Open Session - SWISS GEOSCIENCE MEETINGs

112
Symposium 2: Mineralogy-Petrology-Geochemistry
2.38
New Experiments of Feldspar Hydrolysis and Implications for
Interpretations of Weathering Rates
Zhu C.*, Fu, Q.**, Lu P.*, Seyfried W.E.**
*Indiana University, Department of Geological Sciences, Bloomington, IN 47405, USA (chenzhu@indiana.edu)
**Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA
We conducted a series of new batch reactor experiments for alkali-feldspar dissolution and secondary mineral precipitation
at 150 – 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored with a time series of in situ fluid samples
(e.g., Fig. 1) following the method discussed in Seyfried et al. (1987). Solid reaction products were retrieved from experiments,
some terminated after different durations for identical experiments, to examine the intermediate and final reaction
products. FEG-SEM and HR-TEM examinations revealed dissolution features and secondary mineral coverage on feldspar
surfaces. Boehmite, kaolinite, and muscovite were identified as secondary minerals by XRD and TEM for different experiments.
In order to evaluate the complex interplay between dissolution and precipitation reaction kinetics, we performed
speciation and solubility geochemical modelling to compute the saturation indices (SI) and to trace the reaction paths on
equilibrium activity-activity diagrams. The speciation and solubility modelling results demonstrated: (1) the experimental
aqueous solutions were supersaturated with respect to product minerals for almost the entire duration of the experiments;
(2) the aqueous solution chemistry did not evolve along the phase boundaries but crossed the phase boundaries at oblique
angles; and (3) the earlier precipitated product minerals did not dissolve but continued to precipitate even after the solution
chemistry had evolved into the stability fields of minerals lower in the paragenesis sequence. These three lines of evidence
suggest that product mineral precipitation is a slow kinetic process and product minerals were not in partial equilibrium
with aqueous solution in these experiments.
The newly acquired experimental data and geochemical modelling results support the new hypothesis put forward by Zhu
et al. (2004a,b) on the apparent discrepancy between field derived feldspar dissolution rates and dissolution rates measured
in laboratory at purportedly similar conditions (Blum and Stillings, 1995). The new hypothesis proposes that one important
mechanism controlling the slow feldspar dissolution rates is the slow precipitation kinetics of clays. The precipitation of
clays removes solutes from solution, resulting in undersaturation with respect to feldspars in the aqueous solution and
making additional feldspar dissolution possible. This hypothesis, in which clay precipitation kinetics controls the overall
feldspar dissolution rate, presents several dilemmas for the traditional application of kinetic theories to weathering and
diagenesis systems. The hypothesis shifts the paradigm from the century-old debate about feldspar dissolution rates and
mechanisms to the formation mechanisms of secondary phases.
Figure 1. Activity – activity diagram showing the phase relations in the system K 2 O-(Al 2 O 3 )-SiO 2 -H 2 O-HCl at 200 °C and 300 bars. Symbols re-
present experimental aqueous solutions of alkali feldspar dissolution in 0.2 m KCl and 0.05 m CO 2 solution at 200 °C and 300 bars. The
red line is prediction from reaction path modelling.