Olivine

The silicate with the formula (Mg,Fe)₂SiO₄. It is one of the most common minerals on Earth, and has also been identified in meteorites and on the Moon, Mars, and comet Wild 2.

The ratio of magnesium and iron varies between the two endmembers of the nickel commonly are the additional elements present in highest concentrations.

Olivine gives its name to the group of minerals with a related structure (the olivine group) which includes monticellite (CaMgSiO₄), and kirschsteinite (CaFeSiO₄).

Identification and paragenesis

    Olivine is usually named for its typically olive-green color (thought to be a result of traces of luster. It is transparent to translucent.

Transparent olivine is sometimes used as a gold and stone. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad island in the Red Sea.

Olivine/peridot occurs in both sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.

Fe-rich olivine is relatively much less common, but it occurs in orthopyroxene ((Mg,Fe)₂Si₂O₆).

Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km within Earth. Because it is thought to be the most abundant mineral in Earth’s mantle at shallower depths, the properties of olivine have a dominant influence upon the rheology of that part of Earth and hence upon the solid flow that drives plate tectonics. Experiments have documented that olivine at high pressures (e.g., 12 GPa, the pressure at depths of 360 kilometers or so) can contain at least as much as about 8900 parts per million (weight) of water, and that such water contents drastically reduce the resistance of olivine to solid flow; moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than contained in Earth’s oceans.^[1]

Mg-rich olivine has also been discovered in meteorites, on Mars, and on Earth's moon. Such meteorites include chondrites, collections of debris from the early solar system, and pallasites, mixes of iron-nickel and olivine. The spectral signature of olivine has been seen in the dust disks around young stars. The tails of comets (which formed from the dust disk around the young Sun) often have the spectral signature of olivine, and the presence of olivine has recently been verified in samples of a comet from the Stardust spacecraft. ^[2]

Crystal structure

Minerals in the olivine group crystallize in the ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions.

There are three distinct oxygen sites (marked O1, O2, and O3 in figure 1), two distinct metal sites (M1 and M2), and only one distinct silicon site. O1, O2, M2, and Si all lie on mirror planes, while M1 exists on an inversion center. O3 lies in a general position.

High pressure polymorphs

At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about 410 km olivine undergoes a spinel structure. These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods.

The pressure at which these phase transitions occur depends on temperature and iron content (Deer et al. 1992). At 800°C the pure magnesium end member, forsterite, transforms to wadsleyite at 11.8 gigapascals (118 kbar) and to ringwoodite at pressures above 14 GPa (140 kbar). Increasing the iron content decreases the pressure of the phase transition and narrows the mole fraction fayalite, olivine transforms directly to ringwoodite over the pressure range 10–11.5 GPa (100–115 kbar). Fayalite transforms to Fe₂SiO₄ spinel at pressures below 5 GPa (50 kbar). Increasing the temperature increases the pressure of these phase transitions.

Historical and mythical uses

The Septuagint names chrysolithos as a stone on the Hoshen in the verse Exodus 28:20; the masoretic text has the word tarshish, which has uncertain meaning, in the same place. According to the New International Version and Rebbenu Bachya, the word tarshish refers to chrysolite (olivine) and Rebbenu Bachya claims it was the stone representing the tribe of Asher. However, Chrysolite took its modern meaning much more recently, and in Greek times just meant golden stone (chryso-lithos), and could refer not only to yellowish olivine, but also to lapis lazuli which has golden flecks within its mainly blue surface and fits with the targum descriptions of the tarshish stone as being sea-colored. Tarshish probably refers to Tarshish, a place, though this doesn't identify the stone much more. In the Biblical account, there is a stone, on an earlier row, that scholars think was translucent and yellow, so scholars think that chrysolithos/tarshish here is unlikely to refer to olivine, because that would place two translucent stones next to each other, and be quite jarring; instead scholars favour yellow Jasper or Serpentine. There is a wide range of views among traditional sources about which tribe the stone refers to.

Uses

A worldwide search is on for cheap processes to sequester CO₂ by mineral reactions. Removal by reactions with olivine is an attractive option, because it is widely available and reacts easily with the (acid) CO₂ from the atmosphere . When olivine is crushed, it weathers completely within a few years, depending on the grain size. All the CO₂ that is produced by burning 1 liter of oil can be sequestered by less than 1 liter of olivine. The reaction is exothermic but slow. In order to recover the heat produced by the reaction to produce electricity, a large volume of olivine must be thermally well isolated. Then it can produce power, while at the same time removing CO₂ The end-products of the reaction are silicon dioxide, magnesium carbonate and small amounts of iron oxide.^{[citation needed]}

See also

• List of minerals
• Bowen's reaction series

References

1. ^ Smyth, J. R., Frost, D. J., Nestola, F., Holl, C. M., Bromiley, G., Olivine hydration in the deep upper mantle: Effects of temperature and silica activity. Geophysical Research Letters, v. 33, L15301, doi:10.1029/2006GL026194, 2006
2. ^ Press Release 06-091. Jet Propulsion Laboratory Stardust website, retrieved May 30, 2006.

Other sources

• Klein, Cornelis; and C. S. Hurlburt (1985). Manual of Mineralogy (21rst ed.). New York: John Wiley & Sons, 681 pp. ISBN 0-471-80580-7. 

• Deer, W. A.; R. A. Howie, and J. Zussman (1992). An Introduction to the Rock-Forming Minerals (2nd ed.). London: Longman, 696 pp. ISBN 0-582-30094-0.