Pyroxenes are a group of chain silicates that occur as fundamental Mg, Fe, Ca, and Na-bearing minerals in many igneous and metamorphic rocks. The term ‘pyroxene’ derives from the Greek πυρ ξένος, ‘fire stranger, and was coined by the French mineralogist Rene Just Haüy in 1796 in reference to their finding as crystals within glassy lava: the leading idea was that these ‘fire strangers’ were impurities of the glass, not formed by the action of heat. Nowadays, it is well known that phenocrysts of pyroxene in lava often represent product of early crystallization of magma at depth, i.e. before the eruption.
Struttura e chimica
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 c-axis of the mineral (figure below). The top vertices of the tetrahedrons of each individual chain are always pointing in the same direction, whereas adjacent tetrahedral chains in the structure alternatively point up and down along the short crystal axis (a axis). The tetrahedron chains, although not linked laterally, form ‘tetrahedral layers’ in the structure, oriented parallel to the intermediate axis (b-axis). These ‘tetrahedral layers’ alternate in the structure with layers of octahedral sites (M-sites or XY sites). Two types of octahedral sites occur in the structure of pyroxene: the smaller M1 (or X) octahedral sites are located where vertices of opposite tetrahedron chains point towards each other, whereas the larger M2 (or Y) octahedral sites are found adjacent to the bases of opposite tetrahedron chains (figure below). The M1 sites and its adjacent tetrahedral chains form a series of small T-O-T ‘sandwiches’ (called I-beams) whose internal bonds are stronger than the bonds between neighboring tetrahedral chains or between tetrahedral chains and M2 sites. This produces two planes of weakness in the structure intersecting at about 87-88° or 92-93° that correspond macroscopically to the sfaldatura planes of the pyroxenes.
From a chemical point of view, pyroxenes can be distinguished based on the content of X and Y sites in three main subgroups: magnesium-iron pyroxenes (XY = Fe, Mg), calcic pyroxenes (Y = Ca), and sodium pyroxenes (Y = Na). Additionally, other important groups in rocks are represented by calcium-sodium pyroxenes (Y = Na, Ca) and lithium pyroxenes (i.e. spodumene, LiAlSi2O6).
Crystal of augite, a type of pyroxene. 35 mm across. Photo by Siim Sepp (sandatlas.org).
Durezza: 5 – 6 (spodumene: 7)
Sfaldatura: two perpendicular sets of prismatic cleavage
Geminazione: some species show multiple and simple twins
Colore: black, pale to dark green, brown, yellow, metallic
Lucentezza: vitreous, metallic
Struscio: white, grey, greenish, yellowish-grey
Alterazione: amphibole, chlorite, talc, smectite, clay minerals, serpentine
In sezione sottile...
Colore: colorless to pale green, brown, yellow, reddish, greenish and dark green
Pleocroismo: present in colored varieties
Birifrangenza (δ): 0.007-0.060 (low to high interference colors)
Segno ottico: + o -
*optical properties vary significantly in different pyroxene types
The pyroxene quadrilateral
Magnesium-iron pyroxenes form a solid-solution between enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6). These are also known as orthopyroxenes, because of their orthorhombic crystal structure. Similarly, calcium pyroxenes consists of a solid-solution between the diopside (CaMgSi2O6) and hedenbergite (FeMgSi2O6), which are also known as clinopyroxenes, because of their monoclinic crystal structure and consequent oblique extinction at crossed polarized light. The compositional space between enstatite-ferrosilite and diopside-hedenbergite is known as the pyroxene quadrilateral and comprises two additional clinopyroxenes: augite e pigeonite. Pigeonite lies closer to the orthopyroxenes and it is characterized by a Ca content between 5 and 20%, whereas augite contains between 20 and 45% in its octahedral sites. The miscibility between the enstatite-ferrosilite series and the diopside-hedenbergite series is hindered by the large ionic radius of the Ca cation with respect to Fe, Mg, which makes the solid solution impossible at low temperature. Indeed, the Ca content of orthopyroxenes is normally less than 5%. Augitic and pigeonitic compositions can form at higher temperatures of crystallization (T = 900-1000 °C) at which the extent miscibility between orthopyroxenes and Ca-pyroxenes is greater. However, a miscibility gap between about 15 and 25% Ca persists up to very high temperatures and, for this reason, pigeonite commonly develops exsolution lamellae of augite and orthopyroxene after its formation.
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, due to the different morphology of the tetrahedral chains.
Sodium and calcium-sodium pyroxenes
Sodium pyroxenes, all monoclinic, form a solid solution between jadeite (NaAlSi2O6) and aegirine (NaFe3+Si2O6). This subgroup of pyroxenes is normally characterized by an extensive solid solution with Ca-pyroxenes (diopside, hedenbergite, augite): the substitution of Na for Ca is balanced by the coeval substitution of (Al, Fe3+) for (Fe2+, Mg). If Ca in the Y site exceeds 20%, the mineral is considered a sodium-calcium pyroxene, which is termed omphacite when Al-rich, and as aegirine-augite, when it is richer in Fe3+. If the Ca content of Y exceeds 80%, the mineral belongs to the pyroxene quadrilateral (see above).
Caratteristiche di terreno
Pyroxenes are a large group of minerals that occur predominantly in igneous and metamorphic rocks, showing variable color and mineral properties. However, all members of this group generally tend to show (1) prismatic habit and (2) at least two sets of prismatic cleavage planes intersecting at nearly 90° on the basal face, oriented at ~ 45° with respect to the (100) and (010) crystal faces (figure above). Pyroxenes are generally dark-colored (black, brown, to dark green) and show metallic luster, although light-colored varieties exist. The hardness of pyroxenes is comprised between 5 and 6 (weaker than quartz), but some, i.e. spodumene, may reach 7 on the Mohs scale. Pyroxene can be confused with anfiboli, especially in situations where cleavage planes are not evident.
Pyroxenes in thin section
The identification of pyroxene in thin section is straightforward based on the same criteria used to recognize the mineral in the field: prismatic habit and presence of at least two orthogonal sets of perfect prismatic sfaldatura planes. The basal section of pyroxene, where cleavage planes are visible (shown below) is 8-sided. Orthopyroxenes and clinopyroxenes can be identified based on the angle of extinction of prismatic section at CPL: orthopyroxenes show straight extinction, whereas clinopyroxenes show oblique extinction at variable angle.
At PPL, pyroxenes range from colorless (diopside, jadeite, enstatite) to pale green, brown, yellow (ferrosilite, pigeonite, hedenbergite, augite), sometimes dark green (aegirine). Pleocroismo is normally weak, but the most colored varieties (in general Fe-rich pyroxenes like ferrosilite and aegirine) show stronger pleochroism. At CPL, orthopyroxenes typically show low, first-order interference colors, whereas most clinopyroxenes display 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, which, however, generally shows higher interference colors.
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. Width: 7 mm. Photo © 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. They can also be found in diorites, andesites, and feldspathoid-bearing rocks. More rarely, pyroxenes occur together with quartz in rocks like tonalites. Calcium pyroxenes occur also in metamorphic rocks, especially in metamorphosed impure carbonate rocks, carbonate hornfelses, and skarns, where they coexist with a plethora of other calc-silicates. Orthopyroxenes are important constituents of migmatites and granulites, where they form as a result of the breakdown of biotite. High-pressure metamorphic rocks like eclogites contains 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.
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