For a solid-solid-plus-fluid decarbonation reaction like calcite + quartz ⇌ wollastonite + CO2, all the solid phases (calcite, quartz, wollastonite) are essentially pure end-member minerals with mole fractions equal to 1 (invariant), so the equilibrium temperature at a given total pressure is controlled entirely by the composition (mole fraction) of the fluid phase — specifically the mole fraction of CO2 in a mixed CO2-H2O fluid, since diluting CO2 with H2O shifts the equilibrium temperature (T-XCO2 relationships are fundamental in metamorphic petrology of calc-silicate rocks).