A total of 34 solubility experiments using the ``model''-pelite
mineral assemblage microcline + low albite + andalusite + quartz were
performed at 600 degreesC and 0.2 GPa over a total chloride range of
0.03-2.9 molal. The concentrations of sodium, potassium, aluminum, and
silica were measured and the results are compared with four different
thermodynamic datasets. The K/Na ratio was approached from below and
above for the thermodynamically buffered mineral assemblage microcline +
low albite + andalusite + quartz. Tight brackets were obtained for
experiments performed in up to I molal chloride concentration. From 0.03
to similar to1 molal chloride concentration, a constant K/Na ratio of
0.33 was obtained. At 3 molal chloride concentration the K/Na ratio
decreases to 0.28. Experiments accidentally running out of quartz show a
different trend. The K/Na ratio systematically decreases from 0.39 to
0.27 from low to high chloride concentrations, Silica and aluminum
concentrations were only approached from below. The silica concentration
of 0.1 molal is constant over the investigated total chloride
concentration. Aluminum is more concentrated at low Cl-tot, decreasing
from 0.001 to 0.0005. Comparison of experimental results with computed
aqueous speciation generated with four different thermodynamic databases
shows good qualitative agreement. For the most part. predicted total
concentrations using the extended Debye-Huckel activity model for
charged solutes, and setting the Setchenow coefficient to zero for
neutral species, are generally within an order of magnitude of those
measured, which is likely within the uncertainties of the calculations.
Copyright (C) 2001 Elsevier Science Ltd.