『Abstract
Steady-state muscovite dissolution rates have been measured at
temperatures from 60 to 201 ℃ and 1≦pH≦10.3 as a function of reactive
solution K, Si, and Al concentration. The pegmatitic muscovite
used in these experiments has a composition consistent with (Na0.09,K0.86)Fe0.05Al2.92Si3.05O10(OH1.95,F0.06). All experiments
were performed in titanium mixed-flow reactors. All experiments
were performed at far-from-equilibrium conditions with respect
to muscovite. All reactive solutions were undersaturated with
respect to secondary product phases other than for some experiments
which were supersaturated with respect to bohemite and diaspore;
steady-state dissolution was stoichiometric for all experiments
that were undersaturated with respect to these phases.
The variation of rates with reactive solution composition depends
on the solution pH. At pH≦7 rates were found to decrease significantly
with increasing reactive fluid Al activity but be independent
of aqueous SiO2 activity. pH＜7 rates measured
in the present study from 60 to 175℃ are consistent with
r+,K-muscovite,pH＜7 (mol/cm2/s)
= (10-6.53 - exp (-58.2 kJ/mol／RT))(aH+3/aAl3+)0.50
where r+ refers to the far-from-equilibrium
muscovite dissolution rate, R designates the gas constant,
T signifies absolute temperature and ai
represents the activity of the subscripted aqueous species. In
contrast at basic pH muscovite dissolution rates depend on both
reactive fluid Al and Si activity consistent with
r+,muscovite,150℃,8.5>pH>10.5
(mol/cm2/s) = (10-9.195 - exp (-89.1 kJ/mol／RT))(aH+3/aAl3+)0.5(aSiO2)-1
These contrasting behaviors suggest a change in dissolution mechanism
with pH. At acidic pH rates appear to be controlled by the breaking
of tetrahedral Si-O bonds after adjoining tetrahedral Al have
been removed by proton exchange reaction. At basic pH rates may
be controlled by the breaking of octahedral Al-O bonds after adjoining
tetrahedral Al and Si have been removed from the muscovite structure.』
1. Introduction
2. Theoretical considerations
3. Sample preparation and experimental methods
4. Experimental results
5. Discussion
6. Conclusions
Acknowledgments
References