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Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates

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Nesosilicates: independent SiO 4 tetrahedra Olivine (100) view blue = M1 yellow = M2 b c projection

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Olivine (001) view blue = M1 yellow = M2 M1 in rows and share edges M2 form layers in a-c that share corners Some M2 and M1 share edges b a Nesosilicates: independent SiO 4 tetrahedra

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Olivine (100) view blue = M1 yellow = M2 b c M1 and M2 as polyhedra

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l Olivine – complete solid solution F Forsterite-Fayalite Fo x Fa y Fayalite – Fe end-member Forsterite – Mg end-member Olivine Occurrences: F Principally in mafic and ultramafic igneous and meta-igneous rocks F Fayalite in meta-ironstones and in some alkalic granitoids F Forsterite in some siliceous dolomitic marbles Monticellite CaMgSiO 4 Ca M2 (larger ion, larger site) High grade metamorphic siliceous carbonates

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Olivine minerals l Solid solution forsterite-fayalite, tephroite- glaucochroite, monticellite-kirschsteinite l Not in between no forsterite-tephroite series Larnite – Ca 2 SiO 4

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Distinguishing Forsterite-Fayalite l Petrographic Microscope F Index of refraction careful of zoning!! F 2V different in different composition ranges F Pleochroism/ color slightly different l Spectroscopic techniques – many ways to determine Fe vs. Mg l Same space group (Pbnm), Orthorhombic, slight differences in unit cell dimensions only

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Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Diopside: CaMg [Si 2 O 6 ] b a sin Where are the Si-O-Si-O chains??

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) b a sin

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) b a sin

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) b a sin

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) b a sin

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) b a sin

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Perspective view

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Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) SiO 4 as polygons (and larger area) IV slab VI slab b a sin

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Inosilicates: single chains- pyroxenes M1 octahedron

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Inosilicates: single chains- pyroxenes M1 octahedron

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Inosilicates: single chains- pyroxenes M1 octahedron (+) type by convention (+)

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Inosilicates: single chains- pyroxenes M1 octahedron This is a (-) type (-)

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Inosilicates: single chains- pyroxenes TM1T Creates an “I-beam” like unit in the structure.

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Inosilicates: single chains- pyroxenes TM1T Creates an “I-beam” like unit in the structure (+)

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The pyroxene structure is then composed of alternating I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation (+) (+) (+) (+)(+) Inosilicates: single chains- pyroxenes Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

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The pyroxene structure is then composed of alternation I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation (+) (+) (+) Inosilicates: single chains- pyroxenes (+) (+)

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Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms Inosilicates: single chains- pyroxenes

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The tetrahedral chain above the M1s is thus offset from that below The M2 slabs have a similar effect The result is a monoclinic unit cell, hence clinopyroxenes Inosilicates: single chains- pyroxenes c a (+) M1 (+) M2

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Orthopyroxenes have alternating (+) and (-) I-beams the offsets thus compensate and result in an orthorhombic unit cell Inosilicates: single chains- pyroxenes c a (+) M1 (-) M1 (-) M2 (+) M2

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Pyroxene Chemistry The general pyroxene formula: W 1-P (X,Y) 1+P Z 2 O 6 Where F W = Ca Na F X = Mg Fe 2+ Mn Ni Li F Y = Al Fe 3+ Cr Ti F Z = Si Al Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

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Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Diopside Diopside CaMgSi 2 O 6 Hedenbergite CaFeSi 2 O 6 CaFeSi 2 O 6 Wollastonite Ca 2 Si 2 O 6 Enstatite Mg 2 Si 2 O 6 Ferrosilite Fe 2 Si 2 O 6 orthopyroxenes clinopyroxenes pigeonite Orthopyroxenes – solid soln between Enstatite-FerrosiliteOrthopyroxenes – solid soln between Enstatite-Ferrosilite – solid soln between Diopside-HedenbergiteClinopyroxenes – solid soln between Diopside-Hedenbergite Joins – lines between end members – limited mixing away from join

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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) Diopside Diopside CaMgSi 2 O 6 Hedenbergite CaFeSi 2 O 6 CaFeSi 2 O 6 Wollastonite Ca 2 Si 2 O 6 Enstatite Mg 2 Si 2 O 6 Ferrosilite Fe 2 Si 2 O 6 orthopyroxenes clinopyroxenes pigeonite (Mg,Fe) 2 Si 2 O 6 Ca(Mg,Fe)Si 2 O 6 pigeonite clinopyroxenes orthopyroxenes Solvus 1200 o C 1000 o C 800 o C OPX CPX CPX OPX

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Orthopyroxene – Clinopyroxene solvus T dependence l Complex solvus – the ‘stability’ of a particular mineral changes with T. A different mineral’s ‘stability’ may change with T differently… l OPX-CPX exsolution lamellae Geothermometer… Miscibility Gap Fs En Di Hd Fs En Di Hd OPX OPX CPX CPX pigeonite augite orthopyroxene Pigeonite + orthopyroxene orthopyroxene Subcalcic augite pigeonite augite Miscibility Gap 800ºC 1200ºC

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Pyroxene Chemistry “Non-quad” pyroxenes Jadeite NaAlSi 2 O 6 Ca(Mg,Fe)Si 2 O 6 Aegirine NaFe 3+ Si 2 O 6 Diopside-Hedenbergite Ca-Tschermack’s molecule CaAl2SiO 6 Ca / (Ca + Na) 0.2 0.8 Omphacite aegirine- augite Augite Spodumene: LiAlSi 2 O 6

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Pyroxenoids “Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites Wollastonite (Ca M1) (Ca M1) 3-tet repeat Rhodonite MnSiO 3 5-tet repeat Pyroxmangite (Mn, Fe)SiO 3 (Mn, Fe)SiO 3 7-tet repeat Pyroxene 2-tet repeat 7.1 A 12.5 A 17.4 A 5.2 A

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Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. CHAPTER 16 Orthosilicates.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. CHAPTER 16 Orthosilicates.

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