Download presentation

Presentation is loading. Please wait.

Published byAmina Hewes Modified over 3 years ago

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) 0.2 0.8 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

Similar presentations

Presentation is loading. Please wait....

OK

Back to silicate structures:

Back to silicate structures:

© 2018 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google