Majorite

minerals

diamond

ringwoodite

troilite

Formula: Mg3(MgSi)(SiO4)3
Nesosilicate (insular SiO4 groups), garnet group, forms a series with pyrope
Specific gravity: 4
Hardness: 7 to 7½
Streak: White
Colour: Brown, yellow
Common impurities: Cr,Ni,Ca,Na
Environments

Metamorphic
Extraterrestrial

Majorite occurs as minute grains formed from low-calcium, high-aluminium pyroxene, olivine and shock-induced glass, by high-pressure impact metamorphism in bolides (large meteors which explode in the atmosphere) (Webmin, HOM,Dana). Associated minerals include pyroxene, ringwoodite, olivine, iron variety kamacite, goethite and troilite (HOM).
Majorite is believed to be an abundant mineral in the lower transition zone and uppermost lower mantle of the Earth at depths of 550 to 900 km. It forms complex solid solutions with other aluminium, iron and calcium-bearing garnets in this region (Wiki). Majorite crystallises at pressures between 16 and 23 GPa and temperatures above 1600oC. It is thought to be a major constituent of the transition zone of the Earth's mantle (AM 78.1165-1173). Single crystals of calcium-bearing majorite have been synthesised at 18.2 GPa and 2050oC (AM 79.581-584).

Localities

At the type locality, the Coorara meteorite, Rawlinna, Dundas Shire, Western Australia, majorite occurs as purple grains often coarsely interwoven with pyroxene. Other associated minerals include ringwoodite, olivine, iron variety kamacite and goethite (AM 55.1815).

On Malaita Island, Malaita Province, Solomon Islands, rocks containing high-pressure mineral assemblages derived from the mantle transition zone between depths of about 400 and 670 kilometers occur as xenoliths and megacrysts. Observed ultrahigh pressure minerals include majorite, calcium- and magnesium- perovskite, aluminous silicates and diamond. Majoritic garnets in these xenoliths record pressures of up to 22 GPa. The occurrence of material with perovskite chemistry and several enigmatic aluminous phases indicates pressures of up to 27 GPa. Samples were brought to the surface about 34 million years ago by potassic ultramafic magmas, which evidently originated in the lower mantle (AAAS 288, Issue 5469.1215-1223).

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