Читать книгу Geochemistry - William M. White - Страница 85

Phases are real substances that are homogeneous, physically distinct, and (in principle) mechanically separable. For example, the phases in a rock are the minerals present. Amorphous substances are also phases, so glass and opal would be phases. The sugar that won't dissolve in your iced tea is a distinct phase from the tea, but the dissolved sugar is not. Phase is not synonymous with compound. Phases need not be chemically distinct: a glass of ice water has two distinct phases: water and ice. Many solid compounds can exist as more than one phase. Nor need they be compositionally unique: plagioclase, clinopyroxene, olivine, and so on, are all phases even though their composition can vary. A fossil in which the aragonite (CaCO₃) is partially retrograded into calcite (also CaCO₃) consists of two phases, which, although they might be chemically identical, have different crystal structures and hence different properties. Systems and reactions occurring within them that consist of a single phase are referred to as homogeneous; those systems consisting of multiple phases, and the reactions occurring within them, are referred to as heterogeneous.