Experimental study of polycrystal growth from an advecting supersaturated fluid in a model fracture

We present real-time observations of polycrystal growth experiments in transmitted light in an accurately controlled flow system with the analogue material alum [KAl(SO(4))(2).12H(2)O]. The aim of the experiments is to obtain a better insight into the evolution of vein microstructures. A first series of experiments shows the evolution of a polycrystal at supersaturations between 0.095 and 0.263. The average growth rate of the crystals is influenced by growth competition and the depletion of the solute along fracture length. Growth competition is controlled by crystallographic orientation, crystal size and crystal location. In addition, the growth rate of an individual crystal facet also shows variations depending on the facet index, facet size and flow velocity. These variations can influence the morphology of the grain boundaries and the microstructures. The aim of the second series of experiments is to investigate the growth evolution of rough/dissolved facets in detail. The growth distance required for the development of facets is around 15 mu m. In all the experiments, we observe that the measured growth rates have a much larger range than predicted by alum single-crystal growth kinetics. This is due to the combined effect of the facet index and the crystal size. Furthermore, at high supersaturations, the facet growth rate measurements do not fit the same growth rate equation as for the experiments at lower supersaturations (< 0.176). This can be explained by a change in the growth mechanism at high supersaturations with more influence of volume diffusion, relative to advection of the bulk solution on the growth rate. This effect can also cause a more homogeneous sealing pattern over fracture length. At high supersaturations, the larger crystals in these experiments incorporate regularly spaced fluid inclusion bands and we propose that these can be used as an indicator for high palaeo-supersaturation. The final microstructures of the experiments show no asymmetry with respect to the flow direction.