THE STABILITY OF GROSSULAR IN H₂O-CO₂ MIXTURES

Abstract

The stability field of grossular in H₂O-CO₂ mxitures has been calculated on the basis of previous work in the system CaO-A1₂O₃-SiO₂-H₂O-CO₂. The following reactions restricting the grossular field are, from high to low temperatures:
(1) grossular+CO₂=calcite+anorthite+wollastonite
(2) grossular+CO₂=calcite+anorthite+quartz
(3) grossular+CO₂+H₂O=calcite+zoisite+quartz
(4) grossular+CO₂+H₂O=calcite+prehnite.
These reactions give three isobaric invariant points, that is, point A (calcite-wollastonite-grossular-anorthite-quartz): T=564°C and X_CO2=18.8 mole % at 1000 bars, and T=575°C and X_CO2 =13.9 mole % at 2000 bars; point B (calcite-grossular-zoisite-anorthite-quartz): T=461°C and X_CO°C=5.18 mole % at 1000 bars, and T=503°C and X_CO2=6.30 mole % at 2000 bars; and point C (calcite-grossular-zoisite-prehnite-quartz): T=406°C and X_CO2=1.60 mole % at 1000 bars, and T=405°C and X_CO2=0.919 mole % at 2000 bars.
In the system containing iron, at the temperatures where the assemblage prehniteiron oxide is not stable, the reaction.
(3') grandite+CO₂+H₂O=calcite+epidote+quartz+hematite (or magnetite), places the strict limit on the condition of formation of grandite garnet growing in equilibrium with a H₂O-CO₂ fluid. The upper and lower limits on the carbon dioxide conditions of formation of ore-bearing skarn consisting of calcite, grandite and quartz are approximately placed by Reaction (3') and the reaction,
calcite+quartz=wollastonite+CO₂
respectively.