1</sub>/''a'' for pure MgSiO<sub>3</sub>
| unit cell   = a = 11.52&nbsp;Å; Z&nbsp;=&nbsp;8
| color       = Purple, pale yellowish brown, colorless
| colour      =
| habit       = Microcrystalline aggregates; acicular to equant grains in narrow veinlets
| twinning    =
| cleavage    = None
| fracture    =
| tenacity    =
| mohs        = 7–7.5
| luster      = Vitreous
| streak      =
| diaphaneity = Semitransparent
| gravity     = Measured at about 4; 3.51 (calculated for pure MgSiO<sub>3</sub>)
| density     =
| polish      =
| opticalprop = Isotropic
| refractive  = n = 1.87 – 1.92 (calculated)
| birefringence = slight
| pleochroism =
| 2V          =
| dispersion  =
| extinction  =
| references  = <ref name=HBM/><ref name=Mindat/><ref name=Webmin>[http://www.webmineral.com/data/Majorite.shtml Majorite on Webmineral]</ref><ref name=IMA>[http://rruff.info/ima/ The IMA Mineral List]</ref>
}}
'''Majorite''' is a [[mineral]] found in the [[Earth's mantle|mantle]] of the Earth. Its chemical formula is Mg<sub>3</sub>(MgSi)(SiO<sub>4</sub>)<sub>3</sub>. It is a type of [[garnet]], distinguished from other garnets in having [[silicon]] in octahedral as well as tetrahedral [[Coordination geometry|coordination]]. Majorite was first described in 1970 from the [[Coorara Meteorite]] of [[Western Australia]] and has been reported from various other [[meteorite]]s in which majorite is thought to result from an extraterrestrial high pressure shock event.<ref name=HBM>[http://rruff.geo.arizona.edu/doclib/hom/majorite.pdf Handbook of Mineralogy, Mineral Data Publishing]</ref><ref name=Mindat>[http://www.mindat.org/min-2546.html Majorite on MinDat]</ref> Mantle-derived [[xenolith]]s containing majorite have been reported from potassic [[ultramafic]] [[magma]]s on [[Malaita]] Island on the [[Ontong Java Plateau]] in the southwest Pacific Ocean.<ref>Kenneth D. Collerson, et al., ''Rocks from the Mantle Transition Zone: Majorite-Bearing Xenoliths from Malaita, Southwest Pacific'', Science 19 May 2000: Vol. 288. no. 5469, pp. 1215–1223 [http://www.sciencemag.org/cgi/content/abstract/288/5469/1215 Abstract]</ref>
==Synthetic magnesium endmember majorite==
Pure synthetic magnesium majorite (MgSiO<sub>3</sub>) is [[Polymorphism (materials science)|polymorphous]] with [[enstatite]], and [[akimotoite]]. 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 MgSiO<sub>3</sub> majorite is [[tetragonal]].<ref>Angel, R. J. (1989) Structure and twinning of single-crystal MgSiO3 garnet synthesized at 17 GPa and 1800-degrees-C, American Mineralogist, 74, 509-512.</ref>
==In the mantle==
Majorite is believed to be an abundant mineral in the lower transition zone and uppermost lower [[Mantle (geology)|mantle]] of the Earth at depths of {{convert|550|-|900|km|abbr=on}}.  It forms complex [[solid solution]]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]]. 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]]. However, when the temperature and pressure decrease as would occur when the majorite is drawn up toward the surface of the Earth by [[Mantle convection|convection currents]] 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.<ref>Bryner, Jeanna: ''Huge Stockpile of Oxygen Found Deep Inside Earth'', LiveScience, 1 October 2007 08:38 am ET [http://www.livescience.com/environment/071001_mineral_sponge.html]</ref>
==See also==
* [[Glossary of meteoritics]]
==References==
{{reflist}}
{{Meteorites}}
[[Category:Nesosilicates]]
[[Category:Meteorite minerals]]
[[Category:Cubic minerals]]
[[Category:Minerals in space group 230]]
[[Category:Garnet group]]