Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co., Arizona, USA.

The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure comprised 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. Image:Peridot in basalt. ... Image:Peridot in basalt. ... Earth cutaway from core to exosphere. ... Peridotite Peridotite is a dense, coarse grained ultrabasic rock, consisting mainly of the minerals olivine and pyroxene. ... The xenolith (Greek: foreign rock) is a rock which becomes enveloped in a larger rock during the latters development and hardening. ... Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ... The silicate minerals make up the the largest and most important class of rock-forming minerals. ... Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means change in form (from the Greek words meta, change, and morphe, form). The protolith is subjected to extreme heat (greater than 150 degrees Celsius) and pressure causing... Sedimentary, volcanic, plutonic, metamorphic rock types of North America. ... In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. ... In crystallography, the orthorhombic crystal system is one of the 7 lattice point groups. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Atomic mass 40. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ... General Name, Symbol, Number manganese, Mn, 25 Chemical series transition metals Group, Period, Block 7, 4, d Appearance silvery metallic Atomic mass 54. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/gray Atomic mass 6. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... General Name, Symbol, Number scandium, Sc, 21 Chemical series transition metals Group, Period, Block 3, 4, d Appearance silvery white Atomic mass 44. ... General Name, Symbol, Number titanium, Ti, 22 Chemical series transition metals Group, Period, Block 4, 4, d Appearance silvery metallic Atomic mass 47. ... General Name, Symbol, Number vanadium, V, 23 Chemical series transition metals Group, Period, Block 5, 4, d Appearance silvery gray metallic Atomic mass 50. ...

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 they were impurities in the glass, hence the name "fire strangers". However, they are simply early forming minerals that crystallized before the lava erupted. The materials definition of a glass is a uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form. ...

The upper mantle of Earth is composed mainly of olivine and pyroxene. A piece of the mantle is shown in Figure 1 (the pyroxene is the black crystal) showing the expected composition of 60% olivine and 30% pyroxene. Earth cutaway from core to exosphere. ... Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ...

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 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). Image File history File links Mg-Fe-Ca-pyroxene nomeclature File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Mg-Fe-Ca-pyroxene nomeclature File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... An ion is an atom or group of atoms with a net electric charge. ...

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 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 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. The pyroxene silicate minerals enstatite (MgSiO3) and ferrosilite (FeSiO3) form a complete solid solution series and are common rock-forming minerals found in igneous and metamorphic rocks and meteorites. ... In crystallography, the orthorhombic crystal system is one of the 7 lattice point groups. ... In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. ... Augite is a mafic mineral described chemically as (Ca, Na)(Mg, Fe, Al)(Al, Si)2O6 or calcium sodium magnesium iron aluminium silicate. ... Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. ... Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. ...

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 baring pyroxenes who's nomenclature is 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 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. Image File history File links Na-pyroxene nomenclature File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Na-pyroxene nomenclature File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Jadeite is a pyroxene mineral with composition NaAlSi2O6. ... Aegirine is an inosilicate member of the clinopyroxene group. ...

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

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 Aluminium Silicate)
  • Clinoenstatite (Magnesium Silicate)
  • Diopside (Calcium Magnesium Silicate, CaMgSi₂O₆)
  • Esseneite (Calcium Iron Aluminium Silicate)
  • Hedenbergite (Calcium Iron Silicate)
  • Hypersthene (Magnesium 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
  • Donpeacorite, (MgMn)MgSi₂O₆
  • Enstatite, Mg₂SiO₆
  • Ferrosilite, FeMgSi₂O₆
  • Nchwaningite (Hydrated Manganese Silicate)

• Schefferite, Ca(Mg,Fe,Mn)Si₂O₆
• Zinc schefferite, Ca(Mg,Mn,Zn)Si₂O₆
• Jeffersonite, Ca(Mg,Fe,Mn,Zn)Si₂O₆
• Leucaugite, Ca(Mg,Fe,Al)(Al,Si)₂O₆

In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. ... Aegirine is an inosilicate member of the clinopyroxene group. ... Augite is a mafic mineral described chemically as (Ca, Na)(Mg, Fe, Al)(Al, Si)2O6 or calcium sodium magnesium iron aluminium silicate. ... Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. ... Hedenbergite, or calcium iron pyroxene, is a black mineral closely allied to diopside and, owing to the isomorphous replacement of iron by magnesium, there is no sharp line of division between them. ... Hypersthene is a common rock-forming mineral belonging to the group of orthorhombic pyroxenes. ... Jadeite is a pyroxene mineral with composition NaAlSi2O6. ... Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate - LiAl(SiO3)2 - and is a source of lithium. ... In crystallography, the orthorhombic crystal system is one of the 7 lattice point groups. ... Hypersthene is a common rock-forming mineral belonging to the group of orthorhombic pyroxenes. ... The pyroxene silicate minerals enstatite (MgSiO3) and ferrosilite (FeSiO3) form a complete solid solution series and are common rock-forming minerals found in igneous and metamorphic rocks and meteorites. ... The pyroxene silicate minerals enstatite (MgSiO3) and ferrosilite (FeSiO3) form a complete solid solution series and are common rock-forming minerals found in igneous and metamorphic rocks and meteorites. ... Jeffersonite is a dark green mineral, a Manganese Zinc enriched form of Augite, sometimes compared to Aegerine. ...

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

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