HYDROTHERMAL SYNTHESIS OF PYROPHYLLITE FROM SI-AL GELS AND SALT-SOLUTIONS - SOME IMPLICATIONS FOR THE FORMATION OF PYROPHYLLITE IN LOW-GRADE METAMORPHIC ROCKS RICH IN ALUMINOUS SMECTITES

Pyrophyllite, kaolinite + quartz and beidellite were synthesized at 30 0 and 350 degrees C at 1 kbar within a period of 10 days. For the crys tallization silica-alumina gels in salt solutions were used as startin g materials. In all cases lowering the temperature from 350 to 300 deg rees C results in the formation of only kaolinite and quartz instead o f pyrophyllite and sometimes kaolinite. In NaOH and NaF solutions beid ellite is formed accompanying pyrophyllite. Some additional experiment s with gels containing the cations Na+, K+, Li+, or Ca2+ resulted in t he formation of beidellite with sometimes kaolinite. The anions presen t in the solution and the state and quantity of the interlayer cation and especially the counter anion in solution are important factors det ermining the phyllosilicate formed under relatively low grade metamorp hic conditions and hydrothermal alteration.