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Chemical Substitution

Chemical Substitution. Or Solid Solution. The example sphalerite. Zn and Fe substitute for one another in the crystal lattice without changing the internal structure of the mineral. What determines whether solid solution will take place or not?. Size Charge Ionization potential.

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Chemical Substitution

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  1. Chemical Substitution Or Solid Solution

  2. The example sphalerite • Zn and Fe substitute for one another in the crystal lattice without changing the internal structure of the mineral

  3. What determines whether solid solution will take place or not? • Size • Charge • Ionization potential

  4. Goldschmidt’s Rules 1.The ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than approximately 15%. 2. Ions whose charges differ by one unit substitute readily for one another provided electrical neutrality of the crystal is maintained. If the charges differ by more than one unit, substitution is generally slight. 3. When two different ions can occupy a particular position in a crystal lattice, the ion with the higher ionic potential forms a stronger bond with the anions surrounding the site.

  5. Ringwood’s Modifications (1955) Substitutions may be limited, even when the size and charge criteria are satisfied, when the competing ions have different electronegativities and form bonds of different ionic character. This rule was proposed in 1955 to explain discrepancies with respect to the first three Goldschmidt rules. For example, Na+ and Cu+ have the same radius and charge, but do not substitute for one another.

  6. Size intro.chem.okstate.edu/.../Ionic%20Radii2.GIF

  7. Electronegativity • http://en.wikipedia.org/wiki/Electronegativity

  8. Types of Solid Solution • Simple • Coupled • Omission • ‘Stuffed’ or Interstitial

  9. What is the effect of P and T on solid solution? • Let’s do some predictions here

  10. Physical Properties that change with solid solution • Color • Specific Gravity • Hardness • Lattice Parameters • Melting T

  11. Some Definitions • Essential vs Vicarious constituents of a mineral • Dispersed elements—may appear in trace amounts throughout minerals (ppm) and not found in any particular minera. • Rb+ for K+ or Sr+2 for Ca+2

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