Conversion of fluor-phlogopite to hydroxy-phlogopite in NaOH and LiOH-hydrothermal solutions

Exchange of fluorine for hydroxide ions in synthetic fluor-phlogopite (KMg3AlSi3O10F2) was studied in hydrothermal solutions of various concentrations of NaOH and LiOH. The pressure applied was fixed at 100MPa, temperatures were 300, 450 and 600°C, and the experimental duration was 7 days. The dependence of the alkali species on the exchange efficiency was studied in comparison with that of KOH, as reported previously. The conversion rate at 600°C in the different alkaline solutions was higher in the order of NaOH > KOH > LiOH. In the case of NaOH, conversion of 90% or more was attained, and the crystallinity of the mica produced was higher than that in other alkaline solutions. Occasionally, euhedral crystals of about 0.5mm were obtained. Several secondary phases were found in some conditions: (1) sodium phlogopite-like phase in NaOH solution at low temperature (3), (2) 1:1 phases, probably halloysite (500°C), in concentrated NaOH solution (=10M), (3) Mg(OH)2 (500°C) in LiOH solution. The solubility of fluorine in the OH-rich phlogopite decreased with increasing temperature over 500°C in NaOH and LiOH solutions. This substitution behavior is in contrast with that found in KOH solutions, and is explained by the coexistence of the secondary phases described above.