Ringwoodite

High-pressure phase of magnesium silicate

Ringwoodite

Crystal (~150 micrometers across) of Fo90 composition blue ringwoodite synthesized at 20 GPa and 1200 °C.

General Category

Nesosilicates
Spinel group

Formula
(repeating unit)

Magnesium silicate (Mg2SiO4)

IMA symbol

Rgd

Strunz classification

9.AC.15

Crystal system

Cubic

Crystal class

Hexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)

Space group

Fd3m

Unit cell

a = 8.113 Å; Z = 8

Identification Colour

Deep blue, also red, violet, or colourless (pure Mg2SiO4)

Crystal habit

Microcrystalline aggregates

Diaphaneity

Semitransparent

Specific gravity

3.90 (Mg2SiO4); 4.13 ((Mg0.91,Fe0.09)2SiO4); 4.85 (Fe2SiO4)

Optical properties

Isotropic

Refractive index

n = 1.8

Birefringence

none

Pleochroism

none

References

Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth\’s mantle between 525 and 660 km (326 and 410 mi) depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).

Ringwoodite is notable for being able to contain hydroxide ions (oxygen and hydrogen atoms bound together) within its structure. In this case two hydroxide ions usually take the place of a magnesium ion and two oxide ions.

Combined with evidence of its occurrence deep in the Earth\’s mantle, this suggests that there is from one to three times the world ocean\’s equivalent of water in the mantle transition zone from 410 to 660 km deep.

This mineral was first identified in the Tenham meteorite in 1969, and is inferred to be present in large quantities in the Earth\’s mantle.

Olivine, wadsleyite, and ringwoodite are polymorphs found in the upper mantle of the earth. At depths greater than about 660 kilometres (410 mi), other minerals, including some with the perovskite structure, are stable. The properties of these minerals determine many of the properties of the mantle.

Ringwoodite was named after the Australian earth scientist Ted Ringwood (1930–1993), who studied polymorphic phase transitions in the common mantle minerals olivine and pyroxene at pressures equivalent to depths as great as about 600 km.

2SiO
4), brucite (Mg(OH)
2), and silica (SiO
2) so as to give the desired final elemental composition. Putting this under 20 gigapascals of pressure at 1,523 K (1,250 °C; 2,282 °F) for three or four hours turns this into ringwoodite, which can then be cooled and depressurized.

Crystal structure

Ringwoodite has the spinel structure, in the isometric crystal system with space group Fd3m (or F43m). On an atomic scale, magnesium and silicon are in octahedral and tetrahedral coordination with oxygen, respectively. The Si-O and Mg-O bonds have mixed ionic and covalent character. The cubic unit cell parameter is 8.063 Å for pure Mg2SiO4 and 8.234 Å for pure Fe2SiO4.

Chemical composition

Ringwoodite compositions range from pure Mg2SiO4 to Fe2SiO4 in synthesis experiments. Ringwoodite can incorporate up to 2.6 percent by weight H2O.

Physical properties

Molar volume vs. pressure at room temperature for ringwoodite γ-Mg2SiO4
Molar volume vs. pressure at room temperature for ahrensite γ-Fe2SiO4

The physical properties of ringwoodite are affected by pressure and temperature. At the pressure and temperature condition of the Mantle Transition Zone, the calculated density value of ringwoodite is 3.90 g/cm3 for pure Mg2SiO4; 4.13 g/cm3 for (Mg0.91,Fe0.09)2SiO4 of pyrolitic mantle; and 4.85 g/cm3 for Fe2SiO4. It is an isotropic mineral with an index of refraction n = 1.768.

The colour of ringwoodite varies between the meteorites, between different ringwoodite bearing aggregates, and even in one single aggregate. The ringwoodite aggregates can show every shade of blue, purple, grey and green, or have no colour at all.

A closer look at coloured aggregates shows that the colour is not homogeneous, but seems to originate from something with a size similar to the ringwoodite crystallites. In synthetic samples, pure Mg ringwoodite is colourless, whereas samples containing more than one mole percent Fe2SiO4 are deep blue in colour. The colour is thought to be due to Fe2+–Fe3+ charge transfer.

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