DISPROPORTIONATION IN MINERAL SOLID-SOLUTIONS - SYMMETRY CONSTRAINTS ON PRECIPITATE ORIENTATION AND MORPHOLOGY - IMPLICATIONS FOR THE STUDY OF ORIENTED INTERGROWTHS

Precipitation (exsolution) from mineral solid solutions is studied from the group-theoretical point of view. The symmetry of the oriented intergrowth (matrix and precipitates), including the precipitate morphology, is described by the intersection group, a subgroup of both the matrix and precipitate respective point groups. The index of the intersection group into the matrix group defines: (i) the number of orientation variants of the precipitate; (ii) the order of the subgroup of the matrix group, the symmetry elements of which generate the variants. Examples from the literature, in oxides and silicates, illustrate these principles and show how they can help to identify precipitation processes and derive genetic information from oriented intergrowths.