Ringwoodite
Encyclopedia
Ringwoodite is a high-pressure polymorph
of olivine
, and it is stable at high temperatures and pressures like those in the Earth's mantle near 600 km depth. This mineral was first identified in the Tenham Meteorites
in 1969, and it is inferred to be present in large quantity in the earth’s mantle. It was named after the Australia
n earth scientist Alfred E. Ringwood
who studied polymorphic phase transitions in the common mantle minerals, olivine and pyroxene, at pressures equivalent to depths as great as about 600 km. Olivine
, wadsleyite
, and ringwoodite are polymorphs found in the upper mantle of the earth; at depths greater than about 660 km; other minerals, including some with the perovskite
structure, are stable. The properties of these minerals determine many of the properties of the mantle.
structure. Spinel-group minerals crystallize in the isometric system with an octahedral habit. Olivine is most abundant in the upper mantle, above about 400 km; the olivine polymorphs, wadsleyite
and ringwoodite, are thought to dominate the transition zone of the mantle, a zone present from about 400 to 660 km depth. Ringwoodite is thought to be the most abundant mineral phase in the lower part of Earth’s transition zone. The physical and chemical property of this mineral partly determine properties of the mantle at those depths. The pressure range for stability of ringwoodite lies in the approximate range from 18 to 23 GPa.
Apart from the mantle, natural ringwoodite had been found in many shocked chondritic meteorites
, in which the ringwoodite occurs as fine-grained polycrystalline (Chen et al., 2004: Goresy et al., 2004; Gillet et al., 2004).
(Mineral Data Publishing, 2001). In our planet, olivine is located in mantle at depths less than about 410 km, and ringwoodite is inferred to be present within the transition zone from about 520 km to 660 km depth. Seismic discontinuities at about 410, 520, and 660 km depth have been attributed to phase changes involving olivine and its polymorphs. The 520-km discontinuity is generally believed to be caused by the transition of the olivine polymorph, wadsleyite
(P-phase) to ringwoodite (y-phase), while the 660-km discontinuity by the phase transformation of ringwoodite (y-phase) to a perovskite-structured mineral plus magnesiowüstite (Deuss et al., 2001: Woodhouse et al., 2001).
Ringwoodite in the lower half of the transition zone is inferred to play a pivotal role in mantle dynamics, and the plastic properties of ringwoodite are thought to be critical in determining flow of material in this part of the mantle (Xu et al., 2003; Weidner et al., 2003; Chen et al., 2003; Vaughan et al., 2003; Wang et al., 2003; Uchida et al., 2003). The solubility of hydroxide
in ringwoodite is important because of the effect of hydrogen upon rheology
. Ringwoodite synthesized at conditions appropriate for the transition zone has been found to contain up to 1.1 weight percent water (Smyth et al., 2003). Because the transition zone between the Earth’s upper and lower mantle helps govern the scale of mass and heat transport throughout the Earth, the presence of water within this region, whether global or localized, may have a significant effect on mantle rheology and therefore mantle circulation (Kavner, 2003). In regions of subduction zones, the ringwoodite stability field hosts high levels of seismicity (Xu et al., 2003; Weidner et al., 2003; Chen et al., 2003; Vaughan et al., 2003; Wang et al., 2003; Uchida et al., 2003).
Fd3m. It has cubic symmetry at low temperatures. On the atomic scale, Mg and Si are in octahedral and tetrahedral coordination with oxygen, respectively. The Si-O and Mg-O bonds are both ionic and covalent.
The cubic unit cell parameter is 8.068Å.
Colour: 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 they 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 (Stöffler et al., 1991; Lingemann et al., 1994).
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, and it is stable at high temperatures and pressures like those in the Earth's mantle near 600 km depth. This mineral was first identified in the Tenham Meteorites
Tenham Meteorites
Tenham meteorites are the fragments of a larger meteorite that fell in 1879 in a remote area of Australia near the Tenham station, South Gregory, in western Queensland. Although the fall was seen by a number of people its exact date has not been established. Bright meteors were seen to be moving...
in 1969, and it is inferred to be present in large quantity in the earth’s mantle. It was named after the Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n earth scientist Alfred E. Ringwood
Ted Ringwood
Alfred Edward "Ted" Ringwood FRS was an Australian experimental geophysicist and geochemist, and the 1988 recipient of the Wollaston Medal....
who studied polymorphic phase transitions in the common mantle minerals, olivine and pyroxene, at pressures equivalent to depths as great as about 600 km. Olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, wadsleyite
Wadsleyite
Wadsleyite is a high-pressure polymorph of olivine, an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In the phase transformations from Mg2SiO4-Fe2SiO4 , olivine is transformed to wadsleyite β-Mg2SiO4 and then to a spinel-structured γ-Mg2SiO4 with increasing pressure...
, and ringwoodite are polymorphs found in the upper mantle of the earth; at depths greater than about 660 km; other minerals, including some with the perovskite
Perovskite
A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
structure, are stable. The properties of these minerals determine many of the properties of the mantle.
Introduction
Ringwoodite is the polymorph of olivine, (Mg, Fe)2SiO4, with the spinelSpinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
structure. Spinel-group minerals crystallize in the isometric system with an octahedral habit. Olivine is most abundant in the upper mantle, above about 400 km; the olivine polymorphs, wadsleyite
Wadsleyite
Wadsleyite is a high-pressure polymorph of olivine, an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In the phase transformations from Mg2SiO4-Fe2SiO4 , olivine is transformed to wadsleyite β-Mg2SiO4 and then to a spinel-structured γ-Mg2SiO4 with increasing pressure...
and ringwoodite, are thought to dominate the transition zone of the mantle, a zone present from about 400 to 660 km depth. Ringwoodite is thought to be the most abundant mineral phase in the lower part of Earth’s transition zone. The physical and chemical property of this mineral partly determine properties of the mantle at those depths. The pressure range for stability of ringwoodite lies in the approximate range from 18 to 23 GPa.
Apart from the mantle, natural ringwoodite had been found in many shocked chondritic meteorites
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...
, in which the ringwoodite occurs as fine-grained polycrystalline (Chen et al., 2004: Goresy et al., 2004; Gillet et al., 2004).
Geological occurrences
Ringwoodite occurs in the veinlets cutting the matrix of meteorites and replacing olivine probably produced during shock metamorphismShock metamorphism
Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorphic effects unequivocally associated with explosions...
(Mineral Data Publishing, 2001). In our planet, olivine is located in mantle at depths less than about 410 km, and ringwoodite is inferred to be present within the transition zone from about 520 km to 660 km depth. Seismic discontinuities at about 410, 520, and 660 km depth have been attributed to phase changes involving olivine and its polymorphs. The 520-km discontinuity is generally believed to be caused by the transition of the olivine polymorph, wadsleyite
Wadsleyite
Wadsleyite is a high-pressure polymorph of olivine, an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In the phase transformations from Mg2SiO4-Fe2SiO4 , olivine is transformed to wadsleyite β-Mg2SiO4 and then to a spinel-structured γ-Mg2SiO4 with increasing pressure...
(P-phase) to ringwoodite (y-phase), while the 660-km discontinuity by the phase transformation of ringwoodite (y-phase) to a perovskite-structured mineral plus magnesiowüstite (Deuss et al., 2001: Woodhouse et al., 2001).
Ringwoodite in the lower half of the transition zone is inferred to play a pivotal role in mantle dynamics, and the plastic properties of ringwoodite are thought to be critical in determining flow of material in this part of the mantle (Xu et al., 2003; Weidner et al., 2003; Chen et al., 2003; Vaughan et al., 2003; Wang et al., 2003; Uchida et al., 2003). The solubility of hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
in ringwoodite is important because of the effect of hydrogen upon rheology
Rheology
Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....
. Ringwoodite synthesized at conditions appropriate for the transition zone has been found to contain up to 1.1 weight percent water (Smyth et al., 2003). Because the transition zone between the Earth’s upper and lower mantle helps govern the scale of mass and heat transport throughout the Earth, the presence of water within this region, whether global or localized, may have a significant effect on mantle rheology and therefore mantle circulation (Kavner, 2003). In regions of subduction zones, the ringwoodite stability field hosts high levels of seismicity (Xu et al., 2003; Weidner et al., 2003; Chen et al., 2003; Vaughan et al., 2003; Wang et al., 2003; Uchida et al., 2003).
Crystal structure
Ringwoodite is in the isometric crystal system and has space groupSpace group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
Fd3m. It has cubic symmetry at low temperatures. On the atomic scale, Mg and Si are in octahedral and tetrahedral coordination with oxygen, respectively. The Si-O and Mg-O bonds are both ionic and covalent.
The cubic unit cell parameter is 8.068Å.
Physical properties
The physical properties of ringwoodite are affected by the pressure and temperature. The calculated density value of ringwoodite is 3.9 g/cm3. It is an isotropic mineral with an index of refraction n = 1.768. It is isotropic and has high surface relief.Colour: 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 they 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 (Stöffler et al., 1991; Lingemann et al., 1994).