For sustainable geologic CO2 sequestration (GCS), it is important to understand the effects of temperature and CO2 pressure on mica's dissolution and surface morphological changes under saline hydrothermal conditions. Batch experiments were conducted with biotite (Fe-end member mica) under conditions relevant to GCS sites (35-95 degrees C and 75-120 atm CO2), and 1 M NaCl solution was used to mimic the brine. With increasing temperature, a transition from incongruent to congruent dissolution of biotite was observed. The dissolution activation energy based on Si release was calculated to be 52 +/- 5 kJ mol(-1). By comparison with N-2 experiments, we showed that CO2 injection greatly enhanced biotite's dissolution and its surface morphology evolutions, such as crack formation and detachment of newly formed fibrous illite. For biotite's dissolution and morphological evolutions, the pH effects of CO2 were differentiated from the effects of bicarbonate complexation and CO2 intercalation. Bicarbonate complexation effects on ion release from biotite were found to be minor under our experimental conditions. On the other hand, the CO2 molecules in brine could get into the biotite interlayer and cause enhanced swelling of the biotite interlayer and hence the observed promotion of biotite surface cracking. The cracking created more reactive surface area in contact with brine and thus enhanced the later ion release from biotite. These results provide new information for understanding CO2-brine-mica interactions in saline aquifers with varied temperatures and CO2 pressures, which can be useful for GCS site selection and operations.