sodic clinopyroxene groupsodic-calcite clinopyroxene groupNov. 14, 2003Mineralogy 231Lecture 29: minerals of the upper mantle and crust: pyroxenesWednesday: spiral chains, gangue minerals: pyroxenoidsreadings: N 274-277; K 456-457, 520-523; DHZ, 203-215rock types from the uppermost mantleperidotite: olivine with clinopyroxene and orthopyroxene, minor amounts of garnet or spineleclogite: high pressure basaltic rock primarily comprised of garnet and clinopyroxeneminerals in basalts, gabbrosclinopyroxene and/or orthopyroxene, plagioclase, ± olivine ± hornblendepyroxenes also abundant in other crustal rocksclinopyroxenes in marbles, amphibolites, skarnsclinopyroxene and/or orthopyroxene in granulites, some granitespyroxenes one of a very few important rock-forming mineral groupsbasic structure of pyroxenessingle chain silicate: 50% polymerization of silica tetrahedrachain forms a straight line with all tetrahedra pointing the same way Fig. 14.1achains point toward each other with VI site (M1) between them forming "I-beams" Fig. 14.csecond VI site (M2) between tetrahedra pointing away from each other Fig. 14.1 bchemical classification of pyroxenesferromagnesian pigeonite group (mkl), hypersthene group (ortho)calcic diopside, hedenbergite, augitesodic aegirine, jadeitesodic-calcic omphacite, aegirine-augitelithium spodumene (very rare)Ca-Mg-Fe pyroxenescomposition space CaSiO3 - MgSiO3 - FeSiO3Figs. 14.2a, 14.6calcic diopside-hedenbergitesubcalcic augiteCa-bearing Fe-Mg pigeonite grouporthopyroxene hypersthene groupstructural classification of pyroxenesspace group crystal system name occurrencePbca orthorhombic hypersthene group gabbros, basalts, granulite facies rocksenstatite peridotitesferrosilite fayalite granites, iron formations (rare)Pn monoclinic omphacite eclogites, some blueschistsP21/c monoclinic pigeonite layered mafic intrusionsC2/c monoclinic all others many igneous and metamorphic rockshypersthene group (Mg,Fe)SiO3 (orthopyroxene, opx)continuous solid solution between enstatite MgSiO3 and ferrosilite FeSiO3medium to high RI increasing with Fe content, 2V + (enstatite), - (hypersthene) Fig. 14.52V + for Mg-rich opx, goes – with increasing Fe, then positive again Fig. pigeonite group (Mg,Fe,Ca)SiO3 (pig)distinguished from other pyroxenes by low 2V Fig. on p. 267similar range of RI to opx, stable only in high T mafic igneous rocksclinopyroxene group (cpx)continuous solid solution between diopside CaMgSi2O6 (Di) and hedenbergite CaFeSi2O6 (Hd)augite (Ca,Mg,Fe)2Si2O6, subcalcic pyroxeneplots below Di-Hd join on pyroxene quadrilateraloptics vary with composition, 2V decreases with decreasing Wo (Ca) Fig. 14.7sodic clinopyroxene groupaegirine, NaFeSi2O6 (Ac)jadeite, NaAlSi2O6 (Jd) Fig. on p. 271aegirine-jadeite: NaAlSi2O6 - NaFeSi2O6 solid solutionsodic-calcite clinopyroxene groupomphacite NaCaMgAlSi4O12, intermediate between Jd and Di Fig. on p. 272aegirine-augite: NaFeSi2O6 - Ca,(Mg,Fe)Si2O6 solid solution Fig. on p. 269composition diagram for Na pyroxenespyroxenes in the composition space NaAlSi2O6 - NaFeSi2O6 - Ca(Mg,Fe)Si2O6Fig. 14.9phase equilibria of pyroxenesdiopside - enstatite: CaMgSi2O6 - MgSiO3quadrilateral pyroxenes: CaMgSi2O6 - CaFeSi2O6 - MgSiO3  FeSiO3jadeite - quartz - albite: NaAlSi2O6 - SiO2ReferencesDeer, W.A., Howie, R.A. & Zussman, J. (1978, reprinted with corrections 1997) Volume 2A, Rock-Forming Minerals: Single-Chain Silicates, 2nd. ed., The Geological Society, London, 383 p.Deer, W.A., Howie, R.A. & Zussman, J. (1991) An Introduction to The Rock-Forming Minerals, 2nd ed.,Longman, NY, 696