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Pyroxene

From Academic Kids

The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure which comprises of single chains of silica tetrahedra and they crystalise in the monoclinic and orthorhombic system. 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 commonly substitutes for silicon in other silicates, it is not a common substitution in the pyroxene.

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

The mantle of Earth is composed mainly of olivine and pyroxene.


Contents

Chemistry and nomenclature of the pyroxenes

Missing image
Pyrox_names.png
Figure 1: 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 octahedral X (or M1) and Y (or M2) sites and the tetrahederal T site. 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 only silicon in the tetrahedral site and ions with a charge of +2 in both of the octahedral (X and Y) sites giving the formular XYT2O6. The names of the common calcium - iron - magnesium pyroxenes are defined in the 'pyroxene quadrilateral' shown in figure 1. 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 cxalcium 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 calcum 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 pigenite 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.

Missing image
Na_pyrox_trig.png
Figure 2: 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 baring pyroxenes who's nomenclature is shown in figure 2. 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 X 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 1.

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+
X Al Fe3+ Ti4+ Cr V Ti3+ Zr Sc Zn Mg Fe2+ Mn
Y 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 X site and bulkier ions on the Y 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 Y and X sites respectively. For example Na and Al give the jadeite (NaAlSi2O6) composition.
  2. Coupled substitution of a 1+ ion on the Y site and a mixture of equal numbers of 2+ and 4+ ions on the X site. This leads to e.g. NaFe2+0.5Ti4+0.5Si2O6.
  3. The Tschermak substitution where a 3+ ion ocupies the X 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 Aluminum Silicate)
    • Clinoenstatite (Magnesium Silicate)
    • Diopside (Calcium Magnesium Silicate, CaMgSi2O6)
    • Esseneite (Calcium Iron Aluminum Silicate)
    • Hedenbergite (Calcium Iron Silicate)
    • Hypersthene (Magnesium Iron Silicate)
    • Jadeite (Sodium Aluminum 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 Aluminum Silicate)
    • Petedunnite (Calcium Zinc Manganese Iron Magnesium Silicate)
    • Pigeonite (Calcium Magnesium Iron Silicate)
    • Spodumene (Lithium Aluminum Silicate)
  • Orthopyroxenes (Orthorhombic)
    • Hypersthene
    • Donpeacorite, (MgMn)MgSi2O6
    • Enstatite, Mg2SiO6
    • Ferrosilite, FeMgSi2O6
    • Nchwaningite (Hydrated Manganese Silicate)


References

Morimoto, N., J. Fabries, A.K. Ferguson, I.V. Ginzburg, M. Ross, F.A. Seifeit and J. Zussman (1989) "Nomenclature of pyroxenes" Canadian Mineralogist Vol.27 pp143-156 http://www.mineralogicalassociation.ca/doc/abstracts/ima98/ima98(12).pdf


External links

ja:輝石 pl:Pirokseny

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