Pyroxene
The pyroxenes are an important group of silicates that form part of many igneous and metamorphic rocks. Their name comes from the words "pyro" and "xeno" from ancient Greek meaning "fire" and "strange". This name was given by René Just Haüy because he considered its occurrence in lavas as something alien.
Chemical and physical characteristics
They have a common structure consisting of simple chains of silica tetrahedrons. Its general formula is XY(Si, Al)2O6, where "X" represents calcium, sodium, iron2+, manganese, lithium or magnesium, and "Y" represents smaller ions such as chromium, aluminum, iron2+, iron3+, magnesium, manganese or titanium. Among the different pyroxenes according to their chemistry there is a great amount of miscibility. Varieties rich in iron, magnesium, and calcium are most common.
With a vitreous luster, they are unalterable by hydrochloric acid. Pyroxenes with iron are dark, those without this element are usually whitish, gray or light green. Pyroxenes usually occur in prismatic or small needle forms. In appearance and chemistry they resemble amphiboles but pyroxenes differ in that they lack a hydroxyl (OH) in their crystal structure and lack the "A" site. 3. 4; from amphiboles and therefore cannot accommodate large alkaline elements. Another easier difference to detect is that pyroxenes have a plane of cleavage at approximately 93° and 87°, while amphibole has them at 56° and 124°.
Based on their symmetry, pyroxenes are subdivided into ortho and clinopyroxenes. The former are orthorhombic and the latter are monoclinic.
Classification
The group of pyroxenes includes two subgroups, depending on the crystallization system. Clinopyroxenes crystallize in the monoclinic system (such as augite, diopside, and spodumene), while orthopyroxenes crystallize in the orthorhombic system (such as bronzite, enstatite, and hypersthene).
Clinopyroxenes
Name of mineral chemical formula Egyptian NaFe3+Yeah.2O6 Augita (Ca, Mg, Fe)2(Yes, Al)2O6 Clinoenstatita MgSiO3 Clinoferrosilita Fe2+Yes3 Cosmochloro NaCrSi2O6 Diopsy CaMgSi2O6 Onfacita (Ca,Na)(Mg,Fe, Al)Si2O6 Esseneita CaFe3+Al SiO6 Grossmanita Cain3+Al SiO6 Hedenbergita CaFe2+Yeah.2O6 Jadeita NaAlSi2O6 Jervisita NaScSi2O6 Johannsenita CaMn2+Yeah.2O6 Kanoita MnMg(SiO3)2 Kushiroita CaAl(AlSiO6) Namansilita NaMn3+Yeah.2O6 Natalyita NaV3+Yeah.2O6 Petedunnita CaZnSi2O6 Pigeonite (Mg, Fe, Ca)SiO3 Espodumena LiAlSi2O6 Wollastonita CasiO3 - Ca3Yes3O9)
Orthopyroxenes
Name of mineral chemical formula Donpaacorita Mn2+Mg(SiO3)2 Enstatita MgSiO3 Ferrosilita (Federal2+)2(SiO3)2 Hiperstena (Fe, Mg)2Yeah.2O6 Broncita (Mg, Fe)SiO3
Paragenesis
Pyroxenes are part of many igneous and metamorphic rocks. In igneous rocks, pyroxenes form in magmas at temperatures of 1,000 to 1,300 °C, being one of the first phases to crystallize. Its rapid weathering means that it is not usually part of sedimentary rocks. Some of the igneous rocks where it is usually found pyroxene are basalt, gabbro and peridotite.
In igneous systems, enstatite can be formed when olivine in a magma comes into contact with quartz or its constituent SiO2, components that cannot normally coexist in chemical equilibrium. This situation is expressed in the following chemical reaction:
- Mg2SiO4(orlivinor)+SiO2(silice)→ → Mg2Si2O6(enstatita)- Yeah.
Omphacite, a pyroxene of sodium and aluminum, is found only in eclogites, rocks that have undergone very high temperature and pressure metamorphism. Eclogites containing omphacite can be found in exhumed subduction zones. In South Africa eclogite nodules with omphacite on peridotite have been found in kimberlite chimneys.
Diopside is usually found in calcium-rich rocks, such as limestone and dolomite, that have undergone contact metamorphism (also called thermal metamorphism) or regional metamorphism. In skarn areas, diopside is generated by contact metamorphism and is associated with wollastonite, vesuvianite, grossular and tremolite. The chemical reaction that generates diopside during contact metamorphism of calcium-rich rocks is a decarbonatization process that can be expressed in the following formula:
- CaMg(CO3)2(dorlormita)+SiO2(silice)→ → CaMgSi2O6(diorpsidor)+2CO2(diorxidordecarbornor){displaystyle CaMg(CO_{3})_{2}(dolomite) + SiO_{2}(silice)rightarrow CaMgSi_{2}O_{6}(diopsido) + 2CO_{2}(dioxidodecarbono)}}
Another variety of pyroxene that can form during contact metamorphism is hedenbergite, it appears when iron-rich sediments undergo this process.
Weathering
Chemical weathering of pyroxene produces clay minerals as a final product. Since clay minerals are heterogeneous, it can be said that the composition of said minerals will depend on the initial composition of the pyroxene. It is considered that in the initial phases of weathering they are regulated by the pyroxene structure and that complex biopyriboles are produced. In a particular study, the transformation of pyroxene into talc-like structures by iron was observed. In the same study it could be seen that talc-like structures separated from iron in advanced stages of weathering producing iron oxides and pure talc on a microscopic scale.
Augite weathering and alteration can produce both actinolytic (uralitic) amphiboles and chlorite and more rarely epidote and carbonates. The actinolytic amphiboles produced by the alteration commonly occur in the form of aggregates of small prismatic crystals but also more infrequently in the form of a single crystal. The alteration of the augite usually begins at its edges or in the cleavage zones, producing small areas of discolored pyroxene speckled with tiny plates of amphibole.