chrysolite n : a brown or yellow-green olivine found in igneous and metamorphic rocks and used as a gemstone

English

Noun

1. olivine

The mineral olivine (when gem-quality also called peridot) is a magnesium iron 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 solid solution series: forsterite (Mg-endmember) and fayalite (Fe-endmember). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). Forsterite has an unusually high melting temperature at atmospheric pressure, almost 1900°C, but the melting temperature of fayalite is much lower (about 1200°C). The melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen, silicon, magnesium, and iron. Manganese and 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 tephroite (Mn2SiO4), 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 nickel), though it may alter to a reddish color from the oxidation of iron. It has a conchoidal fracture and is rather brittle. The hardness of olivine is 6.5–7, its relative density is 3.27–3.37, and it has a vitreous luster. It is transparent to translucent.

Transparent olivine is sometimes used as a gemstone called peridot, the French word for olivine. It is also called chrysolite, from the Greek words for 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 mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks. Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica. That magma crystallizes to mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite, and dunite can be residues left after extraction of magmas, and typically they are more enriched in olivine after extraction of partial melts. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and olivine is one of the Earth's most common minerals by volume. The metamorphism of impure dolomite or other 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 igneous rocks in small amounts in rare granites and rhyolites, and extremely Fe-rich olivine can exist stably with quartz and tridymite. In contrast, Mg-rich olivine does not occur stably with silica minerals, as it would react with them to form 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.

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.

Crystal structure

Minerals in the olivine group crystallize in the orthorhombic system (space group Pbnm) with isolated silicate tetrahedra, meaning that olivine is a nesosilicate. In an alternative view, the atomic structure can be described as a hexagonal, close-packed array of oxygen 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 phase transition to the sorosilicate, wadsleyite and, at about 520 km depth, wadsleyite transforms into ringwoodite, which has the 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 wadsleyite stability field. At about 0.8 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 Topaz, Amber, yellow Jasper, yellow Serpentine, or even 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.

See also

• List of minerals
• Bowen's reaction series

References