Pyroxenes are a group of chain silicates constituting fundamental femic minerals in many igneous and metamorphic rocks. The name ‘pyroxene’ derives from the Greek πυρ ξένος, fire strangers, as early authors considered them to be impurities in glassy lava.
Structure and chemistry
Pyroxenes are chain silicates with general formula XYSi2O6. Their structure consists of (Si,Al)O4 tetrahedrons linked on opposite vertices to form infinite chains parallel to the long axis of the mineral. The top vertices of the tetrahedrons of each chain always face the same direction and subsequent chains in the structure have opposite polarity of these vertices, facing alternatively up and down with regularity. The tetrahedron chains are separated by layers of octahedral sites. The smaller M1 (or X) octahedral site occurs where vertices of opposite tetrahedron chains point one against the other, the larger M2 (or Y) octahedral site lies in contact with the bases of opposite tetrahedron chains. The bonds between tetrahedral chains and M1 sites are stronger than the bonds between neighboring tetrahedral chains and M2 sites and this determines the presence of two planes of weakness in the structure intersecting at about 87-88° or 92-93° (dashed lines in figure), corresponding to the cleavage planes of the pyroxenes. Pyroxenes can be classified in three main subgroups based on the content of X and Y sites: magnesium-iron pyroxenes (XY = largely Fe, Mg), calcic pyroxenes (Y occupied by Ca), and sodium pyroxenes (Y largely containing Na). Other important groups include calcium-sodium pyroxenes (containing Na and Ca in Y) and lithium pyroxenes (i.e. spodumene, LiAlSi2O6).
Hardness: 5 – 6 (spodumene: 7)
Cleavage: two perpendicular sets of prismatic cleavage
Twinning: some species show multiple and simple twins
Color: black, pale to dark green, brown, yellow, metallic
Luster: vitreous, metallic
Streak: white, grey, greenish, yellowish-grey
Alteration: amphibole, chlorite, talc, smectite, clay minerals, serpentine
In thin section…
Color: colorless to pale green, brown, yellow, reddish, greenish and dark green
Pleochroism: present in colored varieties
Birefringence (δ): 0.007-0.060 (low to high interference colors)
Optic sign: + or –
*optical properties vary significantly in different pyroxene types
The pyroxene quadrilateral
Magnesium-iron pyroxenes are a solid-solution between enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6) and are also known as orthopyroxenes, due to their orthorhombic crystal structure. Clinoenstatite and clinoferrosilite, the low-temperature monoclinic polymorphs of the orthopyroxenes, exist in nature but are rather uncommon. Calcium pyroxenes represent a solid solution between the monoclinic diopside (CaMgSi2O6) and hedenbergite (FeMgSi2O6), also known as clinopyroxenes.
Along the enstatite – ferrosilite series, the substitution of Ca in Y is normally less than 5%, due to the large size of this cation. However, there is some miscibility with the diopside – hedenbergite series described by the pyroxene quadrilateral (figure). Clinopyroxenes containing between 5 and 20% Ca are classified as pigeonite and, if they contain between 20 and 45% Ca, as augite. The miscibility between Fe-Mg pyroxenes and Ca-pyroxenes is greater at higher temperature (T = 900-1000 °C), although there is a miscibility gap between about 15 and 25% Ca in the formula and very limited miscibility close to the ‘pure’ Fe- and Mg-rich end-members of the two series.
Above the diopside – hedenbergite series (i.e. more than 50% Ca), there is no miscibility with wollastonite (CaSiO3), which has a different crystal structure (triclinic) and is not part of the pyroxene group.
Sodium and calcium-sodium pyroxenes
Sodium pyroxenes, all monoclinic, constitute a solid solution between jadeite (NaAlSi2O6) and aegirine (NaFe3+Si2O6). This subgroup of pyroxenes is normally characterized by extensive solid solution with Ca-pyroxenes (diopside, hedenbergite, augite), where the substitution of Na for Ca is accompanied by the substitution of (Al, Fe3+) for (Fe2+, Mg). If Ca in the Y site exceeds 20%, we pass to the sodium-calcium pyroxenes: the Al-rich varieties are known as omphacite and the Fe3+-rich varieties as aegirine-augite. If Ca in Y exceeds 80% (hence Na < 20%), the pyroxene belongs to the pyroxene quadrilateral (see above).
Pyroxenes are a wide group of minerals that show variable color and properties depending on their composition. However, they all show (1) prismatic habit and (2) two sets of prismatic cleavage planes that intersect at 90° on basal faces, making a ~ 45° angle with the long side of the basal face. They are usually dark-colored and show metallic luster, with colors ranging from black to green and brown, although pale colored varieties exist. Pyroxenes have hardness comprised between 5 and 6, with some (e.g. spodumene) reaching 7 on the Mohs scale. In igneous and metamorphic rocks, pyroxene can be confused with amphiboles, especially when cleavage traces are not well visible.
Pyroxenes in thin section
The identification of pyroxene in thin section is based mainly on the same features that can be recognized in the field: prismatic habit and presence of two perpendicular sets of perfect prismatic cleavage planes. The angle of extinction allows then to identify orthopyroxenes (with straight extinction on prismatic sections) from clinopyroxenes, whose angle of extinction varies widely. At PPL, members of the pyroxene group then range from colorless (diopside, jadeite, enstatite) to pale green, brown, yellow (ferrosilite, pigeonite, hedenbergite, augite), sometimes dark green (aegirine). Pleochroism is normally weak to moderate in colored varieties. In general, colors and pleochroism increase with increasing Fe content. Pyroxenes have high positive relief. Orthopyroxenes have low interference colors, while most clinopyroxenes show high second to third order interference colors. Pyroxenes can be confused with wollastonite (which occurs mostly in contact-metamorphosed limestones). Small pyroxene grains, lacking discernible cleavage planes, can be confused with olivine.
Above: Prismatic pyroxene (aegirine) crystals with hourglass zoning in a fine-grained groundmass of feldspars. Some crystals show well-developed cleavage planes. Alkaline syenite from Mt. Flora, Lovozero massif, Kola Peninsula, Russia. Field of view: 7 mm. Photo by Alessandro da Mommio/Strekeisen.
Pyroxenes occur predominantly in intermediate and mafic igneous rocks and in high-grade metamorphic rocks. Quadrilateral pyroxenes are fundamental constituents of basalts, gabbroic rocks, and peridotites and may occur in diorites, andesites, and more evolved quartz- or feldspathoid-bearing magmas. Calcium pyroxenes can be found also in metamorphic rocks, as a result of contact or regional metamorphism of impure limestones and dolomites. Orthopyroxenes are important minerals in migmatites and granulites, where they form as a result of the breakdown of biotite. High-pressure metamorphic rocks like eclogites are dominated by sodium and sodium-calcium pyroxenes like omphacite and jadeite that form beyond the stability field of plagioclase. Aegirine and aegirine-augite are products of crystallization of alkaline and peralkaline magmas and can be found in Na-rich alkali granites and syenites, either in the presence of quartz or nepheline.
References and Further Reading
Cameron, M., & Papike, J. J. (1981). Structural and chemical variations in pyroxenes. American Mineralogist, 66(1-2), 1-50.
Cloutis, E. A., & Gaffey, M. J. (1991). Pyroxene spectroscopy revisited: Spectral‐compositional correlations and relationship to geothermometry. Journal of Geophysical Research: Planets, 96(E5), 22809-22826.
Lindsley, D. H. (1983). Pyroxene thermometry. American Mineralogist, 68(5-6), 477-493.
Morimoto, N. (1989). Nomenclature of pyroxenes. Mineralogical Journal, 14(5), 198-221.