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( )F. Gerard et al.r Chemical Geology 151 1998 247258248

complexes are in state of local equilibrium Wieland.et al., 1988; Guy and Schott, 1989 . Until recently,

precursor complexes for multi-oxide minerals were

believed to consist of either hydrogenated, hydroxe-

nated, or hydrated surface sites. Taking account of

these assumptions, TST dictates that at far-from-

equilibrium, dissolution rates only depend on pH.

The kinetic effects of solution pH on silicate dissolu-tion has been thoroughly documented Helgeson et

al., 1984; Knauss and Wolery, 1986, 1989; Tole et

al., 1986; Murphy and Helgeson, 1987; Carroll and

Walther, 1990; Acker and Bricker, 1992; Hellmann,.1994 . The abundance of data has driven most hy-

drochemical and geochemical code developers to

implement the first order, pH-dependent, rate expres-

sion in their codes to forecast silicate dissolution.Nonetheless, except for the silica phases Rimstidt

.and Barnes, 1980 , the first order approach of the

overall dissolution rate toward equilibrium had notbeen experimentally validated until recently. More-

over, other catalyticrinhibitor effects due to dis-

solved inorganic and organic species have beenpointed out for several years Wogelius and Walther,

1991; Amrhein and Suarez, 1992; Welch and Ull-

man, 1993; Bennett and Casey, 1994; Stillings et al.,.1996 . However, they provide no mechanistic kinetic

expression or their applicability range is too narrow

to be effective in waterrock interaction modelling.

Recently, a new mechanistic kinetic rate law con-

sistent with TST and surface chemistry was proposedin the literature Gautier et al., 1994; Oelkers et al.,

.1994; Schott and Oelkers, 1995 . This new rate law

takes account of the inhibitor effect of dissolved

aluminium on alkali-feldspars, kaolinite, muscovite,

and kyanite dissolution. In addition, alkali-feldspars

and kaolinite apparently have a larger dissolution

rate dependence on chemical affinity than previously

assumed.

Several convincing arguments lead us to believe

that the new rate law might significantly affect the

chemical evolution of natural waterrock systems.

Above all, aqueous aluminium effects appear strong

enough to induce an apparent chemical affinity de-

pendence on rates at far-from-equilibrium conditions.

Moreover, dissolved aluminium is common in aque-

ous solutions and has a significant concentration

range. Finally, the rate of approach toward equilib-

rium is intrinsically slowed due to its larger chemical

affinity dependence. Given alkali-feldspars, and to a

lesser extent muscovite, are ubiquitous rock-forming

minerals, most waterrock reaction paths will be

modified by these effects.

The aluminium speciation-dependent dissolution

rate equation has been introduced into the KIRMATreactive transport code Gerard et al., 1996, 1997;

.Gerard, 1997 , which also contains the standard ki-netic rate expression. This innovation permits the

study of effects of the aluminium speciation depen-

dent rate law in hydrochemical modelling. The large

number of factors that now influence aluminosilicate

dissolution rates suggest its effects are case depen-

dent. The goal of this study is to estimate the upper

and lower boundaries of the effect of aqueous alu-

minium on dissolution rates. These results allow

evaluation of the trends and potential implications of

this dissolution rate law on waterrock systems.

2. Theory

2.1. Kinetic equations

Mineral dissolution rate laws are commonly de-

rived from TST, assuming both that Hq, H O, and2OHy are the only aqueous species involved in the

reaction forming the activated complex, and that one

mole of activated complex is formed from each moleof mineral. These assumptions lead to the following

equation:

EA an y

qr s k Sa 1 y exp y A exp . 1 .RTd d H / /RTWith

A s yRT ln QrK 2 . .

y1 .where r is the dissolution rate mol s , k thed d y2 y1.

dissolution rate constant mol cm s ,S

thereactive surface area of the mineral, a q the in-Hhibitorrcatalytic term for pH effects and n is an

experimental exponent, A denotes the chemical y1 . affinity kcal mol , R the gas constant 1.99=

y3 y1 y1.10 kcal mol K , A the pre-exponential factor

of the Arrhenius rate law, E the apparent activationa y1 .energy of the reaction kcal mol , which is a


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( )F. Gerard et al.r Chemical Geology 151 1998 247258 249

function of solution pH. Finally, Q is the ionic .activity product IAP of the mineral and K its

thermodynamic equilibrium constant.

According to TST, the exponent n is equal to the

number of protons required to form the activatedcomplex Helgeson et al., 1984; Murphy and Helge-.son, 1987 . The rate constant k and the exponent nd

usually have different values depending on pH re-

gion: pH delimits the acid from the neutral pH1region and pH the neutral from the basic pH region.2This pH dependence leads to the well-known U-shape

function. Many efforts have been made to relate pH 1with the pH of the point of zero net proton charge,

commonly noted pH see for example, Blum andpznpc.Stillings, 1995; Hochella and Banfield, 1995 . Al-

though some consistent results are obtained on sim-ple oxides such as SiO e.g., Brady and Walther,2.1989, 1990; Bennett, 1991 , inconsistent results have

been obtained for more complex oxides such asalkali-feldspars and kaolinite see for instance the

.discussion in Chen and Brantley, 1997 . Moreover,as proposed by several authors e.g., Chou and Wol-

.last, 1985; Garnor et al., 1995 , the temperaturedependence of the exponent n see Carroll and

Walther, 1990; Hellmann, 1994; compare Chou and.Wollast, 1985 to Knauss and Wolery, 1986 might

be due to the presence of some unrecognized in- .hibitor effects thus pointing out the failure of Eq. 1

to explain the dissolution kinetic of the complex

oxides studied. Although this deviation from thestandard theory can be understood with the help of

surface chemistry see Brady and Walther, 1992;.Casey and Sposito, 1992 , both discrepancies can be

explained by taking into account the effect of dis-

solved aluminium. The presence of an aluminium-

deficientrsilica-rich rate controlling precursor com- .plex was considered by Gautier et al. 1994 ; Oelkers

. .et al. 1994 , Oelkers and Schott 1995 , Schott and . .Oelkers 1995 , and Oelkers 1996 . It was proposed

that this precursor complex is formed by the follow-

ing exchange reaction:

K8q 3q3 m H q M OmP8q m Al 3 .

where MO represents a potentially reactive surface .site metaloxygen , m is a stoichiometric coeffi-

cient, P8 the precursor species, and K8 the thermody-

namic constant of the precursor formation reaction.

The rate law derived from the above formation reac- .tion is given by for details see Oelkers et al., 1994 :

a3 mqHm

3qaAlr s k Sd d 3 m

qK8

aH

gerard 1998 chemical geology

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