Majorite

Majorite is a type of garnet mineral found in the mantle of the Earth. Its chemical formula is Mg3(MgSi)(SiO4)3. It is distinguished from other garnets in having Si in octahedral as well as tetrahedral coordination. Majorite was first described in 1970 from the Coorara Meteorite of Western Australia and has been reported from various other meteorites in which majorite is thought to result from an extraterrestrial high pressure shock event.[2][3] Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.[6]

Majorite
General
CategoryNesosilicates
Formula
(repeating unit)		Mg3(MgSi)(SiO4)3
IMA symbolMaj[1]
Strunz classification9.AD.25
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupIa3d or tetragonal I41/a for pure MgSiO3
Unit cella = 11.52 Å; Z = 8
Identification
ColorPurple, pale yellowish brown, colorless
Crystal habitMicrocrystalline aggregates; acicular to equant grains in narrow veinlets
CleavageNone
Mohs scale hardness7-7.5
LusterVitreous
DiaphaneitySemitransparent
Specific gravityMeasured at about 4; 3.51 (calculated for pure MgSiO3)
Optical propertiesIsotropic
Refractive indexn = 1.87 - 1.92 (calculated)
Birefringenceslight
References[2][3][4][5]

Synthetic magnesium endmember majorite

Pure synthetic magnesium majorite (MgSiO3) is 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 MgSiO3 majorite is tetragonal.[7]

Majorite 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 550–900 km (340–560 mi). It forms complex solid solutions 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 convection currents the mineral breaks down and releases the oxygen. Recent 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.[8]

See also

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Handbook of Mineralogy, Mineral Data Publishing
  3. Majorite on MinDat
  4. Majorite on Webmineral
  5. The IMA Mineral List
  6. 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 Abstract
  7. 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.
  8. Bryner, Jeanna: Huge Stockpile of Oxygen Found Deep Inside Earth, LiveScience, 1 October 2007 08:38 am ET
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