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P CO2 in low-grade metamorphism; zeolite, carbonate, clay mineral, prehnite relations in the system CaO-Al2O3-SiO2-CO2-H2O

Research paper by Alan Bruce Thompson

Indexed on: 01 Jun '71Published on: 01 Jun '71Published in: Contributions to mineralogy and petrology. Beitrage zur Mineralogie und Petrologie



Abstract

Equilibria for several reactions in the system CaO-Al2O3-SiO2-CO2-H2O have been calculated from the reactions calcite+quartz=wollastonite+CO2 (5) and calcite+Al2SiO5+quartz=anorthite+CO2 (19) and other published experimental studies of equilibria in the systems Al2O3-SiO2-H2O and CaO-Al2O3-SiO2-H2O.The calculations indicate that the reactions laumontite+CO2=calcite+kaolinite+2 quartz+2H2O (1) and laumontite+calcite=prehnite+quartz+3H2O+CO2 (3) in the system CaO-Al2O3-SiO2-CO2-H2O, are in equilibrium with an H2O-CO2 fluid phase having \({\text{X}}_{{\text{CO}}_{\text{2}} } \)∼-0.0075 for Pfluid=Ptotal=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO2.Using the above reactions as model equilibria, several probelms of p CO2 in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).