12.109 Lecture Notes September 15, 2005 Rock Forming Minerals III Structure and composition of: PYROXENES Pyroxenes = chain silicates, with tetrahedral and octahedral chains Pyroxene structure combines tetrahedral chains + octahedral strip Æ I-beams Diopside CaMgSi2O6 Monoclinic, due to symmetry of octahedra “Octahedral ducks” The symmetry of a pyroxene can depend on the facing direction of the octahedra. To remember this, we picture the octahedron as a duck, and signify as positive “ducks facing forwards” and as negative “ducks facing backwards.” How to make an orthorhombic pyroxene Pyroxene structure is a polytype Polytypism – one basic building unit put together in different ways Orthopyroxene (OPX) Stack I-beams in rows alternating directionality of octahedronsPyroxene polytypes + monoclinic c·2/c + CPX + clinopyroxene, diopside + + + - orthorhombic Pbcn + protoenstatite -+ + orthorhombic Pbcn + OPX - orthopyroxene -+ + Pyroxene quadrilateralOther types of pyroxenes NaAlSi2O6 jadeite NaFeSi2O6 aegirine LiAlSi2O6 spodumene CaAl(Al, Si)O6 CaTs Calcium Tschermaks Fassaite = >50% CaTs MgAl(Al, Si)O6 MgTs Magnesium Tschermaks Augites can have substantial amounts of Ti in M1 site Æ Ti-CaTs (R2+TiAl2O6, where R can be any of a number of 2+ elements) Or with Cr, Cr-CaTs, distinctive green pyroxenes (R2+CrAlSiO6) Protoenstatite almost pure Mg pyroxene, like enstatite found in meteorites, enstatite chondrites boninites – discovered 1989, contain protoenstatite produced in subduction zones compositional variation in pyroxenes due to temperature when you look at pyroxenes in thin section, commonly you see evidence for exsolution – unmixing of components, chemical separation important in petrology because provides a record of temperature history In thin section, see blebs of unmixed mineral, esp. in plutonic rocks (slowly cooled igneous intrusives) or in high T igneous rocks where minerals grow directly from melt Can tell about rate of cooling @ low T, pigeonite phase disappears, reacts to form OPX and augite In plutonic rocks, get inverted pigeonite, OPX with augite lamelli Pigeonite Æ augite + OPX makes martensite, similar to phase transition in carbon-iron