Conversion from fluor- to hydroxyl-phlogopite in KOH solution

Synthetic fluor-phlogopite, KMg3Si3AlO10F2, was treated by KOH hydrothermal solution under the following conditions: a temperature range from 300 to 800°C, KOH concentration from 1 to 10M, and duration from 3 hours to 216 days under a constant external pressure of 100 MPa. Solid products were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and electron microprobe analyzer (EPMA). The aqueous phase was chemically analyzed. In the entire range examined, (OH)-rich phlogopite was always produced in addition to the original F-phlogopite. The composition of the newly formed phlogopite varied with temperature and KOH concentration. The compositional gap between the two phlogopites was explained as an apparent immiscibility gap that appears in the temperature-composition section of the three parameter system of temperature, composition, and KOH concentration. The present study suggests practical conditions for the complete conversion from F- to OH-phlogopite as follows: [KOH]/[F-phlogopite]>8 and a temperature range of 500–600°C. SEM observations of the experimental products indicated that the F/OH exchange mechanism was not an ion exchange between solid and solution, but rather a dissolution and precipitation reaction in a KOH hydrothermal solution.