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

Precipitation (exsolution) from mineral solid solutions is studied fro m the group-theoretical point of view. The symmetry of the oriented in tergrowth (matrix and precipitates), including the precipitate morphol ogy, is described by the intersection group, a subgroup of both the ma trix and precipitate respective point groups. The index of the interse ction group into the matrix group defines: (i) the number of orientati on variants of the precipitate; (ii) the order of the subgroup of the matrix group, the symmetry elements of which generate the variants. Ex amples from the literature, in oxides and silicates, illustrate these principles and show how they can help to identify precipitation proces ses and derive genetic information from oriented intergrowths.