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Disilicates and Ring Silicates: Structure, Chemistry, and Occurrence

Learn about disilicates and ring silicates, including the structure, chemistry, and occurrences of minerals like epidote, zoisite, allanite, tourmaline, beryl, and cordierite. Explore the complex linking of silica tetrahedrons and their distribution in various rock types.

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Disilicates and Ring Silicates: Structure, Chemistry, and Occurrence

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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

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