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_2S + 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_2S + 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.

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