Earth Materials. John O'Brien

Чтение книги онлайн.

Figure 3.5 Limited substitution and miscibility gap in calcium–magnesium carbonates with compositional ranges of low‐Mg calcite, high‐Mg calcite, and dolomite.

      3.1.3 Limited ionic substitution

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.

3.2 PHASE STABILITY (EQUILIBRIUM) DIAGRAMS

The behavior of materials in Earth systems can be modelled using thermodynamic calculations and/or empirical results from laboratory investigations. The results of such calculations and/or investigations are commonly summarized on phase stability diagrams. A phase is a mechanically separable part of the system. Phase stability (equilibrium) diagrams display the stability fields (conditions under which a phase is stable) for various phases in a system of specified composition. These fields are separated by phase stability boundary lines that represent the conditions under which phase changes from one phase to another occur. Phase stability diagrams related to igneous systems and processes summarize relationships between liquids (melts) and solids (crystals) in a system. Such diagrams usually have temperature increasing upward on the vertical axis and composition shown on the horizontal axis. At high temperatures the system is completely melted. The stability field for 100% liquid is separated from the remainder of the phase diagram by a phase boundary line called the liquidus that represents the temperature above which the system exists as 100% melt and below which it contains some crystals. The low temperature stability field for 100% solid crystals is separated from higher temperature conditions by a phase boundary line called the solidus. At intermediate temperatures between the solidus and liquidus, the system consists of two types of stable phases in equilibrium, both liquid and solid crystals. Phase equilibrium diagrams, based on both theoretical and laboratory analyses, exist for a variety of multicomponent systems. A one‐component and five representative two‐component systems related to the discussion of igneous rocks and processes (Chapters 7–10) are discussed below. Metamorphic phase diagrams are discussed in Chapter 18. For discussions of systems that are beyond the scope of this text, including three‐ and four‐component systems, the reader is referred to mineralogy books by Wenk and Bulakh (2016), Nesse (2016), Dyer et al. (2008) and Klein and Dutrow (2007) and to petrology books by Frost and Frost (2019), Philpotts and Ague (2009), Winter (2009) and Best (2000). Some of the more important terms you will encounter in this discussion are defined in Table 3.1.

Table СКАЧАТЬ