![]() Only the total time to arrive at the point of K-feldspar equilibrium is required. In the third approach, a kinetic rate expression is used to calculate the reaction path, using a step-size adjusting algorithm which takes care of phase boundary transitions by automatically decreasing the time interval when necessary. In the second approach, only one simulation is sufficient, but the appropriate amounts of reaction must be known beforehand. In the first approach, no knowledge of the amounts of reaction is needed, but a number of simulations are necessary to find the appropriate phase-boundary intersections. PHREEQC can be used to solve this problem in three ways: the individual intersections of the reaction path and the phase boundaries on a phase diagram can be calculated (example 6A), the reaction path can be calculated incrementally (6B), or the reaction path can be calculated as a kinetic process (6C). In this example, the thermodynamic data for the phases (table 21, PHASES keyword) are derived from Robie and others (1978) and are the same as test problem 5 in the PHREEQE manual (Parkhurst and others, 1980). The reaction path for this set of phases was originally addressed by Helgeson and others (1969). Only a limited set of phases-K-feldspar, gibbsite, kaolinite, and K-mica (muscovite)-is considered in this example. ![]() In this example, the precipitation of phases as a result of incongruent dissolution of K-feldspar (microcline) is investigated.
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