The phase transitions of mixed crystais at sub-solidus conditions are discussed from a theoretical physico-chemical point of view. Two different types of sub-solidus transitions are found to exist in rock-forming solid solutions. (1). The eutectic type in which only the one component of a mixed crystal series changes into another modification, decomposes, or reacts with other minerals. Under these conditions the transformation or reaction point (or rather-line) in the P,T-diagram varies with varying composition of the solid solution in an analogous manner as the melting or saturation point in eutectic melts or liquid soiutions. And (2.) the isodimorphic type by which both end members of a two component mixed crystal change to other modifications, decompose, or react with other minerals. In that case the sub-solidus transition equilibrium is analogous to the melting equilibrium of mixed crystals.
12 examples from rock-forming silicates are discussed broadly. These examples show that the transformations in mixed crystal series can be used as suitable continuous geological P, T -registrators. They further show that there exists gradual rather than discontinuous transitions between the several critical or typomorphic parageneses of the different mineral facies of rocks.