Olivine

The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)₂SiO₄ in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). It gives its name to the group of minerals with a related structure (the olivine group) which includes monticellite and kirschsteinite. Olivine occurs in both mafic and ultramafic igneous rocks, and as a primary mineral in certain metamorphic rocks. It is one of the most common minerals on Earth, and has also been identified on the Moon, Mars, and comet Wild 2.

Identification and paragenesis

Olivine is usually colored olive-green (hence the name), 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. The deep green color is thought to be due to traces of nickel.

Transparent olivine is sometimes used as a gemstone, often 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 crystallizes from magma that is rich in magnesium and low in silica, which forms mafic to ultramafic rocks such as gabbro, basalt, peridotite, and dunite. Olivine or high pressure structural variants also constitute over 50% of the Earth's upper mantle making it 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. Olivine has also been discovered in meteorites, on Mars, and on Earth's moon. 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.Press Release 06-091. Jet Propulsion Laboratory Stardust website, retrieved May 30, 2006.

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 eighths 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 and phase diagram

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 wadslyite and at about 520 km depth wadslyite 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 wadslyite 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 wadslyite 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

According to Rebbenu Bachya, the word "Tarshish" in the verse Exodus 28:20 means "Chrysolite" and was the stone on the Ephod representing the tribe of Asher.

Notes

References

Hurlbut, Cornelius S., 1966 pr, Dana's Manual of Mineralogy, 17th ed., ISBN 0471032883
Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0

Magnesium minerals | Iron minerals | Nesosilicates