Ionic Substitution in Minerals/ Crystals

Rules of substitution

1. The ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than about 15%.
2. Ions whose charges differ by one unit substitute readily for one another provided electrical neutrality if the crystal us maintained. If the charges of the ions differ by more than one unit, substitution is geneally 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.

• Ringwood’s modification : Substitution 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.
  • Na⁺ and Cu⁺ have same charge and their radii are virtually identical but do not substitute each other. The reason is that Cu forms more covalent bonds than Na, as indicated by their electronegativities .

1. Goldschmidt’s First Rule : The extent of substitution depends on the concentration of the ions in the medium in which crystals are forming , on the temperature, on the capability of their bonds and coordination in crystals.
   • Therefore, the concentration of trace elements in crystals can be used to estimate the temperature of formation of certain minerals. (crystals forming at high temperature are more tolerant to foreign ions than that of low temperature)
   • Ex – The concentration of Fe⁺² in sphalerite (Fe, Zn S) increases with the temperature provided that enough iron was available to saturate the sphalerite.
2. Goldschmidt’s Second Rule : When the charge difference is greater than one, substitution is limited because of the difficulty in maintaining electrical neutrality.
   • charge deficiencies that result from substitution of ions of unequal charge must be compensated by a second substitution involving an ion having a different charge. This process is called coupled substitution . It contributes of many minerals.
   • An alternative to coupled substitution displayed by the clay minerals is absorption of ions on the charged surface of small crystals.
3. Goldschmidt’s Third Rule : Ions having a higher ionic potential (charge/ radius) form a stranger bond than their competitors and are therefore preferentially incorporated into the crystal.
   • Consequently, ions with high ionic potentials are concentered in early-formed crystals in a cooling magma, where those with low ionic potential are concentrated in the residual magma and enter late-forming crystals.
4. Fourth Rule : When two minor ions are similar in size and charge compete for the same site, the one whose electronegativity is more similar to that of the major ion is preferred because the bonds are more compatible.
   • A minor ion can replace a major ion only when their electronegativities are similar.

Terminology related to partitioned between the solid & liquid phase

1. Camouflage :- Camouflage occurs when the minor element has the same charge and a similar ionic radius as the major element it is replacing.
   • Camouflage is displayed by Zr⁺⁴ (0.80 Å) and Hf⁺⁴ (0.79 Å) because hafnium rarely forms it’s own minerals and is always present in the mineral zircon (ZrSiO₄)
2. Capture :- Capture takes place when a minor element enters a crystal prefentially because it has a higher ionic potential than the ion of the major element.
   • Capture occur in the formation of feldspar crystals, which may capture Ba⁺² (1.44 Å) or Sr⁺² (1.21 Å) in place of K⁺ (1.46 Å).
   • As a result, the concentrations of these elements in the residual magma decrease during the crystallization of K-feldspar.
   • However, the replacement of the univalent K⁺ ion by a divalent ion requires a coupled substitution of Al⁺³ for Si⁺⁴ in order to preserve the electrical neutrality of the crystal lattice.
3. Admission :- Admission involves the energy of a foreign ion that has a lower ionic potential than the major ion because it has either a lower charge or a larger radius or both.
   • The occurrence of Rb⁺ (1.57 Å) in K⁺ (1.46 Å) feldspar.
   • Replacement of Ca⁺² (1.08 Å) by Sr⁺² (1.21 Å) in calcite.
   • Substitution of Cl^– (1.72 Å) by Br^– (1.88 Å) in chlorides.

Distribution coefficients and Geothermometers

Distribution coefficient (D) = C^s/ C^l

where, C^s = the concentration of a minor element in the crystal of a mineral

C^l = the concentration of that element in the liquid from which the crystal formed under equilibrium conditions

The value of D describes the type of substitution ,

• D > 1 ; Capture
• D < 1 ; Admission
• D = 1 ; Camouflage

The distribution coefficients are in general temperature dependent and are also affected by the compositions of the liquid and the crystal as well as by pressure.