Components, phases, and criteria of chemical equilibrium in rocks

For mineral assemblages resulting from regional metamorphism, the components H ~2~ O and CO ~2~ have commonly been treated as perfectly mobile, so that their concentrations may be ignored. Alternatively, the concentration of H ~2~ O (or CO ~2~ ) in a rock might be thought of as significantly influencing the resulting mineral assemblages. Neither theory alone could explain all the observations. On the other hand, O has generally been treated as an inert component in metamorphic mineral assemblages, especially in Fe deposits. However, the possible control of oxidation state of Fe by the content of carbonaceous matter has to be considered; if such material is not included in the mineral assemblage, then self-consistency demands that oxygen be considered as a perfectly mobile component. Studies of mineral assemblages require correct counting of phases. Two of the chief problems are zoned minerals and the hyperfine mixtures such as the mixed-layer clays. An unbreached zoned mineral may be treated as though its independent chemical components belonged to an open system. The phase status of hyperfine mixtures must be tested by direct, chemical methods because physical measurements (e. g. lattice spacings) do not necessarily test the properties that define a phase. Existing criteria of chemical equilibrium in rocks are based on negative evidence: lack of incompatible phases and nonviolation of the Gibbs Phase Rule. More stringent screening tests may include: 1) Evidence for textural equilibrium, with its presumed corollary of at least local chemical equilibrium; 2) separate analyses of the same minerals from different parts of the same rock for possible crossing of tie-lines; and 3) study of subassemblages of minerals that consist of only a few components and thus are easier to interpret in terms of the metamorphic process.