Framework Silicates

1. Framework Silicates • 2/3 of crust is framework silicates • Quartz and feldspars are most common • All with similar structures • TO4 tetrahedral framework • T = Si or Al • Each oxygen is shared with another tetrahedron • Sharing of highly charged cation (Si4+) make open structures

2. Consequences of open framework • Compostional • Accommodate large cations – Ca, Na, and K • Charge balance maintained by exchange of Al and Si

3. Physical properties • Specific gravity lower than most other minerals • E.g. quartz = 2.65, olivine = 3.27 • Not stable at high P • Restricted to crust

4. Four major groups: • Silica group (SiO2) • Feldspars • Feldspathoids • Zeolites

5. Silica group • Eight possible polymorphs • one other synthetic variety • Rare forms – High P • Stishovite – Si in octahedral coordination • Coesite • Common forms • Quartz, a & b • Tridymite, a & b • Cristobalite, a & b

6. Fig. 12-1

7. Structures • Reconstructive polymorphism between qtz, tridymite and cristobalite • Displacive polymorphism between a and b varieties • Tridymite and Cristobalite not stable at surface P and T • Will convert to quatz

8. Varieties of quartz • Microcrystalline varieties • Chert – occurs as nodules or beds in limestone • Black version – flint • Red version – jasper • Chalcedony – fibrous microcrystalline quartz • Color bands or irregular color called agate

9. Coarsely crystalline varieties • Amethyst – biolet or purple from trace amounts of Fe • Rose quartz – pink colored, may be caused by mineral inclusions • Citrine – yellow quartz from Fe, radiation or combination • Smoky quartz – irradiation and small amounts of Al • Milky quartz – minute fluid inclusions

10. Feldspar Group • Three compositional end members • K-feldspar (KAlSi3O8) – Ks or Or • Sanidine • Orthoclase • Microcline • Albite (NaAlSi3O8) – Ab • Anorthite (CaAl2Si2O8) – An • Plagioclase – An and Ab • Alkali – Ab and Ks (Or)

11. Plagioclase feldspars • At high T, continuous solid solution • CaAl substitutes for NaSi • Generally described as fraction of An, assuming An + Ab = 100%

12. Compositions divided into ranges and given names • Albite: An0 – An10 • Oligoclase: An10 – An30 • Andesine: An30 – An50 • Labradorite: An50 – An70 • Bytownite: An70 – An90 • Anorthite: An90 – An100

13. Alkali feldspars • Continuous solid solution at high T • K and Na same charge – compensates for difference in size • K and Ca don’t have solid solution because different size and charge

14. Fig. 12-5 Names of compositional variations caused by solid solution in feldspar minerals Influence of temperature on the allowed solid solution of feldspar minerals

15. Fig. 5-23 Origin of miscibility gap – variations in solid solutions

16. Feldspathoids • Similar to feldspars • Common minerals • Nepheline Na3K(Al4Si4O16) • Leucite KAlSi2O6 • Sodalite group Na8Al6Si6O24Cl2 • Nepheline is the most common

17. Chemically different from feldspars • Less Si relative to Na and K • Rarely found with quartz (too much Si) • Found in alkali-rich, silica poor igneous rocks

18. Structurally similar to feldspars • 4 and 6 member rings • Linked to form framework • More open than feldspars • Lower specific gravity than feldspars

19. Feldspathoid minerals compared to feldspars minerals Shaded regions are allowed solid solution

20. Zeolites • Very common group of minerals • Over 40 naturally occurring varieties • Over 600 synthetic ones • Largest single group of silicate minerals • Most commonly alteration products of basaltic and andasitic volcanic rocks

21. Commonly too fine grained for identification from physical properties • Requires x-ray diffraction • Often considered “clay minerals’ • Tectosilicates, not phyllosilicates

22. Composition • Hydrated framework silicates • General formula • MxDy(Alx+2ySin-x-2yO2n) MH20 • Si/Al ratio varies from 1 to 6 • M usually monovalent Na or K • D usually divalent Ca, Mg or others

23. Structure • Open framework of Al/Si tetrahedral • Link to form open channels and voids • Geometry varies from one to the other • Water and cations often in voids and weakly bonded • Create important properties of minerals

24. Fig. 12-22 Heulandite

26. Occurrence • All but analcime are secondary • Analcime may be a primary igneous mineral, late crystallization in basalts • Can be used as low-T geothermometers

27. Variety of Uses • Desiccants: hydrated, but water easily exchanges so can dessicate gasses such as CO2, freon, and organic chemicals • Molecular sieves: if dehydrated, other molecules fill voids, e.g. separate N from O, purify O • Water softener: Na-rich zeolites will remove Ca from water and replace with Na

28. Water purification: heavy metals in acid mine drainage, isotopes from radioactive waste, contaminated soils, remove NH4 from wastewater and cat litter • Soil conditioner: agriculture for water and cations, slow release of N, carrier of pesticides

29. Feed: pigs, cattle, chicken, turkeys, improve feed efficiency, reduce waste smell, increase N retention • Petroleum refining, cleaning spills, filters in paper processing

30. Environments of formation (increasing depth of burial) • Weathering with high pH • Diagenesis of ash, lakes and marine • Alteration from ground water • Hydrothermal alteration • Contact metamorphism • Burial and low grade regional metamorphism