Pyroxene


Pyroxene
Figure 1: A sample of pyroxenite, a rock consisting mostly of pyroxene minerals.

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY(Si,Al)2O6 (where X represents calcium, sodium, iron+2 and magnesium and more rarely zinc, manganese and lithium and Y represents ions of smaller size, such as chromium, aluminium, iron+3, magnesium, manganese, scandium, titanium, vanadium and even iron+2). Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes.

The name pyroxene comes from the Greek words for fire (πυρ) and stranger (ξένος). Pyroxenes were named this way because of their presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic glass; it was assumed they were impurities in the glass, hence the name "fire strangers". However, they are simply early-forming minerals that crystallized before the lava erupted.

Mantle-peridotite xenolith from San Carlos Indian Reservation, Gila Co., Arizona, USA. The xenolith is dominated by green peridot olivine, together with black orthopyroxene and spinel crystals, and rare grass-green diopside grains. The fine-grained gray rock in this image is the host basalt.(unknown scale)

The upper mantle of Earth is composed mainly of olivine and pyroxene. A piece of the mantle is shown at right (orthopyroxene is black, diopside (containing chromium) is bright green, and olivine is yellow-green) and is dominated by olivine, typical for common peridotite. Pyroxene and feldspar are the major minerals in basalt and gabbro.

Contents

Chemistry and nomenclature of the pyroxenes

Figure 2: The nomenclature of the calcium, magnesium, iron pyroxenes.

The chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are primarily defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X (or M2) site, the Y (or M1) site, and the tetrahederal T site. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size. Twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 previously used names have been discarded (Morimoto et al., 1989).

A typical pyroxene has mostly silicon in the tetrahedral site and predominately ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT2O6. The names of the common calcium – iron – magnesium pyroxenes are defined in the 'pyroxene quadrilateral' shown in Figure 2. The enstatite-ferrosilite series ([Mg,Fe]SiO3) contain up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite (and the ferrosilite equivalents). Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite ([Mg,Fe,Ca][Mg,Fe]Si2O6) only crystallises in the monoclinic system. There is not complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. This leads to a miscibility gap between pigeonite and augite compositions. There is an arbitrary separation between augite and the diopside-hedenbergite (CaMgSi2O6 – CaFeSi2O6) solid solution. The divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineral wollastonite has the formula of the hypothetical calcium end member but important structural differences mean that it is not grouped with the pyroxenes.

Figure 3: The nomenclature of the sodium pyroxenes.

Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to nomenclature shown in Figure 3. The inclusion of sodium, which has a charge of +1, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In jadeite and aegirine this is added by the inclusion of a +3 cation (aluminium and iron(III) respectively) on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron(II) components are known as omphacite and aegirine-augite, with 80% or more of these components the pyroxene falls in the quadrilateral shown in figure 2.

Table 1 shows the wide range of other cations that can be accommodated in the pyroxene structure, and indicates the sites that they occupy.

Table 1: Order of cation occupation in the pyroxenes
T Si Al Fe3+
Y Al Fe3+ Ti4+ Cr V Ti3+ Zr Sc Zn Mg Fe2+ Mn
X Mg Fe2+ Mn Li Ca Na

In assigning ions to sites the basic rule is to work from left to right in this table first assigning all silicon to the T site then filling the site with remaining aluminium and finally iron(III), extra aluminium or iron can be accommodated in the Y site and bulkier ions on the X site. Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above and there are several alternative schemes:

  1. Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively. For example Na and Al give the jadeite (NaAlSi2O6) composition.
  2. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site. This leads to e.g. NaFe2+0.5Ti4+0.5Si2O6.
  3. The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g. CaAlAlSiO6.

In nature, more than one substitution may be found in the same mineral.

Pyroxene minerals

  • Clinopyroxenes (monoclinic)
    • Aegirine (Sodium Iron Silicate)
    • Augite (Calcium Sodium Magnesium Iron Aluminium Silicate)
    • Clinoenstatite (Magnesium Silicate)
    • Diopside (Calcium Magnesium Silicate, CaMgSi2O6)
    • Esseneite (Calcium Iron Aluminium Silicate)
    • Hedenbergite (Calcium Iron Silicate)
    • Jadeite (Sodium Aluminium Silicate)
    • Jervisite (Sodium Calcium Iron Scandium Magnesium Silicate)
    • Johannsenite (Calcium Manganese Silicate)
    • Kanoite (Manganese Magnesium Silicate)
    • Kosmochlor (Sodium Chromium Silicate)
    • Namansilite (Sodium Manganese Silicate)
    • Natalyite (Sodium Vanadium Chromium Silicate)
    • Omphacite (Calcium Sodium Magnesium Iron Aluminium Silicate)
    • Petedunnite (Calcium Zinc Manganese Iron Magnesium Silicate)
    • Pigeonite (Calcium Magnesium Iron Silicate)
    • Spodumene (Lithium Aluminium Silicate)
  • Orthopyroxenes (orthorhombic)
    • Hypersthene (Magnesium Iron Silicate)
    • Donpeacorite, (MgMn)MgSi2O6
    • Enstatite, Mg2Si2O6
    • Ferrosilite, Fe2Si2O6
    • Nchwaningite (Hydrated Manganese Silicate)

See also

References

External links


Wikimedia Foundation. 2010.

Synonyms:

Look at other dictionaries:

  • pyroxène — [ pirɔksɛn ] n. m. • 1801; de pyro et gr. xenos « étranger », c. à d. « étranger au feu, non igné » ♦ Minér. Minéral constituant un des groupes des silicates essentiels. L amiante est un pyroxène. ● pyroxène nom masculin Silicate naturel de… …   Encyclopédie Universelle

  • Pyroxene —   [zu griechisch xénos »Gast«, »Fremder«] Plural, wichtige Gruppe gesteinsbildender Minerale (zum Teil auch als Augite bezeichnet), meist grün, braun bis schwarz; Härte nach Mohs um 6, Dichte 3 3,5 g/cm3; Kristalle säulig bis stängelig, stets… …   Universal-Lexikon

  • Pyroxene — Pyr ox*ene, n. [F. pyrox[ e]ne, from Gr. ? fire + ? a stranger; so called because it was supposed to the be a stranger, or of rare occurrence, in igneous rocks,] (Min.) A common mineral occurring in monoclinic crystals, with a prismatic angle of… …   The Collaborative International Dictionary of English

  • Pyroxene — Pyroxène Les pyroxènes (du grec πυρ, feu, et ξενος, étranger, « étranger au feu »[1]) sont une famille de minéraux du groupe des inosilicates. Ce sont des composants courants des roches ignées et métamorphiques. Ils sont apparentés aux… …   Wikipédia en Français

  • pyroxene — [pī räk′sēn΄] n. [Fr pyroxène < Gr pyr, FIRE + xenos, stranger: from its being foreign to igneous rocks] any of a group of monoclinic or orthorhombic silicate minerals usually containing iron or magnesium but not the hydroxyl radical, and… …   English World dictionary

  • Pyroxene — Als Pyroxengruppe bezeichnet man Minerale aus der Mineralklasse Silicate und Germanate, sowie der Abteilung der Kettensilicate, deren Kristallstruktur sich durch Einfachketten aus eckenverknüpften SiO4 Tetraedern auszeichnet und deren… …   Deutsch Wikipedia

  • pyroxene — pyroxenic /puy rok sen ik/, adj. /puy rok seen, peuh , puy rok seen /, n. any of a very common group of minerals of many varieties, silicates of magnesium, iron, calcium, and other elements, occurring as important constituents of many kinds of… …   Universalium

  • Pyroxène — Figure 1 : Manteau xénolithique péridotite de la réserve indienne de San Carlos, Gila Co., Arizona, États Unis. Le xénolithique est dominé par l olivine péridot vert, avec de l orthopyroxène (Pyroxène) noir et des cristaux de spinelle, et de …   Wikipédia en Français

  • pyroxene — noun Etymology: French pyroxène, from Greek pyr + xenos stranger Date: 1800 any of a group of igneous rock forming silicate minerals that contain calcium, sodium, magnesium, iron, or aluminum, usually occur in short prismatic crystals or massive… …   New Collegiate Dictionary

  • PYROXÈNE — n. m. T. de Minéralogie Nom donné aux Silicates de chaux, de magnésie et de fer, que l’on rencontre accidentellement dans les produits volcaniques …   Dictionnaire de l'Academie Francaise, 8eme edition (1935)


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.