OLIVINE

any member of a group of common magnesium, iron silicate minerals. any member of a group of common magnesium, iron silicate minerals. Olivines occur in many basic and ultrabasic igneous rocks and comprise a major constituent of the Earth's upper mantle. They also have been found in some (type A) lunar rocks and in many meteorites. All olivines have the same basic orthorhombic structure in which isolated SiO4 tetrahedra are linked by chains of metal-bearing octahedra. The oxygen atoms of the structure are in approximate hexagonal close-packing, and the two octahedral sites are not equivalent but equal in number. Olivine compositions can be generally described as A2BO4 in which the A octahedral ions are most commonly magnesium (Mg) and iron (Fe) with minor amounts of manganese, nickel, and calcium also present; the B tetrahedral ion is silicon. Natural olivine compositions range from nearly pure Mg2SiO4 (forsterite) to Fe2SiO4 (fayalite) with complete solid solution. Nickel may be present in magnesium-rich olivines, calcium is usually present in all compositions, and manganese is common in iron-rich compositions. No appreciable substitution occurs for silicon. Olivine forms vitreous yellow to greenish yellow equidimensional crystals having no obvious cleavage. Density ranges from 3.2 grams per cubic cm for forsterite to 4.4 grams per cubic cm for fayalite. Olivine is a particularly interesting mineral for interpreting the origin of igneous rocks. Because it is generally the first mineral to crystallize from basic and ultrabasic rocks, its composition should reflect to some extent that of the parent magma. In particular, the magnesium-iron ratio of olivine bears a relation to that of its magma as established experimentally and by the study of natural rocks. Minor element concentrations can also be used to derive concentrations in the parent liquid; they also may be used to a limited extent as an indicator of crystallization environment. The ultrabasic rocks dunite and peridotite generally have olivines with between 92 and 88 mole percent forsterite, while basic rocks such as gabbro and basalt have olivines of 85 to 40 mole percent forsterite. Because olivine crystallizes early, thick accumulations may result from gravity settling, especially in igneous intrusions. Compositional zoning within one crystal is very common and represents the changing magma composition as crystallization progresses. Iron-rich olivines are common in quartz-bearing syenites and they occur occasionally in granites. Rather pure forsterite occurs in metamorphosed limestone and dolomites. Olivine is a stable phase only at high temperatures; in the presence of water at low temperatures, it readily undergoes hydrothermal alteration, forming serpentine, chlorite, magnetite, or talc. Common olivine is relatively infusible, withstanding temperatures of more than 1,500 C (2,700 F), and is sometimes employed in refractory brick. Transparent green olivine is called peridot and is used as a gemstone. Totally serpentinized olivine rocks are commonly utilized as ornamental stone. Additional reading W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (196263), with a 2nd ed. in progress (1978 ); Manual of Mineralogy (after James D. Dana), 20th ed. by Cornelis Klein and Cornelius S. Hurlbut, Jr. (1985); J.J. Papike and M. Cameron, Crystal Chemistry of Silicate Minerals of Geophysical Interest, Reviews of Geophysics and Space Physics, 14(1):3780 (1976); and Paul H. Ribbe (ed.), Orthosilicates, 2nd ed. (1982). William B. Simmons