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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University Press 621 pp.

      1  3.1 Atomic (ionic) substitutio

      2  3.2 Phase stability (equilibrium) diagrams

      3  3.3 Isotopes

      Minerals are composed of atoms or ions that occupy structural sites in a crystal structure (Chapter 2). Different ions can occupy the same structural site if (1) they have similar size, (2) have similar charge, and (3) are available in the environment in which the mineral is forming. This process of one ion replacing another ion is called ionic substitution. In mineral formulas, ions that commonly substitute for one another are generally placed within a single set of parentheses. In the olivine group, iron and magnesium can freely substitute for one another in the sixfold, octahedral site. As a result, the formula for olivine is commonly written as (Mg,Fe)2SiO4.

Schematic illustration of criteria for substitution are (a) similar size, (b) similar charge and (not shown) availability.

      Substitution is favored for ions that are widely available in the environment in which the mineral is growing (Figure 3.1). As minerals grow, coordination sites will preferentially select ions with the appropriate radii and charge that are available in the vicinity of the growing crystal. The ions that occupy a coordination site in a mineral provide vital clues to the chemical composition of the system and environmental conditions under which crystallization occurred.

      3.1.1 Simple ionic substitution

Schematic illustration of olivine complete substitution solid solution series. Schematic illustration of compositions of carbonate minerals expressed in terms of the proportions of iron, magnesium, and manganese; that is of the three components: siderite (Sd), magnesite (Ms), and rhodochrosite (Rc) plotted on a ternary diagram.