Abstract
The kinetics of the Al,Si exchange in alkali feldspars has been investigated for the two topochemically monoclinic samples sanidine (Or84) and anorthoclase (Or28). The crystals were first disordered at 1050 °C and then reordered at 850 °C, P(H2O) = 0.5 kbar; at 750 °C, 1 kbar; and at 650 °C, 1 kbar. Disordering experiments were performed with natural, untreated sanidine at the same conditions to bracket the equilibrium ordering state. The Al,Si distribution was determined from the optic axial angle 2VX, as measured on a spindle stage. The kinetic data were evaluated with the Mueller-Ganguly formalism. It was found that the ordering kinetics in sanidine and anorthoclase are similar. Steady states were reached after ≈ 10 d at 850 °C, after ≈ 100 d at 750 °C, and would be reached after several years at 650 °C. Activation energies are 223.0 (± 11.3) kJ/mol for sanidine and 244.3 (± 56.1) kJ/mol for anorthoclase. Their respective apparent equilibrium temperatures Tac are 685(38) °C and 798(33) °C, and probably represent temperatures prior to the volcanic eruption. Model calculations of cooling paths show that, if cooling starts at T0 > (Tae + 50 °C), then the cooling rate at T ≈ (Tae + 50 °C) can be recovered from the final ordering state achieved. The actual temperature T0 at which cooling begins and the cooling rate between T0 and Tae + 50 °C do not influence the final state of order. However, if continuous cooling begins at a temperature T0 in the interval between Tae + 50 °C and Tae, T0 must be known to determine the cooling rate.
