CRYSTAL-GROWTH AND DISSOLUTION KINETICS OF GYPSUM AND FLUORITE - AN IN-SITU SCANNING FORCE MICROSCOPE STUDY
European Journal of Mineralogy 7(2): 267-276
Scanning Force Microscopy was used to study the dissolution and growth in situ on cleaved surfaces of gypsum and fluorite in aqueous solution at room temperature. Three different dissolution processes can be distinguished: Displacement of cleavage steps parallel to the investigated sample surface (process I) and the formation of terrace vacancies which correlate with point defects and linear defects (process II). Both these processes can be observed on the gypsum (010) and fluorite (111) surfaces. The formation of etch pits (process III) probably related to screw dislocations, occurs frequently only on the fluorite (111) surfaces. On the (111) surface on fluorite, an in situ change of the degree of undersaturation of the applied solution indicates that the effects of structural defects - i.e. the occurrence of pits - strongly depends on the composition of the solution. A higher degree of undersaturation (i.e. lower concentration) is required for the formation of pits related to terrace vacancies, compared to deep etch pits which are probably correlated with screw dislocations. After the pits generated by terrace vacancies are established, the velocity of monolayer steps is drastically reduced, despite the higher degree of undersaturation in this stage of the experiment. Therefore, we suggest that the rate of dissolution of the (111) fluorite surface is dominated by a high density of point defects via surface topography. On the (010) surface of gypsum, terrace vacancies occur only during the early stage of dissolution experiments. Later on, new etch pits occur rarely and the (010) surface appears to be quite stable in undersaturated solution. Growth on the (010) surface of gypsum is a layer-by-layer process. Monomolecular steps advance parallel to the surface with a certain velocity depending on the degree of supersaturation and the crystallographic orientation. The velocity of step displacement of  steps increases faster with increasing degree of supersaturation compared to  steps. Dissolution is dominated by the retreat of monolayer steps, i.e. the reverse growth mechanism. In summary, local growth and dissolution rate are strongly affected by surface topography.