Majorite: Properties and Occurrences

Majorite is a type of garnet mineral found in the mantle of the Earth. It is a silicate of magnesium, iron, and aluminum, found in the upper mantle of the Earth, that stores oxygen when under intense pressure. Its chemical formula is Mg3(MgSi)(SiO4)3. It is a member of the garnet group.

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.

General Information

  • Category: Nesosilicates
  • Formula(repeating unit): Mg3(MgSi)(SiO4)3
  • Crystal system: Cubic
  • Crystal class: Hexoctahedral (m3m)
  • Color: Purple, pale yellowish brown, colorless.

Properties

It is a purple mineral of the garnet group containing magnesium and iron, crystallizing in the cubic system. 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.

  • Crystal habit: Microcrystalline aggregates; acicular to equant grains in narrow veinlets
  • 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

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.

Occurrence: Formed from low-calcium, high-aluminum pyroxene, olivine, and shock-induced glass, by high-pressure impact metamorphism in bolides.

Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.

Association: Pyroxene, ringwoodite, olivine, kamacite, goethite, troilite.

 

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