Читать книгу Earth Materials - John O'Brien - Страница 63

As noted previously, substitution is limited by significant differences in the ionic radii or charge of substituting ions. Ions of substantially different size limit the amount of substitution so that only a limited solid solution can exist between end member components. This situation can be illustrated in the rhombohedral carbonates by the limited solid solution series that exists between calcite (CaCO₃) and magnesite (MgCO₃). Once again, the potential solid solution series can be represented as a line between the two end members, and the composition of any calcium–magnesium‐bearing, rhombohedral carbonate may be represented by a formula, by its position on the tie line or by the proportion of an end member component (calcite = Ct or magnesite = Ms). However, because calcium cations (Ca⁺²) are more than 30% larger than magnesium (Mg⁺²) cations, the substitution between the two end members is limited. Because the amount of substitution is limited, many potential compositions do not exist in nature. Such gaps in a solid solution series are called miscibility gaps by analogy with immiscible liquids that do not mix in certain proportions. In this series, a miscibility gap exists between approximately Ms₂₅ = Ct₇₅ and Ms₄₀ = Ct₆₀ (Figure 3.5). To the left of this miscibility gap, a partial solid solution series exists between Ms₀ = (Ca_1.0,Mg_0.0)CO₃ and Ms₂₅ = (Ca_0.75,Mg_0.25)CO₃. Many organisms secrete shells in this compositional range (Chapter 14). Within this range, we can define low magnesium calcite and high magnesium calcite in terms of their proportions of calcite (Ct) and magnesite (Ms) end members. Low magnesium calcites generally contain less than 4% magnesium (Mg⁺²) substituting for calcium (Ca⁺²) in this structural site and so have compositions in the range Ct_96–100 = Ms_0–4 (Figure 3.5). High magnesium calcites have more than 4% magnesium substituting for calcium and therefore have compositions in the range Ct_75–96 = Ms_4–25. Some workers further subdivide these compositions into medium magnesium and high magnesium calcite with a boundary at 10% magnesium (James and Jones 2016). Compositions from Ms_40–55 = Ct_45–60 actually have a different structure – that of the double carbonate mineral dolomite whose average composition is CaMg(CO₃)₂. Many other examples exist of limited substitution series with miscibility gaps. The importance of mineral compositional variations that result from variations in substitution can be more fully understood in the context of phase stability diagrams, as discussed in the following section.