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Mineral Composition Variability

2. Ionic Substitution - Size. Size: Fe2 ? Mg2 ? Ni2 (0.86

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Mineral Composition Variability

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    1. 1 Mineral Composition Variability GLY 4200 Fall, 2011

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

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

    4. 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 More Information: http://www.todayinsci.com/1/1_27.htm Source: http://www.todayinsci.com/cgi-bin/indexpage.pl?http://www.todayinsci.com/1/1_27.htm More Information: http://www.todayinsci.com/1/1_27.htm Source: http://www.todayinsci.com/cgi-bin/indexpage.pl?http://www.todayinsci.com/1/1_27.htm

    5. 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 Source: http://classes.colgate.edu/rapril/geol201/summaries/atsub.htmlSource: http://classes.colgate.edu/rapril/geol201/summaries/atsub.html

    6. 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. 8 Spin State High-spin versus low-spin Source: http://www.dur.ac.uk/a.l.thompson/MainPage/SpinCrossover.htmSource: http://www.dur.ac.uk/a.l.thompson/MainPage/SpinCrossover.htm

    9. 9 Solid Solution

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