The pyroxene group minerals are single-chain silicates with repeat units of two SiO4 tetrahedra (~ 5.2Å) along the chain direction (c-axis). Chemical formulae, using site nomenclature, are given by: M2M1T2O6, where M2 represents medium- to large-size cations, commonly Ca2+, Na+, Fe2+, Mg2+, Mn2+, and Li+; M1 represents small- to medium-size cations like Fe2+, Mg2+, Al3+, Fe3+, and Ti4+; and T represents Si4+ and Al3+ in tetrahedral sites. The minerals develop good {110} cleavage, with cleavage angles near 90 degrees. The pyroxene group is divided further to subgroups according to composition (and symmetry). The common pyroxenes form solid solutions of the Ca-Mg-Fe pyroxenes and are compositionally described (e.g., Morimoto et al., 1988) in the pyroxene quadrilateral with end-members diopside (Di: CaMgSi2O6), hedenbergite (Hd: CaFeSi2O6), enstatite (En: Mg2Si2O6), and ferrosilite (Fs: Fe2Si2O6). The enstatite-ferrosilite solid solution series forms orthopyroxenes (OPX) with orthorhombic (Pbca) symmetry, whereas, the diopside-hedenbergite solid solution series forms clinopyroxenes (CPX) with monoclinic (C2/c) symmetry. Weathering reactions of pyroxene group minerals often produce clay minerals. Pyroxene end-members are: enstatite Mg2Si2O6 (polymorphs clinoenstatite, orthoenstatite, protoenstatite); ferrosilite Fe2Si2O6; diopside CaMgSi2O6; hedenbergite CaFeSi2O6; jadeite NaAlSi2O6; aegirine NaFeSi2O6; spodumene LiAlSi2O6; pigeonite (Mg,Fe,Ca)(Mg,Fe)Si2O6; augite (Ca,Mg,Fe2+,Fe3+,Ti,Al)2(Si,Al)2O6; omphacite (Ca,Na)(Mg,Al)Si2O6; grossmanite CaTiSiAlO6. Refer to individual end members for further descriptions.