A solution saturated by two isomorphic substances and supercooled to a specific temperature, is in a meta-equilibrium with isomorphic-mixed crystals characterized by two fixed compositions. These compositions are drawn together to a perfect coincidence at a point of thermodynamic equilibrium if a supercooling is decreased. A supercooled solution is supersaturated for crystals with intermediate composition but undersaturated for crystals with other compositions located at side fields. These conclusions were confirmed qualitatively by the experimental comparison of equilibrium temperatures in a series of mixed K_2(CrO_4, SO_4) solutions with their "native" crystals and pure K_2CrO_4 or K_2SO_4 crystals. Equilibrium supercooled solutions are interpreted on the basis of the metasomatic reaction theory. Non-equilibrium crystals interact metasomatically with a solution. A supercooling prevents primary dissolution and a reaction in the whole. Hence, crystals prove to be in a meta-equilibrium with a solution. Such crystals belong to two mixed compositions enriched by one or another component relatively to a crystal at a thermodynamic equilibrium. A course of meta-equilibrium crystallisation is calculated using modified Schreinemakers diagrams- Some phenomena are predicted: co-precipitation of two compositional types of mixed crystals, contrast compositional zonality of crystals belonging to these types, slowing-down and delays in the mixed crystal growth, oscillations of growth rates and formation of auto-epitaxial face relief.
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