CRYSTAL-GROWTH AND DISSOLUTION KINETICS OF GYPSUM AND FLUORITE - AN IN-SITU SCANNING FORCE MICROSCOPE STUDY

Scanning Force Microscopy was used to study the dissolution and growth in situ on cleaved surfaces of gypsum and fluorite in aqueous solutio n at room temperature. Three different dissolution processes can be di stinguished: Displacement of cleavage steps parallel to the investigat ed sample surface (process I) and the formation of terrace vacancies w hich correlate with point defects and linear defects (process II). Bot h these processes can be observed on the gypsum (010) and fluorite (11 1) surfaces. The formation of etch pits (process III) probably related to screw dislocations, occurs frequently only on the fluorite (111) s urfaces. On the (111) surface on fluorite, an in situ change of the de gree of undersaturation of the applied solution indicates that the eff ects of structural defects - i.e. the occurrence of pits - strongly de pends on the composition of the solution. A higher degree of undersatu ration (i.e. lower concentration) is required for the formation of pit s related to terrace vacancies, compared to deep etch pits which are p robably correlated with screw dislocations. After the pits generated b y terrace vacancies are established, the velocity of monolayer steps i s 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 densi ty of point defects via surface topography. On the (010) surface of gy psum, terrace vacancies occur only during the early stage of dissoluti on experiments. Later on, new etch pits occur rarely and the (010) sur face 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 dependi ng on the degree of supersaturation and the crystallographic orientati on. The velocity of step displacement of [100] steps increases faster with increasing degree of supersaturation compared to [001] steps. Dis solution is dominated by the retreat of monolayer steps, i.e. the reve rse growth mechanism. In summary, local growth and dissolution rate ar e strongly affected by surface topography.