Symplectites are relatively fine grained mineral aggregates which replace a larger grain of a homogeneous precursor phase in a solid state reaction. Symplectite microstructures are frequently observed in magmatic and metamorphic rocks. Giving evidence of changes in the P-T-X conditions they bear key petrogenetic information. Typically symplectitic replacement occurs at a sharp reaction front, and the symplectites take the form of vermicular or lamellar intergrowth. The symplectite phases alternate at nearly regular intervals, henceforth referred to as the characteristic wavelength λ. The evolution of symplectite microstrcutures is time and temperature dependent and thus has great potential in extracting crystallization/transformation rates. In natural systems the characteristic wavelength of symplectite microstructures may vary over three orders of magnitude, the information content of this variation is still enigmatic.

In terms of microstructure, symplectites show striking similarity to the products of cellular segregation reactions in metals and in polymer systems. In materials science a wealth of experimental and theoretical studies has been done that aim at identifying the selection criteria that decide upon micrsostructure evolution. The aim of the intended reserach is to synthesize symplectites in the CaO–MgO–SiO2 systems under controlled P, T and water fugacity and to parameterize existing models for cellular segregation reactions for use with mineral systems. At the same time, natural symplectites will be investigated with particular focus on analytical techniques with high spatial resolution and 3D access to the microstructure to apply and refine the conceptual models and experimental parameterization. Ultimately, we intend to provide reliable tools for extracting rate information from observed symplectite microstructures and the associated microchemical patterns.

Products from the breakdown of synthetic monticellite initially having excess in forsterite component. Stage I produced forsterite + monticellite with compositions sitting on the boundaries of the (metastable) miscibility gap at given P, T conditions; stage II produced the thermnodynamically stable assemblage merwinite plus forsterite.