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Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates.

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Presentation on theme: "Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates."— Presentation transcript:

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2 Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates

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

4 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

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

6 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

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

8 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

9 Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates

10 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??

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

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

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

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

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

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

17 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 

18 Inosilicates: single chains- pyroxenes M1 octahedron

19 Inosilicates: single chains- pyroxenes M1 octahedron

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

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

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

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

24 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

25 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 (+) (+)

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

27 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

28 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

29 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

30 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

31 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

32 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

33 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) Omphacite aegirine- augite Augite Spodumene: LiAlSi 2 O 6

34 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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