Majorite

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Majorite
General
Category Nesosilicates
Formula
(repeating unit)
Mg3(Fe,Al,Si)2(SiO4)3
Strunz classification 09.AD.25
Crystal symmetry Isometric hexoctahedral
H-M symbol: (4/m32/m) Space Group: Ia3d or tetragonal I41/a for pure MgSiO3
Unit cell a = 11.52 Å; Z=8
Identification
Color Purple, pale yellowish brown, colorless
Crystal habit Microcrystalline aggregates; acicular to equant grains in narrow veinlets
Crystal system Cubic
Cleavage None
Mohs scale hardness 7-7.5
Luster Vitreous
Diaphaneity Semitransparent
Specific gravity Measured at about 4; 3.51 (calculated for pure MgSiO3)
Optical properties Isotropic
Refractive index n = 1.87 - 1.92 (calculated)
Birefringence slight
References [1][2][3]

Majorite is a type of garnet mineral found in the upper mantle of the Earth. Its chemical formula is Mg3(Fe,Al,Si)2(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.[1][2] Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.[4]

Synthetic magnesium endmember majorite[edit]

Pure synthetic magnesium majorite (MgSiO3)is a polymorph of 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.[5]

Majorite in the mantle[edit]

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 kilometres (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.[6]

See also[edit]

References[edit]

  1. ^ a b Handbook of Mineralogy, Mineral Data Publishing
  2. ^ a b Majorite on MinDat
  3. ^ Majorite on Webmineral
  4. ^ 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
  5. ^ 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.
  6. ^ Bryner, Jeanna: Huge Stockpile of Oxygen Found Deep Inside Earth, LiveScience, 1 October 2007 08:38 am ET [1]