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Mineral Composition Variability

Explore the mineral composition variability and the rules governing ionic substitution, including size and charge considerations. Discover the factors affecting solid solution and different types of crystaine substitution. Learn about polymorphous minerals, pseudomorphism, mineraloids, and exsolution phenomena.

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Mineral Composition Variability

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  1. Mineral Composition Variability GLY 4200 Fall, 2012

  2. Ionic Substitution - Size • Size: Fe2+ ↔ Mg2+ ↔ Ni2+ • (0.86Å) (0.80Å) (0.77Å)

  3. Ionic Substitution - Charge • Coupled substitution • Ca2+ & A13+ ↔ Na+ and Si 4+ • Example: Plagioclase feldspar • NaAlSi3O8 ↔ CaAl2Si2O8 • Void • Ca2+ & Void ↔ 2 Na+

  4. Victor M. Goldschmidt • Swiss-born Norwegian mineralogist and petrologist who laid the foundation of inorganic crystal chemistry and founded modern geochemistry • Born 1888, died 1947

  5. Goldschmidt’s Rules - Size • Atomic substitution is controlled by size (i.e., radii) of the ions • Free substitution can occur if size difference is less than ~15% • Limited substitution can occur if size difference is 15 - 30% • Little to no substitution can occur if size difference is greater than 30% • If there is a small difference of ionic radius the smaller ion enters the crystal preferentially

  6. Goldschmidt’s Rules - Charge • Atomic substitution is controlled by charge of the ions --> cannot differ by more than 1 • For ions of similar radius but different charges, the ion with the higher charge enters the crystal preferentially

  7. Other Factors Affecting Solid Solution • Temperature • Minerals expand at higher T • Minerals contract at lower T • Greater tolerance for ionic substitution at higher T • Pressure • Increasing pressure causes compression • Less tolerance for ionic substitution at higher P • Availability of ions – ions must be readily available for substitution to occur

  8. Spin State • High-spin versus low-spin

  9. Solid Solution

  10. Types of Crystalline Solution • 1. Substitutional - Mg2+ for Fe2+ • 2. Omission - Ca2+ & void for 2 Na+

  11. Crystalline Substitution 2 • 3. Vacancy - normally vacant sites can be filled as part of a coupled substitution. An important example is in the mineral group amphibole. An abundant, end-member component of this group of minerals is tremolite which ideally has the formula: • []Ca3Mg5Si8O22(OH)2 • where [] represents a vacant crystallographic site. Minerals can utilize this vacant site in coupled substitutions such as: • [] + Si4+ = Na+ +Al3+

  12. Crystalline Substitution 3 • 4. Interstitial - Atom or ion occupies space in between the normal sites • Often this is H+, a very small cation • In some crystal structures these voids are channel-like cavities.  A good example is the mineral beryl (Be3Al2Si6O18)

  13. Beryl Cavities

  14. Schottky Defect

  15. Frenkel defect

  16. HCP Stacking Defect • ABABABCABAB • H H C H H

  17. CCP Stacking Defect • ABCABCABABCABC • C C H C C

  18. Grain Boundary Defect • Two lattices grow together, with some displacement of ions (shown in blue)

  19. Polymorphous Minerals • All have the formula Al2SiO5

  20. Ditypous Minerals • Top – sphalerite (aka zinc blende) CCP • Bottom – wurzite HCP

  21. Pseudomorphism • Pseudomorphic goethite after cubic pyrite crystals clustered on a terminated aegerine crystal • Group is 4.6cm • Eric Farquharson specimen

  22. Mineraloids • Upper left –amber • Lower left – obsidian • Right – tektite glass

  23. Exsolution • Augitewith pigeonite exsolution lamellae • Pigeonite is a Ca-poor clinopryoxene • Exsolution in pyroxene

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