Hedenbergite, Ca Fe Si₂ O₆, is the iron rich end member of the

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

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. Its 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 A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

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 In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

crystal system. Hedenbergite is found primarily in metamorphic rocks.

Composition and structure

The

quadrilateral easily records the compositions of different pyroxenes contained in

igneous rocks Igneous rock ( ), or magmatic rock, is one of the three main Rock (geology)#Classification, rock types, the others being sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rocks are formed through the cooling and solidifi ...

, such as diopside, hedenbergite, enstatite, ferrosilite. 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 Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

, and

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

. D. H. Lindsley and 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 The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

, and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. 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. Andradite-Hedenbergite-20546

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. Journal of Geophysical Research: Solid Earth and Planets 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

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s and

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s 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 Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...

Occurrence in chondrites

Chondrite A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

s are meteorites that have experienced very little alteration by melting or differentiation since the formation of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

4.56 billion years ago. One of the most studied chondrites in existence is the Allende meteorite. Hedenbergite was found to be the most abundant secondary calcium-rich silicate phase within Allende chondules and is closely associated with other minerals such as sodalite and nepheline. Kimura and Ikeda (1995) also suggest that hedenbergite formation may have been the result of the consumption of CaO and SiO₂ as plagioclases decomposed into sodalite and nepheline as well as alkali-calcium exchange before the condrules' incorporation into the parent body.

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 lithologies. The Nickel Plate

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

skarn deposit of the Hedley District in southern

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

is characterized by hedenbergitic

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

References

*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

External links

{{Commons category-inline Iron(II) minerals Calcium minerals Pyroxene group Monoclinic minerals Minerals in space group 15