Some phases are substantially more ordered at low temperature than at high temperature. The degree of order has a significant effect on their Gibbs free energy. However for most phases the state of order is not known as a function of temperature or even at reference conditions. In the case of gehlenite, K-feldspar, Na-feldspar and spinel there is sufficient experimental data available that these energy effects could be taken into account. Thus at the temperature and pressure conditions of each experiment for which a Kred value had to be determined involving one of these phases the degree of order and its associated energy was calculated.
The degree of order and its associated change in the Gibbs free energy for gehlenite was calculated by the approach taken by Waldbaum (1973), Waldbaum & Woodhead (1975) and Charlu et al. (1981). For the K-feldspars the approach by Thompson et al. (1974) and Hovis (1974) was used. Order-disorder in Na-feldspar was calculated using the model by Saljeet al. (1985). A totally disordered phase was chosen as the standard state for the alkali-feldspars. The values forΔfH° and S° for the ordered state of each of these phases were evaluated using the approaches given above.
In the case of spinel above 900°C and 0.1 MPa the kinetics of the order-disorder process is very fast. At lower temperatures the spinel is not in equilibrium and the disordered state at approximately 900°C is frozen in (Wood et al., 1986). Therefore heat capacity measurements at high temperatures should include the energetics of the order-disorder process, whereas at low temperatures, the heat capacity is that of spinel in a metastable state of disorder because of slow solid state diffusion rates. For spinel formed in hydrothermal experiments (600-900°C), however, it can be assumed that it is already closer to its state of equilibrium order. Therefore the heat capacity of spinel by Robie et al. (1979) was corrected with the order-disorder model by Sack & Ghiorso (1991). The heat capacity for spinel used here is the heat capacity of totally ordered spinel at all temperatures.