Question

1.     State the Goldschmidt’s rules 3 of substitution and explain the characteristics of the elements that allow for following behaviors:

a.     Ni substitutes for fe²⁺ and Mg in olivine

b.     Cr and V substitutes for Fe³⁺ in spinel

c.     Sr substitutes for Ca in plagioclase

d.     Rb substitutes for K in alkali feldspar and biotite

Answer 1

Atomic substitution is controlled by charge of the ions

If charge difference is:

= 1: substitute readily if electrical neutrality is maintained

>1 : substitution is minimal

For ions of similar similar radius but different different charges charges, the ion with the

higher charge enters the crystal preferentially

 Forms a stronger bond with the anions surrounding the site