1 / 33

Back to silicate structures:

Back to silicate structures:. nesosilicates. phyllosilicates. sorosilicates. inosilicates. cyclosilictaes. tectosilicates. b. c. Nesosilicates: independent SiO 4 tetrahedra. projection. Olivine (100) view blue = M1 yellow = M2. b. M1 in rows and share edges

vonda
Télécharger la présentation

Back to silicate structures:

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


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

  2. b c Nesosilicates: independent SiO4 tetrahedra projection Olivine (100) view blue = M1 yellow = M2

  3. b M1 in rows and share edges M2 form layers in a-c that share corners Some M2 and M1 share edges Nesosilicates: independent SiO4 tetrahedra a Olivine (001) view blue = M1 yellow = M2

  4. b Nesosilicates: independent SiO4 tetrahedra c M1 and M2 as polyhedra Olivine (100) view blue = M1 yellow = M2

  5. Olivine – complete solid solution Forsterite-Fayalite  FoxFay Fayalite – Fe end-member Forsterite – Mg end-member Olivine Occurrences: Principally in mafic and ultramafic igneous and meta-igneous rocks Fayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marbles Monticellite CaMgSiO4 Ca  M2 (larger ion, larger site) High grade metamorphic siliceous carbonates

  6. Olivine minerals • Solid solution forsterite-fayalite, tephroite-glaucochroite, monticellite-kirschsteinite • Not in between  no forsterite-tephroite series Larnite – Ca2SiO4

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

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

  9. b Diopside: CaMg [Si2O6] a sin Where are the Si-O-Si-O chains?? Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

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

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

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

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

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

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

  16. SiO4 as polygons (and larger area) IV slab VI slab IV slab a sin VI slab Inosilicates: single chains- pyroxenes IV slab VI slab IV slab b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

  17. M1 octahedron Inosilicates: single chains- pyroxenes

  18. M1 octahedron Inosilicates: single chains- pyroxenes

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

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

  21. T M1 T Creates an “I-beam” like unit in the structure. Inosilicates: single chains- pyroxenes

  22. (+) T M1 T Creates an “I-beam” like unit in the structure Inosilicates: single chains- pyroxenes

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

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

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

  26. Inosilicates: single chains- pyroxenes 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 (+) M2 c a (+) M1 (+) M2

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

  28. 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

  29. Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite Ca2Si2O6 • Orthopyroxenes – solid soln between Enstatite-Ferrosilite • Clinopyroxenes – solid soln between Diopside-Hedenbergite Hedenbergite CaFeSi2O6 Diopside CaMgSi2O6 clinopyroxenes Joins – lines between end members – limited mixing away from join pigeonite orthopyroxenes Ferrosilite Fe2Si2O6 Enstatite Mg2Si2O6

  30. Wollastonite Ca2Si2O6 Hedenbergite CaFeSi2O6 Diopside CaMgSi2O6 clinopyroxenes pigeonite orthopyroxenes Ferrosilite Fe2Si2O6 Enstatite Mg2Si2O6 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 1200oC orthopyroxenes clinopyroxenes 1000oC CPX Solvus 800oC (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6 OPX OPX CPX

  31. Orthopyroxene – Clinopyroxenesolvus 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… CPX CPX Hd Di Di Hd augite augite Subcalcic augite Miscibility Gap Miscibility Gap pigeonite pigeonite orthopyroxene orthopyroxene Fs En Fs En OPX OPX 800ºC 1200ºC Pigeonite + orthopyroxene

  32. Pyroxene Chemistry Jadeite Aegirine NaAlSi2O6 “Non-quad” pyroxenes NaFe3+Si2O6 0.8 Omphacite aegirine- augite Spodumene: LiAlSi2O6 Ca / (Ca + Na) Ca-Tschermack’s molecule 0.2 CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

  33. 17.4 A 12.5 A 7.1 A 5.2 A Pyroxenoids “Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites Pyroxene 2-tet repeat Wollastonite (Ca  M1)  3-tet repeat Rhodonite MnSiO3  5-tet repeat Pyroxmangite (Mn, Fe)SiO3  7-tet repeat

More Related