Abstract
The low-temperature stability of andradite was experimentally investigated as a function of temperature, ХСО2 and ƒO2 at constant Pfluid of 2000 bars. Experimental results indicate that the reaction 3 quartz + 3 calcite + ¼ hematite + ½ magnetite + ⅛ O2 = andradite + 3 CO2 occurs at T = 550 ± 10°C at XCO2 = 0.22, 596 ± 8°C at XCO2 = 0.5, and 640 ± 10°C at XCO2 = 1.0. From these experimental data, the standard (298°K, 1 bar) Gibbs free energy of formation of andradite (,Ad) is -1293.44 ± 1.2 kcal/gfw, and the enthalpy () is -1377.48 ± 1.2 kcal/gfw. These values are slightly less negative than those for and calculated from data of Gustafson (1974). Mean values for and derived from Gustafson's experiments and the present results are, respectively, –1297.80 kcal/gfw and –1382.13 kcal/gfw. The standard entropy of formation of andradite () is -282 ± 4 gb/gfw, and the Third Law entropy (S0) is 68.2 ± 3 gb/gfw, which is close to the oxide sum estimate of 69.0 gb/gfw.
The experimental data for the low-temperature stability of andradite plus other pertinent data on the stabilities of wollastonite, hedenbergite (calculated), clinozoisite (zoisite), epidote, and grossular provide T-XCO2-ƒO2-Pfluid relations which delineate physical-chemical conditions for Ca-Fe-Al-Si skarn formation.
Relative to the stability field of grossular in C-O-H fluids (Gordon and Greenwood, 1971 ), andradite is stable with fluids richer in CO2 at a given temperature and pressure for all values of ƒO2, although the temperatures of reactions which delineate the stability field of andradite are sensitive to slight changes in either XCO2, ƒO2, or both. Like grossular, andradite is stable to lower temperatures with H2O-rich fluids. Addition of Fe3+ to grossular extends the thermal stability limits of grandite plus quartz to both higher and lower temperatures. In natural systems, simple retrograde carbonation of grandite may not occur if the fluid is sufficiently H2O-rich to stabilize epidote.
