VESUVIANITE STABILITY IN WATER AND WATER CARBON-DIOXIDE FLUIDS

Three mineral reactions bordering the upper part of vesuviane-bearing rodingites are studied in the temperature range 400 to 520-degrees-C, pressure 2 to 6 kbar, and low X(CO2) (to 0.1). Natural vesuviane Ca9.5 MgAl5Si9O34(OH)4 is stable only at the very low CO2 concentrations (X( CO2) < 0.007) which increase due to dissolution of solid silicate phas es in the fluid at the given P-T conditions. The following standard th ermodynamic functions of vesuviane were calculated based on the experi mental data: DELTAG(f,298.15)0 = = - 19448 kJ/mol. DELTAH(f,298.15)0 = - 20617 kJ/mol, S298.15(0) = 940 J/mol . K, V298.15(0) = = 42.89 J/ba r, and C(P) = 1566 + 0.2743 . T - 4.3436 . 10(-6) . T2 - 2.1558 . 10(7 ) . T-2 -- 5.6046 . 10(3) . T-0.5. Some geological applications are al so discussed.