An experimental study of the dissolution mechanism and rates of muscovite

Steady-state muscovite dissolution rates have been measured at temperatures from 60 to 201 degrees C 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 (Na-0.09,K-0.86)Fe0.05Al2.92Si3.05O10(OH1.95,F-0.06). 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 degrees C are consistent with r+,K-muscovite, pH<7 / (mol/cm(2)/s) = (10(-6.53) - exp (-58.2 kJ/mol / RT)) (a(H+)(3)/a(Al)(3+))(0.50) where r(+) refers to the far-from-equilibrium muscovite dissolution rate, R designates the gas constant, T signifies absolute temperature and a(i) 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 degrees C,8.5>pH>10.5 / (mol/cm(2)/s) = (10(-9.195) - exp (-89.1 kJ/mol / RT)) (a(H+)(3)/a(Al)(3+))(0.50) (a(SiO2))(-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. (C) 2008 Elsevier Ltd. All rights reserved.