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

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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A on Figure 3.3 lies on the side opposite the magnesite apex and so contains no magnesium. Because it lies halfway between rhodochrosite and siderite, its composition may be written as Rc50Sd50 or as (Mn0.5,Fe0.5)CO3. Any point that lies within the triangle represents a solid solution that contains all three end member components. The precise composition of any three‐component solid solution can be determined by the distance from the point to the three apices of the triangle. Point B in Figure 3.3 lies closest to the rhodochrosite apex and farthest from the magnesite apex and so clearly contains more Mn than Fe and more Fe than Mg. Its precise composition can be expressed as Sd10Ms2Rc88 or as (Fe0.10,Mg0.02,Mn0.88)CO3. Many other examples of three‐component systems with complete solid solution exist; all may be represented in a similar fashion by their position on a triangular diagram.

      3.1.2 Coupled (paired) ionic substitution

      In the plagioclase feldspars, similar size ions of sodium (Na+1) and calcium (Ca+2) can substitute for one another in any proportion in the large cation coordination site. However, when calcium (Ca+2) substitutes for sodium (Na+1), the positive charge of the crystal lattice is increased, and when the reverse occurs, the positive charge of the lattice is decreased. These changes in charge are balanced by a second set of substitutions. This second set of substitutions occurs in the small tetrahedral cation coordination site where aluminum (Al+3) and silicon (Si+4) substitute for one another. When a sodium (Na+1) ion is added to the large cation coordination site, a silicon (Si+4) ion is added to the small cation structural site. The two sites together contain a total charge of +5 that is balanced by the anions in the plagioclase structure. When a calcium (Ca+2) is added to the large cation site, an aluminum (Al+3) is added to the small cation site. Once again, the two sites together contain a total charge of +5 which is balanced by the anions in the plagioclase structure. The two substitutions are paired. If a sodium (Na+1) ion replaces a calcium (Ca+2) ion in the first coordination site, a silicon (Si+4) ion must simultaneously replace an aluminum (Al+3) ion in the second structural site for the two sites to total +5, so that the electrical neutrality of the crystal lattice is maintained. Ideally all substitutions are paired and any change in the proportion of sodium to calcium (Na/Ca) in the large ion site is balanced by a similar change in the proportion of silicon to aluminum (Si/Al) in the small ion site. As a result, the general composition of plagioclase can be represented by the formula (Na,Ca)(Si,Al)AlSi2O8 to emphasize the nature of coupled ionic substitutions.

Schematic illustration of coupled ionic substitution in the plagioclase solid solution series. Schematic illustration of 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