보배광산의 열수변질 점토광상의 K2O-Al2O3-SiO2-H2O계에서의 일라이트 형성

Thermodynamic data of mineral reactions were used to construct a phase diagram for the formation of illite from andalusite at one bar and 325˚C in the hydrothermal alteration. Based on chemical compositions, the free energy of formation of illite coexisting with andalusite was calculated by assuming the ideal mixing ina bunary system consisting of muscovite and pyrophyllite components. For illite with structural formula K0.86Al2.93Si3.03O10(OH)2, its free energy of formation is -1147.727 kcal/mole at the condition under consideration. The stability area of illite is more narrow than that of end-member muscovite and prefers lower activity of silica. Illite was formed by hydration of andalusite while pyrophyllite decomposed. Illitization took place preferentially at margin and/or along fractures of andalusite.