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disilicates and ring silicates n.
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Disilicates and Ring Silicates

Disilicates and Ring Silicates

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Disilicates and Ring Silicates

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  1. Disilicates and Ring Silicates • Disilicates • Not a common group of minerals – not rock forming • Widely distributed • Most common are the epidote group • Epidote • Zoisite • Clinozoisite • Allanite

  2. Characteristic structure • Pair of single tetrahedrons • Share single O • Z/O ratio = 2/7

  3. General Structure • Complex structure • Leads to much substitution and complicated compositions • Links of silica tetrahedrons • Edge sharing Al octahedrons • Octahedrons linked by double silica tetrahedrons • Other cations link tetrahedrons

  4. Structure – EpidoteGp • One edge sharing octahedral chain • M2 sites • Second edge sharing octahedral chain • M1 site with Al • M3 site with other cations (usually Fe and Mn)

  5. Chains and cross linked tetrahedron • produce 7 to 11 fold coordination sites – A sites • Commonly contain Ca • Overall – 5 sites: • Tetrahedron, 3 octahedral sites (M1, M2, & M3), one high coordination number site - A

  6. M2 Fig. 15-2 M1 Cross-linked double tetrahedron M3 A = 7 to 11 fold coordination

  7. Chains are parallel to b axis • Cleavage commonly occurs between b axis • Commonly elongate parallel to b axis

  8. Chemistry – EpidoteGp M3 site • Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) A site M1, M2, & M3 site Tetrahedral Site Variation of cations in A and M3 sites control names

  9. EpidoteGp Minerals

  10. Occurrence • Zoisite • Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks • Amphibolite from mafic igneous rocks

  11. Clinozoisite, epidote • Regional and contact metamorphic rocks • Pelites, metacarbonates, felsic to mafic meta-igneous rocks • Clinozoisite in Al-rich rocks • Epidote in Fe-rich rocks • Resistant to weathering, so they are common detrital grains, heavy minerals

  12. Allanite • Common accessory mineral in felsic igneous rocks • Granite, granodiorite, monzonite, syenite

  13. Ring Silicates • Only three common species • Tourmaline • Beryl • Cordierite • All are 6 member rings • Rings stacked on top of each other Isostructural

  14. Ring – Z/O = 1/3 Fig. 15-1 Beryl and Cordierite Tourmaline

  15. Beryl – Al2Be3Si6O18 • Structure • Rings rotated relative to the ones above and below • Each column are concentric cylinders of oxygen atoms • Columns arranged hexagonally • Al in distorted 6-fold coordination sites • Be in 4-fold sites between Si tetrahedrons • Could be considered tectosilicates

  16. Fig. 15-6 Si tetrahedral rings Be in 4 fold coordination Al in 6 fold coordination

  17. Compositional variations • Inclusion of Li, Na, K, and Cs • Li substitutes for Be and Al • Other located in channels • Charge balance from substitution of Fe, Mg, Li for Al or vacancies in Be sites • Cr substitution makes emerald

  18. Occurrence • Common in granitic pegmatites • Associations are quartz, K-spar, albite, muscovite, biotite, tourmaline • Use • Source of Be • Gemstone: aquamarine (blue-green), emerald (green)

  19. Cordierite • Identical to Beryl except • Substitute Mg2+ for Al3+ in octahedral sites • Substitute Al for Be in cross linked tetrahedral sites • Replace one Si4+ with one Al3+ in rings for charge balance

  20. Tourmaline • Hydrated Na, Mg, Fe, Li, Al, B silicate • Structure • Single 6-member ring • 3 octahedral sites at base with X cations coordinating • One large W cation site in axis of rings • B trigonally coordinated as base • Columns held together with octahedrally coordinated Y cation

  21. All rings point in same direction • No center of symmetry • piezoelectric

  22. Fig. 15-6 W = Na, less commonly Ca, K X = Mg & Fe, less commonly Mn, Li, Al Y = Al, less commonly Fe, Mg Trigonally coordinated B

  23. Composition • General formula • WX3Y6(Si6O8)(BO3)3(O,OH,F)4 • W = Na, less commonly Ca, K • X = Mg and Fe, less commonly Mn, Li, Al • Y = Al, less commonly Fe, Mg

  24. End members • Schorl • NaFe3Al6(Si6O8)(BO3)3(O,OH,F)4 • Dravite • NaMg3Al6(Si6O8)(BO3)3(O,OH,F)4 • Elbaite • Na(Li,Al)3Al6(Si6O8)(BO3)3(O,OH,F)4

  25. Occurrence • Common in pegmatites, other igneous and metamorphic rocks • Resistant to weathering – heavy mineral fractions • Use • Gemstone • Pressure gauges and other electrical components