Advances in physical geochemistry. Volume 4. Metamorphic reactions. Kinetics, textures and deformation / Успехи физической геохимии. Выпуск 4. Метаморфические реакции. Кинетика, текстуры и деформация

It is very common to find that in mineral synthesis experiments the crystals that form first have disordered cations, even when the synthesis conditions are well within the stability field of the ordered state. Some examples are the crystallization of albite from glass starting material (MacKenzie, 1957) or a flux (Woensdregt, 1983), cordierite from glass (Schreyer and Schairer, 1961; Putnis, 1980a), and plagioclase from glass (Eberhard, 1967; Kroll and Muller, 1980). The uncertainties that might arise in the interpretations of natural mineral assemblages (in terms of prevailing PT conditions), if the same metastable disordered states develop also during crystallization processes in nature, could be quite serious (see, for example, Putnis, 1980b). The problem is one of kinetics. It appears, in general, that at conditions of sufficient supersaturation the disordered phase is kinetically favoured over its ordered equivalent, irrespective of their relative thermodynamic stabilities. It so happens that this phenomenon has been studied in other, nonmineralogical situations. Thus we may quote Chernov and Lewis (1967) in this context: "The composition and structure of a crystal formed in a multicomponent system are determined by the equilibrium diagram only if the conditions of crystal growth are close to those of equilibrium. If the departure from equilibrium is considerable, both the composition and actual structure of the crystal will depend on the crystallization kinetics <...>