Geological constraints on the thermodynamic properties of the stilbite-stellerite solid solution in low-grade metabasalts

Standard state thermodynamic data for stilbite (Ca2NaAl5Si13O36*16H(2)O) an d stellerite (Ca2Al4Si14O36*14H(2)O), together with mixing properties of th e stilbite -stellerite solid solution (stilbite SS) are derived through ass essment of geological observations of stilbite SS compositions in metabasal ts, experimental phase equilibria, and calorimetric observations. Measured compositions of stilbite SS in Tertiary metabasalts in Iceland and Icelandi c geothermal systems change systematically towards the stellerite end-membe r with increasing stratigraphic depth and temperature. Standard molal volum es, heat capacities, and entropies for the end-members of the solid solutio n are derived through critical review of published crystallographic and cal orimetric data for natural stilbite SS. Standard molal Gibbs energies of fo rmation at 298.15 K and 1 bar for stilbite (-4,946,475 cal mol(-1)) and ste llerite (-4,762,036 cal mol(-1)) and the mixing properties of the solid sol ution are retrieved from observed phase- and compositional-relations in met abasalts at Berufjordur, Iceland, measured temperatures of zeolite mineral distribution in active geothermal systems, and published observations of re versed phase equilibria. Mixing in stilbite SS can be described with an ath ermal solid solution model. Thermodynamic data resulting from our analysis provide close correlation between compositions of stilbite SS in Icelandic geothermal systems predicted from compositions of geothermal solutions and observed compositions of these minerals in low-grade metabasalts of Iceland , as well as the observed temperature of the stilbite SS to laumontite (leo nhardite) transition in Icelandic geothermal systems. Stilbite SS compositi on in metabasalts is a sensitive function of temperature, fluid composition , coexisting minerals (especially silica polymorphs) and geothermal gradien t. Copyright (C) 2001 Elsevier Science Ltd.