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Inosilicates: single chains- pyroxenes

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

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Inosilicates: single chains- pyroxenes

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

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

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

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

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

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

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

  8. M1 octahedron Inosilicates: single chains- pyroxenes

  9. M1 octahedron Inosilicates: single chains- pyroxenes

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

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

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

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

  14. (+) (+) (+) (+) (+) 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

  15. (+) (+) (+) (+) (+) 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

  16. 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

  17. Inosilicates: single chains- pyroxenes Orthopyroxenes have alternating (+) and (-) I-beams the offsets thus compensate and result in an orthorhombic unit cell This also explains the double a cell dimension and why orthopyroxenes have {210} cleavages instead of {110) as in clinopyroxenes (although both are at 90o) c (-) M1 (+) M2 a (+) M1 (-) M2

  18. Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite (pyroxenoid) pigeonite 1200oC orthopyroxenes clinopyroxenes 1000oC Diopside Hedenbergite clinopyroxenes Solvus Augite Miscibility Gap 800oC pigeonite (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6 orthopyroxenes Ferrosilite Enstatite

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

  20. 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

  21. b Tremolite: Ca2Mg5 [Si8O22] (OH)2 Inosilicates: double chains- amphiboles Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca)

  22. b Hornblende: K,<1(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 Inosilicates: double chains- amphiboles 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+K) little turquoise ball = H

  23. Hornblende: K,<1(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) Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe)

  24. (+) (+) (+) (+) (+) b Hornblende: K,<1(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) Inosilicates: double chains- amphiboles 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,K) little turquoise ball = H

  25. Hornblende: K,<1(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 Inosilicates: double chains- amphiboles 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, K) little turquoise ball = H

  26. Hornblende: K,<1(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 Inosilicates: double chains- amphiboles (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

  27. 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

  28. Amphibole Chemistry Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Tremolite Ferroactinolite Actinolite Ca2Mg5Si8O22(OH)2 Ca2Fe5Si8O22(OH)2 Clinoamphiboles Cummingtonite-grunerite Anthophyllite Fe7Si8O22(OH)2 Mg7Si8O22(OH)2 Orthoamphiboles 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

  29. 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: Na2Mg3 Al2 [Si8O22] (OH)2 • Riebeckite: Na2Fe2+3 Fe3+2 [Si8O22] (OH)2 • Sodic amphiboles are commonly blue, and often called “blue amphiboles”

  30. pyroxene amphibole Inosilicates b a Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them) Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  60-120o cleavages in amphiboles

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