An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar P_fluid

The univariant reaction governing the upper stability of heulandite (CaAl₂Si₇O₁₈·6H₂O), heulandite=laumontite+3 quartz+2H₂O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol^-1 K^-1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are -9722.3±6.3 kJ mol^-1 and -10524.3±9.6 kJ mol^-1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H₂O, defines an invariant point at which a third reaction, stilbite=heulandite+ H₂O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for {Mathematical expression}= P_total.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which {Mathematical expression} is very close to unity.