Abstract
Reversed experiments were performed on the reaction andalusite + water = hydroxyl–topaz component (in solid solution) in the intervals 600–900° C and 650–2000 bars using conventional cold seal apparatus. The results allow discrimination among various activity models for topaz solid solutions and calculation of the thermodynamic properties of (hypothetical) hydroxyl–topaz. A proton-avoidance activity model agrees well with the experimental data and gives reasonable extrapolations to lower temperatures. Combined thermodynamic data for hydroxyl-topaz (OHT), fluor–topaz (FT) (from Barton et al., 1982), and the solid solution are:
| . | . | . | Cp J/K-mol . | a . |
|---|---|---|---|---|
| OHT | −2693251 | 112.04 | 504.413 − 0.08737T + 1358330T−2 − 5869.53T−0.5 | |
| FT | −2910660 | 105.40 | 471.414 − 0.08165T + 1269470T−2 − 5485.54T−0.5 | 2XFT − 1 |
| . | . | . | Cp J/K-mol . | a . |
|---|---|---|---|---|
| OHT | −2693251 | 112.04 | 504.413 − 0.08737T + 1358330T−2 − 5869.53T−0.5 | |
| FT | −2910660 | 105.40 | 471.414 − 0.08165T + 1269470T−2 − 5485.54T−0.5 | 2XFT − 1 |
Calculations based on these data are in good agreement with observation of natural assemblages. In the presence of topaz, muscovite may be stable on the solidi of some granites to pressures as low as 1–2 kbkar. Calcic plagioclase is incompatible with topaz, especially in the presence of potassium feldspar. Hydroxyl–topaz contents of topaz coexisting with other aluminous phases increases with increasing pressure and decreasing. temperature. With decreasing temperature at constant composition topaz removes HF from a water-rich fluid.
