Back to silicate structures nesosilicates sorosilicates cyclosilictaes phyllosilicates inosilicates tectosilicates Nesosilicates independent SiO4 tetrahedra b c projection Olivine 100 view blue M1 yellow M2 Inosilicates single chains pyroxenes T M1 T Creates an I beam like unit in the structure Inosilicates single chains pyroxenes Clinopyroxenes have all I beams oriented the same all are in this orientation The pyroxene structure is then composed of alternating I beams Note that M1 sites are smaller than M2 sites since they are at the apices of the tetrahedral chains Inosilicates single chains pyroxenes Clinopyroxenes have all I beams oriented the same all are in this orientation The pyroxene structure is then composed of alternation I beams Inosilicates single chains pyroxenes Orthoopyroxenes have I beams oriented in alternate direction in different layers The pyroxene structure is then composed of alternation I beams Inosilicates single chains pyroxenes M2 The tetrahedral chain above the M1s is thus offset from that below c a M1 M2 The M2 slabs have a similar effect The result is a monoclinic unit cell hence clinopyroxenes Inosilicates single chains pyroxenes Orthopyroxenes have alternating and I beams c M1 M2 a M1 M2 the offsets thus compensate and result in an orthorhombic unit cell Pyroxene Chemistry The general pyroxene formula W1 P X Y 1 P Z2O6 Where W Ca Na X Mg Fe2 Mn Ni Li Y Al Fe3 Cr Ti Z Si Al Anhydrous so high temperature or dry conditions favor pyroxenes over amphiboles Pyroxene Chemistry The pyroxene quadrilateral and opx cpx solvus Coexisting opx cpx in many rocks pigeonite only in volcanics Wollastonite Ca2Si2O6 Diopside CaMgSi2O6 clinopyroxenes Orthopyroxenes solid soln between Enstatite Ferrosilite Clinopyroxenes solid soln between Diopside Hedenbergite Hedenbergite CaFeSi2O6 Joins lines between end members limited mixing away from join pigeonite orthopyroxenes Enstatite Mg2Si2O6 Ferrosilite Fe2Si2O6 Orthopyroxene Clinopyroxene OPX and CPX have different crystal structures results in a complex solvus between them Coexisting opx cpx in many rocks pigeonite only in volcanics pigeonite Wollastonite Ca2Si2O6 orthopyroxenes cli no py ro xe ne s 1200oC 1000oC Diopside CaMgSi2O6 clinopyroxenes Hedenbergite CaFeSi2O6 CPX Solvus 800oC pigeonite orthopyroxenes Enstatite Mg2Si2O6 OPX Ferrosilite Fe2Si2O6 Mg Fe 2Si2O6 Ca Mg Fe Si2O6 OPX CPX Orthopyroxene Clinopyroxene solvus T dependence Complex solvus the stability of a particular mineral changes with T A different mineral s stability may change with T differently OPX CPX exsolution lamellae Geothermometer Di CPX Hd CPX Di augite augite Miscibility Gap En orthopyroxene OPX 800 C Hd Subcalcic augite Miscibility Gap pigeonite Fs En pigeonite orthopyroxene OPX 1200 C Fs Pigeonite orthopyroxene Pyroxene Chemistry Non quad pyroxenes Jadeite NaAlSi2O6 Aegirine NaFe3 Si2O6 0 8 Omphacite aegirineaugite Spodumene LiAlSi2O6 Ca Ca Na 0 2 Ca Tschermack s molecule CaAl2SiO6 Augite Diopside Hedenbergite Ca Mg Fe Si2O6 Ideal pyroxene chains with 5 2 A repeat 2 tetrahedra become distorted as other cations occupy VI sites Pyroxenoids 17 4 A 7 1 A 12 5 A 5 2 A Pyroxene 2 tet repeat Wollastonite Ca M1 3 tet repeat Rhodonite MnSiO3 5 tet repeat Pyroxmangite Mn Fe SiO3 7 tet repeat Back to silicate structures nesosilicates sorosilicates cyclosilictaes phyllosilicates inosilicates tectosilicates Inosilicates double chains amphiboles b a sin Tremolite Ca2Mg5 Si8O22 OH 2 Tremolite 001 view blue Si purple M1 rose M2 gray M3 all Mg yellow M4 Ca Inosilicates double chains amphiboles b a sin Hornblende Ca Na 2 3 Mg Fe Al 5 Si Al 8O22 OH 2 Hornblende 001 view dark blue Si Al purple M1 rose M2 light blue M3 all Mg Fe yellow ball M4 Ca purple ball A Na little turquoise ball H Inosilicates double chains amphiboles Hornblende Ca Na 2 3 Mg Fe Al 5 Si Al 8O22 OH 2 Same I beam architecture but the I beams are fatter double chains Hornblende 001 view dark blue Si Al purple M1 rose M2 light blue M3 all Mg Fe Inosilicates double chains amphiboles b a sin Hornblende Ca Na 2 3 Mg Fe Al 5 Si Al 8O22 OH 2 Same I beam architecture but the I beams are fatter double chains All are on clinoamphiboles and alternate in orthoamphiboles Hornblende 001 view dark blue Si Al purple M1 rose M2 light blue M3 all Mg Fe yellow ball M4 Ca purple ball A Na little turquoise ball H Inosilicates double chains amphiboles Hornblende Ca Na 2 3 Mg Fe Al 5 Si Al 8O22 OH 2 M1 M3 are small sites M4 is larger Ca A site is really big Variety of sites great chemical range Hornblende 001 view dark blue Si Al purple M1 rose M2 light blue M3 all Mg Fe yellow ball M4 Ca purple ball A Na little turquoise ball H Inosilicates double chains amphiboles Hornblende Ca Na 2 3 Mg Fe Al 5 Si Al 8O22 OH 2 OH is in center of tetrahedral ring where O is a part of M1 and M3 octahedra OH Hornblende 001 view dark blue Si Al purple M1 rose M2 light blue M3 all Mg Fe yellow ball M4 Ca purple ball A Na little turquoise ball H Amphibole Chemistry See handout for more information General formula W0 1 X2 Y5 Z8O22 OH F Cl 2 W Na K X Ca Na Mg Fe2 Mn Li Y Mg Fe2 Mn Al Fe3 Ti Z Si Al Again the great variety of sites and sizes a great chemical range and hence a broad stability range The hydrous nature implies an upper temperature stability limit Amphibole Chemistry Ca Mg Fe Amphibole quadrilateral good analogy with pyroxenes Tremolite Ca2Mg5Si8O22 OH 2 Anthophyllite Mg7Si8O22 OH 2 Actinolite Cummingtonite grunerite Orthoamphiboles Ferroactinolite Ca2Fe5Si8O22 OH 2 Clinoamphiboles Fe7Si8O22 OH 2 Al and Na tend to stabilize the orthorhombic form in low Ca amphiboles so anthophyllite gedrite orthorhombic series extends to Fe rich gedrite in more Na Al rich compositions Amphibole Chemistry Hornblende has Al in the tetrahedral site Geologists traditionally use the term hornblende as a catch all term for practically any dark amphibole Now the common use of the microprobe has petrologists casting hornblende into end member compositions and naming amphiboles after a well represented end member Sodic amphiboles Glaucophane Na2 Mg3 Al2 Si8O22 OH 2 Riebeckite Na2 Fe2 3 Fe3 2 Si8O22 OH 2 Sodic amphiboles are commonly blue and often called blue amphiboles Amphibole Occurrences Tremolite Ca Mg occurs in meta carbonates Actinolite occurs in low grade metamorphosed basic igneous rocks Orthoamphiboles and cummingtonite grunerite all Ca free Mg Fe rich amphiboles are metamorphic and occur in meta ultrabasic rocks and some meta sediments