Olivine
The
mineral olivine is a
magnesium iron silicate with the formula (Mg,Fe)
2SiO
4 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.
|
Peridot olivine in basalt. Location: San Carlos Indian Reservation, Gila Co., Arizona, USA. |
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 6.5â€"7, its
relative density is 3.27â€"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. ] |
Figure 1: The atomic scale structure of olivine looking along the a axis. Oxygen is shown in red, silicon in pink and magnesium/iron in blue. A projection of the unit cell is shown by the black rectangle |
Minerals in the olivine group crystallize in the
orthorhombic system (
space group P
bnm) 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.
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 Fe
2SiO
4 spinel at pressures below 5 GPa (50 kbar). Increasing the temperature increases the pressure of these phase transitions.
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.
*
List of minerals*
Bowen's reaction series*
Calculation of Olivine-Liquid Equilibria by Ford, 1982*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
