Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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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 Ct96–100 = Ms0–4 (Figure 3.5). High magnesium calcites have more than 4% magnesium substituting for calcium and therefore have compositions in the range Ct75–96 = Ms4–25. 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 Ms40–55 = Ct45–60 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.

Terms Definitions
Liquidus Phase boundary (line) that separates the all‐liquid (melt) stability field from stability fields that contain at least some solids (crystals)
Solidus Phase boundary (line) that separates the all‐solid (crystal) stability field from stability fields that contain at least some liquid (melt)
Eutectic Condition under which liquid (melt) is in equilibrium with two different solids
Peritectic Condition under which a reaction occurs between a pre‐existing solid phase and a liquid (melt) to produce a new solid phase
Phase A mechanically separable part of the system; may be a liquid, gas or solid with a discrete set of mechanical properties and composition
Invariant melting Occurs when melts of the same composition are produced by melting rocks of different initial composition
Incongruent melting Occurs when a solid mineral phase melts to produce a melt and a different mineral with a different composition from the initial mineral
Discontinuous reaction Mineral crystals and melt react to produce a completely different mineral; negligible solid solution exists between the minerals
Continuous reaction Mineral crystals and melt react to continuously and incrementally change the composition of both; requires a mineral solid solution series
Solvus Phase boundary (line) that separates conditions in which complete solid solution occurs within a mineral series from conditions under which solid solution is limited

      3.2.1 The phase rule

      The phase rule (Gibbs 1928) governs the number of phases that can coexist in equilibrium in any system and can be written as:

normal upper P equals normal upper C plus 2 minus normal upper F

      where

       P represents the number of phases present in a system. Phases are mechanically separable varieties of matter that can be distinguished from other varieties based on their composition, structure and/or state. Phases in igneous systems include minerals of various compositions, and crystal structures, amorphous solids (glass) and fluids such as liquids or gases. All phases are composed of one or more of the components used to define the composition of the system.

       C designates the minimum number of chemical components required to define the phases in the system. These chemical components are usually expressed as proportions of oxides. The most common chemical components in igneous reactions include SiO2, Al2O3, FeO, Fe2O3, MgO, CaO, Na2O, K2O, H2O, and CO2. All phases in the system can be made by combining components in various proportions.

       F refers to the number of degrees of freedom or variance. Variance means the number of independent factors that can vary, such as temperature, pressure, and the composition of each phase, without changing the phases that are in equilibrium with one another. We will use the first phase diagram in the next section to show how the phase rule can be applied to understanding phase diagrams. A discussion of the phase rule and of phase diagrams related to metamorphic processes is presented in Chapter 18.

      3.2.2 One component phase diagram: silica polymorphs