Читать книгу Geochemistry - William M. White - Страница 120

When we deal with solid solutions, we are again faced with the inadequacy of the purely macroscopic approach of classical thermodynamics. There is little disadvantage to this approach for gases, where the arrangement of molecules is chaotic. But the crystalline state differs from that of gases in that the arrangement of atoms in the crystal lattice is highly ordered, and the properties of the crystal depend strongly on the nature of the ordering. For this reason, we cannot afford to ignore the arrangement of atoms in solids, particularly with respect to solutions.

Solid solutions differ from those of gases and liquids in several respects. First, solution in the solid state inevitably involves substitution. While we can increase the concentration of HCl in water simply by adding HCl gas, we can only increase the concentration of Fe in biotite solid solution if we simultaneously remove Mg. Second, solid solutions involve substitution at crystallographically distinct sites. Thus, in biotite a solid solution between phlogopite (KMg₃AlSi₃O₁₀(OH)₂) and annite (KFe₃AlSi₃O₁₀(OH)₂) occurs as Fe²⁺ replaces Mg²⁺ in the octahedral site; the tetrahedral Si site and the anion (O) sites remain unaffected by this substitution. Third, substitution is often coupled. For example, the solid solution between anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈) in plagioclase feldspar involves not only the substitution of Na⁺ for Ca²⁺, but also the substitution of Al³⁺ for Si⁴⁺. The anorthite–albite solution problem is clearly simplified if we choose anorthite and albite as our components rather than Na⁺, Ca²⁺, Al³⁺ and Si⁴⁺. Such components are known as phase components. Choosing pure phase end members as components is not always satisfactory either because substitution on more than one site is possible, leading to an unreasonably large number of components, or because the pure phase does not exist and hence its thermodynamic properties cannot be measured.

However we choose our components, we need a method of calculating activities that takes account of the ordered nature of the crystalline state. Here we will discuss two ideal solution models of crystalline solids. We tackle the problem of nonideal solid solutions in Chapter 4.