Hedenbergite

Hedenbergite, CaFeSi₂0₆, is the iron rich end member of the pyroxene group having a monoclinic crystal system. The mineral is extremely rarely found as a pure substance, and usually has to be synthesized in a lab. It was named in 1819 after M.A. Ludwig Hedenberg, who was the first to define hedenbergite as a mineral. Contact metamorphic rocks high in iron are the primary geologic setting for hedenbergite. This mineral is unique because it can be found in chondrites and skarns (calc-silicate metamorphic rocks). Since it is a member of the pyroxene family, there is a great deal of interest in its importance to general geologic processes.

Properties

Hedenbergite has a number of specific properties. It’s hardness is usually between five and six with two cleavage plains and conchoidal fracture. Color varies between black, greenish black, and dark brown with a resinous luster. Hedenbergite is a part of a pyroxene solid solution chain consisting of diopside and augite, and is the iron rich end member. One of the best indicators that you have located hedenbergite is the radiating prisms with a monoclinic crystal system. Hedenbergite is found primarily in metamorphic rocks.

Composition and structure

The pyroxene quadrilateral above easily records the compositionsof different pyroxene's contained in igneous rocks, such as diopside, hedenbergite, enstatite, ferrosilite. [Lindsley D. H. and Munoz J. L. (1969) Solidus Relations Along The Join Hedenbergite – Ferrosilite. American Journal of Science. Vol. 267-A, pp. 295-324] Hedenbergite is almost never found isolated. From the chemical formulas above, we can tell that the main differences in the compositions will be in terms of calcium, magnesium, and iron. D. H. Lindsleyand J. L. Munoz (1969) did such an experiment in order to figure out exactly which combinations of temperature and pressure will cause particular minerals to combine. According to their experiment, at 1000 degrees with a pressure less than two kilobars the stable composition is a mixture of hedenbergite, olivine, and quartz. When the pressure moves to twenty kilobars, the composition moves towards the clinopyroxenes, which contains trace amounts of hedenbergite if any. For temperatures of 750 degrees Celsius, the compositions move from hedenbergite with olivine and quartz to ferrosilite with a greater amount of hedenbergite. If you combine the results of both of these sets of data, you can see that the stability of hedenbergite is more dependent on temperature as opposed to pressure.

Effects of chemical composition on elasticity

Pyroxenes are essential to the geologic processes that occur in the mantle and transition zones.Kandelin J. and Weidner D. J. (1988) Elastic Properties of Hedenbergite. American Journal of Science. Vol. 93, pp.1063-1072] One crystal was oriented with the C axis, and another perpendicular to the C axis. The elastic strength of a polyhedron is determined by the cation occupying the central site. As the bond length of the cations and anions decreases the bond strength increases making the mineral more compact and dense. Substitution between ions like Ca²⁺ and Mg²⁺ would not have a great effect on the resistance to compression while substitution of Si⁴⁺ would make it much harder to compress. Si⁴⁺ would be inherently stronger than Ca²⁺ due to the larger charge and electronegativity.

Occurrence in chondrites

Chondrites are meteorites that have not been altered in any way by melting or differentiation when forming. This means that these materials have been the same since the beginning of the solar system, which is approximately 4.55 billion years ago. The most studied CV3 chondorite is the Allende meteorite and is believed to be the most altered. H. Y. McSween (1977) did a vast amount of work with CV3 chondrites and characterized them as "a petrographically complex class of meteorites." It is believed by Hashimoto and Grossman (1987) that these alterations took place in a "highly oxidized solar nebula gas." Palme (1976) suggested that, all CV3 chondrites have been exposed to oxygen produced from the evaporation and condensation of previously existing material before the materials condensed to form the CV3 chondrites. However, there are mixed feelings about these ideas because some of the alteration could come from the shock impact and melting when the chondrite hit earth.

Occurrence in skarns

Hedenbergite can be found in skarns. A skarn is a metamorphic rock that is formed by the chemical alterations of the original minerals by hydrothermal causes. They are formed by large chemical reactions between adjacent lithologys. The Nickel Plate gold skarn deposit is characterized by hedenbergitic pyroxene (Ettlinger, Meinert, Ray, 1992).

References

*Ettlinger A. D., Meinert L. D., and Ray G. E. (1992) Gold Skarn Mineralization and Fluid Evolution in the Nickel Plate Deposite, British Columbia. Economic Geology. Vol. 87, pp. 1541-1565

*Hashimoto A. and Grossman L. (1987) Alteration of Al-Rich Inclusions Inside Ameboid Olivine Aggregates Inside the Allende Meteorite. Geochemica Et Chosmochemica. Acta 51. pp.1685-1704

*Krot A. N., Scott E. R. D, and Zolensky M. E. (1995) Mineralogical and chemical modification of components in CV3 chondrites: Nebular or asteroidal processing? Meteoritics, Journal of Meteoritical Society. Vol 30. pp. 748-775

*Farbe Minerals (2007) Ilvaite with Hedenbergite. www.webmineral.com/specimines/picshow.php?id=2801

*Pilcher R. (1996) Geology and Fieldwork in Oman. Geology Today.Vol. 12 Issue 1. pp. 31-34

*Wenk & Bulakh, (2006) Geos 306, Fall 2006, Lecture 12. http://www.geo.arizona.edu/xtal/geos306/fall06-12.htm