Yugawaralite (CaAl2Si6O16·4H2O) occurs most commonly in active geothermal areas and is associated with other zeolites including laumontite and wairakite. The stability field of yugawaralite was experimentally investigated using conventional hydrothermal apparatus over a P–T range of 200–350° C and 0.25–2 kbar and run duration of 14 to 61 days. The reversal experiments indicate that the P–T curve of the reaction yugawaralite = wairakite + 2SiO2 + 2H2O passes through 225±5° C at 0.25 kbar, 223±5° C at 0.5 kbar, 238±5° C at 1 kbar and 245 ±5° C at 2 kbar Pfluid. The thermodynamic parameters of yugawaralite were calculated: ΔSf = −2175.46±12.01 J/mol-K, ΔGf=8387.69±4.01 kJ/mol, ΔHf = −9036.19±4.01 kJ/mol. The equilibrium curve, yugawaralite = laumontite + 2SiO2, was estimated by employing the thermodynamic data and the calculated slope of 31.3 bars/° C. Present and previous experiments define an invariant point where yugawaralite, wairakite and laumontite are stable. The invariant point was located at 234° C and 550 bars in experimental conditions where PH2O = Ptotal. In geothermal systems where PH2O is about 1/3 Ptotal, the occurrence of yugawaralite is restricted to depths not much greater than 0.5 km. This result is consistent with its limited occurrence in active geothermal areas.

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