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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 Australian 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. Ringwoodite is the polymorph of olivine, (Mg, Fe)2SiO4, with the spinel 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). Geological Occurrences Ringwoodite occurs in the veinlets cutting the matrix of meteorites and replacing olivine probably produced during shock metamorphism (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). Crystal Structure Ringwoodite is in the isometric crystal system and has space group 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). References * R A. Binns, R. J. Davis, and No S. J. B Reed (1969) Ringwoodite, natural (Mg,Fe)2SiO4 spinel in the Tenham meteorite. Nature 221,943 944 (1969) Retrieved from "http://en.wikipedia.org/" |
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