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The epitaxial overgrowth of brushite (CaHP[O.sub.4] x 2[H.sub.2]O) by the interaction of phosphate-bearing, slightly acidic, aqueous solutions with gypsum (CaS[O.sub.4] x 2[H.sub.2]O) was investigated in situ using atomic force microscopy (AFM). Brushite growth nuclei were not observed to form on the {010} gypsum cleavage surface, but instead formed in areas of high dissolution, laterally attached to gypsum [101] step edges. During the brushite overgrowth the structural relationships between brushite (Aa) and gypsum (A2/a) result in several phenomena, including the development of induced twofold twining, habit polarity, and topographic effects due to coalescence of like-oriented crystals. The observed brushite growth is markedly anisotropic, with the growth rate along the main periodic bond chains (PBCs) in the brushite structure increasing in the order [101] > [101] > [010], leading to tabular forms elongated on [101]. Such a growth habit may result from the stabilization of the polar [101] direction of brushite due to changes in hydration of calcium ions induced by the presence of sulfate in solution, which is consistent with the stabilization of the gypsum [101] steps during dissolution in the presence of HP[O.sup.2-.sub.4] ions. The coupling between growth and dissolution was found to result in growth rate fluctuations controlled by the changes in the solution composition. Keywords: Brushite, gypsum, in situ AFM, epitaxy DOI: 10.2138/am.2010.3557

Leucite and analcime have open framework aluminosilicate structures, where ion exchange by cation substitution has been previously used to explain the replacement of one phase by another. Using [sup.18]O-enriched NaCl solutions in hydrothermal reactions and run-product analyses using scanning electron microscopy, infrared and Raman spectroscopy, and time-of-flight secondary ion mass spectrometry, we show that the replacement of leucite by analcime is not a solid-state reaction involving cation exchange by volume diffusion. Textural features such as nano-pores and clusters, as well as the detection of high amounts of [sup.18]O in the framework of analcime, suggest that the reaction proceeds by dissolution of leucite and reprecipitation of analcime, where structural O atoms of the leucite framework are exchanged and a new analcime structure forms at a moving interface through the leucite parent crystal. The characteristic high porosity (on a nano-scale) in the analcime product phase results from some of the parent phase being lost to the solution to give a volume deficit reaction. However, external dimensions are maintained during the process to result in the pseudomorphic replacement of an open framework aluminosilicate structure by a coupled dissolution-reprecipitation mechanism. Keywords: Dissolution-reprecipitation, pseudomorphism, analcime, Raman, TOF-SIMS, zeolite, replacement