The system CaO-SiO 2-CO 2-H 2O: 1. Melting relationships with excess vapor at 1 kilobar pressure

Abstract

Solid-liquid-vapor phase equilibria have been determined for parts of the system CaO-SiO 2-CO 2-H 2O by delineating the phase fields intersected by the composition join CaCO 3-Ca(OH) 2-(SiO 2 + 4.3 weight per cent H 2O) at 1 kilobar pressure between 600°C and 975°C. The phases encountered were: dicalcium silicate (C 2S), Ca 2SiO 4; calcite (CC), CaCO 3; portlandite (CH), Ca(OH) 2; spurrite (Sp), Ca 5(SiO 4) 2(CO 3) 2; caleiochondrodite (Ch), Ca 5(SiO 4) 2(OH) 2; liquid (L), with composition remaining close to the plane Ca 2SiO 4-CaCO 3-Ca(OH) 2; vapor (V), with composition ranging between CO 2 and H 2O. At the pressure of 1 kilobar, the following ternary univariant reactions, (2) and (3), and quaternary univariant reactions, (6), (7) and (8). were established at the temperatures indicated: (2) CH + Ch + V ⇌ L( E 2) at 774 ± 10° C; (3) Ch + V ⇌ C 2 S + L( P 3) at 955 ± 10° C; (6) CC + CH + Ch + V ⇌ L( E 6) at 637 ± 10° C; (7) CC + Ch + V ⇌ Sp + L( P 7) at 677 ± 10° C; (8) Sp + Ch ⇌ C 2 S + L( P 8) + V at 860 ± 10° C. The vapor in each reaction is high in H 2O content. From these results and other published data a schematic equilibrium diagram for the whole system is constructed. A large field for spurrite extends down to 677°C on the vapor-saturated liquidus surface but wollastonite is not stable in equilibrium with both liquid and vapor below 1000°C at this pressure. The plane Ca 2SiO 4-CO 2-H 2O is a thermal divide; original liquids containing more SiO 2 than this plane yield only silicates and vapor on crystallization, whereas original liquids containing less SiO 2 than this plane can yield low temperature liquids precipitating hydrated and carbonated phases (synthetic carbonatite magmas). Applications to carbonatites and the thermal metamorphism of silicious limestones are discussed briefly