the hydrolysis mechanism of anorthite, and to compare these results with those of the alkali feldspars. 2. TI-IEORETICAL CONSIDERATIONS. The dissolution of ...
Geochimica et Cosmochimica Acta, Vol. 59, No. 24, pp. 5039-5053, 1995 Copyright © 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/95 $9.50 + .00
Pergamon
0016-7037( 95 ) 00326-6
Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis ERIC H. OELKERS and JACQUESSCHOTT Laboratoire de Gfochimie, CNRS-Universit6 Paul Sabatier, 38 rue des Trente-six Ponts, 31400 Toulouse, France
(Received February 14, 1995; accepted in revised form August 15, 1995) Abstract--Steady-state dissolution rates of anorthite ( A n 9 6 ) w e r e measured as a function of aqueous Si, AI, and Ca concentration at temperatures from 45 to 950(2 and over the pH range 2.4 to 3.2 using a Ti mixed-flow reactor. All dissolution experiments exhibited stoichiometric dissolution. The concentration of aqueous Si, AI, and Ca ranged from ~ 7 × 10 -5 to ~1 × 10 -3 molal, ~ 6 × 10 s to ~3.4 × 10 3 molal, and ~ 5 x 10 -5 to ~0.1 molal, respectively, corresponding to calculated anorthite chemical affinities ranging from ~ 115 to N65 kJ/mol. Measured anorthite dissolution rates at constant temperature are proL5 where an+ designates the activity of the hydrogen ion, and consistent with an apparent portional to all+, activation energy of 18.4 kJ/mol. Anorthite dissolution rates are independent of aqueous A1 concentration, which is in contrast with the alkali feldspars, whose constant pH, far from equilibrium rates are proportional -0.33 to aA~+~ (Oelkers et al., 1994; Gautier et al., 1994; E. H. Oelkers and J. Schott, unpubl, data). This difference suggests a distinctly different dissolution mechanism. For the case of both types of feldspars it appears that A1 is more readily removed than Si from the aluminosilicate framework. Because it has a Si/ A1 ratio of 3, the removal of A1 from the alkali feldspar framework leaves partially linked Si tetrehedra. Removal of Si still requires the breaking of Si-O bonds, and thus the overall alkali feldspar dissolution rate is controlled by the decomposition of a silica-rich surface precursor. The variation of alkali feldspar dissolution rates with aqueous AI activity stems from the fact that the formation of this precursor requires the removal of A1. In contrast, because it has a Si/A1 ratio of 1, the removal of A1 from the anorthite framework leaves completely detached Si tetrehedra. As a result, the removal of Si does not require the breaking of Si-O bonds, the rate controlling precursor complex is not formed by the removal of AI, and the overall dissolution rate is independent of aqueous AI concentration at far from equilibrium conditions. It can be inferred from these results that the variation of far from equilibrium aluminosilicate dissolution rates on aqueous AI depends on the number and relative strength of different bond types that need to be broken for mineral hydrolysis. 1. INTRODUCTION
concentration (Chou and Wollast, 1984; Wollast and Chou, 1985; Gautier et al., 1994; Oelkers et al., 1994; E. H. Oelkers and J. Schott, unpubl, data). This behavior has been interpreted to result from the control of these rates by a silica-rich, M-depleted precursor complex forming from a hydrogen for Al exchange on feldspar surfaces. To assess if a similar mechanism also controls the dissolution rates of Al-rich feldspars, anorthite steady state dissolution rates have been measured as a function of solution composition at temperatures from 45 to 95°C and over the pH range 2.5 to 3.2. The purpose of this paper is to present the results of these experiments, deduce the hydrolysis mechanism of anorthite, and to compare these results with those of the alkali feldspars.
Owing to the fact that the feldspars comprise over one half of the volume of the crust, a large number of studies have been aimed at understanding the rates and mechanism of their hydrolysis (Morey and Chen, 1955; Nash and Marshall, 1956a,b; Morey and Fournier, 1961; Lagache, 1965, 1976; Wollast, 1967; Petrovic, 1976; Petrovic et al., 1976; Beruer and Holdren, 1977, 1979; Busenberg and Clemency, 1976; Tsuzuki and Suzuki, 1980; Massard, 1981; Aagaard and Helgeson, 1982; Fleer, 1982; Gardner, 1983; Helgeson et al., 1984; Chou and Wollast, 1984, 1985, 1989; Wollast and Chou, 1985, 1992; Holdren and Speyer, 1985a,b, 1987; Knauss and Wolery, 1986; Velbel, 1986, 1989; Mast and Drever, 1987; Murphy and Helgeson, 1987, 1989; Amrhein and Suarez, 1988, 1992; Casey et al., 1988, 1989a,b, 1991; Murphy, 1989; Hellmann et al., 1990; Sjfberg, 1989; Schott, 1990; Blum and Lasaga, 1991; Muir and Nesbitt, 1991; Nesbitt et al., 1991; Rose, 1991; Anbeek, 1992a,b; Gestsd6ttir and Manning, 1992; Hwang and Longo, 1992; Oelkers and Schott, 1992, 1995, unpubl, data; Burch et al., 1993; Welch and Ullman, 1993; Brady and Carroll, 1994; Brantley and Stillings, 1994; Drever et al., 1994; Gautier et al., 1994; Hellmann, 1994a,b, 1995; Oelkers et al., 1994; Oxburgh et al., 1994; Stillings and Brantley, 1995 ). Several of these studies noted that the logarithm of alkali feldspar steady-state dissolution rates vary linearly with the logarithm of aqueous A1
2. TI-IEORETICAL CONSIDERATIONS The dissolution of anorthite at acidic conditions can be represented by the reaction CaAI2SizO s +
8H + ¢* Ca +2 + 2A1+3 + 2SiO2(aq) + 4 H 2 0
(1)
for which the law of mass action can be written Kan
2 2 ~ 2 4 -8 = a c a + aAl+ asio2(aq)aH2oaH ÷,
(2)
where/2.0 refers to the equilibrium constant for pure anorthite and ai designates the activity of the subscripted aqueous species. The chemical affinity for this reaction (A) can thus be expressed by
A = Rrln(
Kaoa~+
2 ~ 4 ), \ a c a +2a AI + a Sit2( aq)a H20//
5039
(3)
5040
E . H . Oelkers and J. Schott
where R corresponds to the gas constant and T refers to absolute temperature. The standard state adopted in this study is that of unit activity for pure minerals and H20 at any temperature and pressure. For aqueous species other than H20, the standard state is unit activity of the species in a hypothetical 1 molal solution referenced to infinite dilution at any temperature and pressure. In accord with transition state theory as applied to minerals, the overall rate of a mineral hydrolysis reaction per unit surface area (r) can be described using (Aagaard and Helgeson, 1977, 1982; Lasaga, 1981) r = r+(1 - e x p ( - A / ~ r R T ) ) ,
(4)
where r+ refers to the forward dissolution rate per unit surface area and tr stands for Temkin's average stoichiometric number equal to the ratio of the rate of destruction of the activated or precursor complex relative to the overall dissolution rate. Note that application of Eqn. 4 is based on the assumption that only a single rate limiting step controls dissolution over the temperature, solution concentration, and chemical affinity range of interest. The form of Eqn. 4 is such that at far from equilibrium conditions, when A >>~rRT, the overall rate, r, is equal to the forward reaction rate r+. This forward reaction rate is equal to the product of two factors, the concentration of a rate controlling surface precursor complex P" and the rate of destruction of this precursor to form reaction products (Wieland et al., 1988; S t u m m and Wieland, 1990). This concept is consistent with r+ = k e . { P ' } ,
(5)
where ke. refers to a rate constant consistent with the P" precursor species and { P ° } stands for the concentration of the precursor complex. The application of Eqns. 3 - 5 to describing anorthite hydrolysis requires knowledge of the precursor complex identity and formation reactions. Such information can be deduced by first measuring anorthite dissolution rates as a function of temperature, solution composition, and chemical affinity. 3. M A T E R I A L P R E P A R A T I O N AND EXPERIMENTAL METHODS Natural Fuggoppe anorthite crystals originating from Hokkaido, Japan, and having an average size of --0.2 cm were obtained from Wards Natural Science. These crystals were handpicked, then ground with an agate mortar and pestle, and subsequently sieved. The size fraction between 50 and 100/zm was ultrasonically cleaned using acetone to remove fine particles, rinsed with distilled water, and dried overnight at 80°C. The specific surface area of the cleaned powder was 414 ± 10% cm2/gm as measured by krypton absorption using the three point BET method. The chemical composition of the feldspar was determined by electron microprobe. The results of this analysis, given in Table 1, indicate that the mineral composition based on eight oxygens per feldspar molecule is (Ca0mNa0.04)A1L94Siz04Fe002Os, which corresponds to approximately 96 mol% endmember anorthite. The mineral surfaces were analyzed using a scanning electron microscope. Photomicrographs of the mineral surfaces prior to and following experiments are shown in Fig. 1. No fine particles are evident on the surfaces prior to the experiments and no secondary minerals are evident following reaction. It can also be seen in Fig. 1 that the mineral surfaces following reaction exhibit considerable corrosion and etch pit formation. Steady-state dissolution experiments were carried out in the mixed flow reactor system illustrated in Fig. 2. All parts in contact with the high temperature reactive fluid are made of Ti. The application of this reactor to mineral dissolution experiments has been described in detail by Berger et al. (1994). The inlet fluid was stored in a sealed polyethylene container during the experiments. This input fluid passed through a 10 # m filter and was injected into the reactor via a Milton Roy high pressure liquid chromatography pump that allows flow rates ranging from 1 to 10 gm/min. The fluid entered the 215 m L Parr pressurized reactor vessel, which is continuously stirred with a Parr magnetically driven stirrer and kept at a constant temperature _+I°C by a Parr controlled furnace. The fluid leaves the reactor through a 10 # m titanium filter, is quenched, and then exits through a back pressure regulator. Neither any corrosion of the reactor nor
Table 1, Normalized atomic composition of the anorthite used in the present study as measured by electron dispersion techniques, The compositions are an average of 8 analyses; the uncertainties are estimated at 2 percent. Element Na K Ca Si AI Fe Mg Ti Cr P O
Atom Percent 0.334 0. 015 7.483 15,690 14,890 0.166 0.038 0.008
0. 002 0.064 61.488
precipitation of mineral phases were observed during the experiments. This reactor system is ideally suited to investigate water/mineral reaction rates. The fluid saturation state and composition can be regulated by either changing the flow rate or the composition of the input solution without dismantling the reactor and/or changing the amount of mineral present. This reactor has several advantages over both ( I ) batch reactors as it permits direct measurement of steady state dissolution rates at constant solute concentration, and (2) plug-flow reactors as it prevents local concentration gradients. These represent a particular advantage when working with rapidly dissolving minerals, such as anorthite, whose dissolution induces a dramatic change in solution concentration, which could readily lead to the precipitation of secondary mineral phases. For example, Fleer (1982) observed the precipitation of kaolinite during anorthite dissolution experiments performed in batch reactors. Dissolution experiments were carried out in fluids comprised of demineralized/degassed H20, Merck reagent grade CaC12, HC1, and AIC13, and n a S i O 4 obtained by the dissolution of amorphous silica for one week at 90°C. The silica and A1 compositions of the output fluids were determined using the molybdate blue method (Koroleff, 1976) and by flame atomic absorption (Perkin Elmer Zeeman 5000), respectively. The reproducibility of these analyses are generally within 2%. The pH of the output fluids were measured immediately after sampling. All pH values reported and used in the present study were those measured at a temperature of 25°C. Species distribution calculations performed using the EQ3NR computer code (Wolery, 1983) indicate that these values differ by less than 0.02 pH units from the corresponding true values at the temperature of the experiments. Experiments were assumed to exhibit a steady-state behavior when the outlet solution concentration of A1 and Si did not vary, within analytical uncertainty, for a period of time equal to that required to pump three reactor volumes of inlet fluid through the system. Validation of these steady-state rates over longer time-frames is not possible owing to anorthite mass changes stemming from relatively fast dissolution rates. The surfaces of both fresh and reacted feldspar samples were analyzed using a VG ESCALAB MklI X-ray Photoelectron Spectrophotometer (XPS) with nonmonochromatic A1-X-rays (A1 K a = 1486.6 eV). A review of the application of XPS to the analysis of mineral surfaces is given by Carlson (1975) and Hochella (1988). Sample preparation and analysis were identical to those previously described in Schott etal. ( 1981 ). 4. RESULTS
4.1. The Attainment of Steady-State Concentration Profiles F o r e a c h i n p u t fluid c o m p o s i t i o n a n d flow rate, the r e a c t i o n w a s a l l o w e d to p r o c e e d until t h e c o m p o s i t i o n o f t h e o u t p u t
Dissolution of anorthite
5041
(a)
(b)
FIG. 1. Photomicrographs showing the surfaces of the anorthite used in the present study: (a) before dissolution experiment, (b) after experiment 94C.
fluid attained a constant value. Two examples of the evolution of output fluid composition during an experiment are depicted in Fig. 3. The symbols in this figure depict the measured silica and Al concentration as a function of time during Experiments 94B and 94C. Each of these experiments were run at a temperature of 60°C using A1 and Si free inlet solutions having a pH of ~2.5. The inlet solution for Experiment 94B contained no Ca, but the inlet solution of 94C contained 0.1 M Ca. It can be seen in this figure that the output solution attained a constant silica and A1 concentration after ~ 4 0 0 min ( ~ 7 h) of reaction, corresponding to steady state. The slight decrease in these concentrations after elapsed times of greater than ~ 1 5 0 0 min is likely attributable to a decrease in anorthite mass. For example, the anorthite mass decreased by ~ 6 %
during the first 3000 rain of experiment 94C, which is approximately equal to the corresponding decrease in outlet A1 and Si concentrations. Because the original grain size ranged from 50 to 100 ktm, this anorthite mass decrease implies that a layer averaging ~ 3 - 5 # m thick dissolved from these surfaces during the experiment. It follows that any anomalously reactive surface feature (e.g., fine particles, dislocations, etc.) caused by the preparation of the solids would be removed during the first few hours of each experiment. As an additional test of the attainment of steady state, a steady-state dissolution rate measurement was replicated at least once for several of the input solutions. In each case the differences between the original and replicated result was ~ 2 0 % or less, corresponding to an uncertainty of _+0.1 log units.
5042
E.H. Oelkers and J. Schon Magnel
Pressure
Regulator
Stirrer
IInput S o l u t i o n ?
J
filter
I
Fluid exit Temperature/ Stirring control
Furnace/
Pump
Titanium mixed flow reactor
FIG. 2. Schematic illustration of the experimental apparatus used in the present study to perform open-system experiments (see text).
The A1/Si ratio of the outlet solutions depicted in Fig. 3 are illustrated in Fig. 4. It can be seen in Fig. 4 that the All Si ratio of the outlet solution is slightly enriched in A1 compared to the composition of the anorthite powder during the first 400 min of each experiment. After this initial time period, this A1/Si ratio attains the same value as the dissolving anorthite. It seems likely that during this brief initial time period the anorthite surfaces, which was prepared using acetone and distilled water at 25°C, are equilibrating with the acidic reactor fluid. This brief initial behavior results in the slight depletion of A1 on the anorthite surfaces during the experiment. The observation of stoichiometric steady-state dissolution in the present study is in contrast with observations of Welch
and Ullman (1993), who reported a preferential steady-state AI release during bytownite dissolution experiments performed in a fluidized bed reactor in acidic solutions at 25°C. The depletion of AI on the surface can be validated using the results of XPS surface compositional analyses. The surface composition obtained by XPS of anorthite following experiments 94B and 94C can be compared with their initial values with the data given in Table 2. It can be seen in this table that the A1 concentration at anorthite surfaces following experiments is depleted relative to the initial mineral, which appears to be consistent with the solution data. It can also be seen in Table 2 that the anorthite surfaces reacted in Ca-rich inlet solutions are enriched, whereas the anorthite reacted in
30 T,-
30 me
X
O3 2O
main •
X 20 O3
•
/ /
m
"6 E
O
Steady State Concentration
. 10
Steady State Concentration
E 1o
Experiment 94B pH= 2.58 T=60 ° C
Experiment 94B ]
pH= 2.58 T=60 ° C 0
1000
2000
3000
0
Elapsed time, m i n u t e s
2000
3000
50
5O
mm
40
o 40
• ;/
X
.~
t000
Elapsed time, m i n u t e s
30
nun X ~ 30 t
•
m
0
Steady State Concentration
E 20
E 2o
Experiment 94C
.~. lO
O
'~
Steady State Concentration
Experiment 94C
10
pH = 2.67, T = 60 ° C
pH = 2.67, T = 60 ~ C
0
0
1000
2000
Elapsed time, minutes
3000
1000
2000
Elapsed time, minutes
FIG. 3. Concentration of A1 and Si in outlet solution as a function of time during experiments 94B and 94C (see text).
000
Dissolution of anorthite 1.1
were calculated using Eqn. 3 together with solute activities computed with the EQ3NR computer program (Wolery, 1983), equilibrium constants for aluminum hydroxide complexes reported by Castet et al. (1993), and an equilibrium constant for reaction 1 obtained from SUPCRT92 (Johnson et al., 1992). In this calculation the chemical affinity of the natural anorthite sample was taken to be equal to that of pure anorthite and the output concentration of Ca was computed assuming stoichiometric dissolution. These same speciation calculations indicated that the output solutions were undersaturated with respect to all minerals other than quartz, which was slightly supersaturated in some cases. No evidence was observed for the precipitation of SiO2 during the experiments. The stoichiometry of these dissolution experiments at steady state can be assessed with the aid of Fig. 5a and b. In Fig. 5a, the difference between the measured input and output aqueous AI concentration is plotted as a function of the corresponding change in aqueous silicon concentration. The linear curve in this figure has a slope of 0.95, which represents the A1 to Si ratio of the dissolving anorthite. The close agreement between this curve and the symbols illustrates that the steady-state dissolution of anorthite measured in the present study was stoichiometric with respect to Si/A1 ratio. A similar comparison is presented in Fig. 5b, where the difference between inlet and outlet A1 concentration is plotted as a function of the corresponding change in hydrogen ion concentration. The slope of the linear curve in this figure is - 4 , which is equivalent to the stoichiometric ratio of these elements in the acidic media pure anorthite dissolution reaction (reaction 1 ). Small deviations between the linear curve and the data points in Fig. 5b may be due to the formation of aqueous aluminum hydroxide complexes, and compositional differences between pure anorthite and the natural anorthite used in the present study. Nevertheless, the close agreement between the symbols and solid line in Fig. 5b confirms that the change in hydrogen ion concentration matches the corresponding stoichiometric change in A1 in accord with reaction 1, and together with the results depicted in Fig. 5a, imply that these dissolution experiments were stoichiometric with all reaction constituents. The variation of the 60°C steady-state dissolution rates obtained in the experiments performed in a variety of different inlet solution compositions are depicted as a function of pH in Fig. 6. It can be seen in this figure that the constant pH steady-state dissolution rates obtained in Si-, AI-, Ca-rich, and AI-, Si-, and Ca-free input solutions are identical within the uncertainty of the data. The slope of the solid line in this figure indicates that these rates are proportional to a~+ where the exponent n is equal to 1.5. Note that although the total pH range obtained in the present study was only ~0.7 log units,
Experiment 94-B pH= 2.58 T=60o C
< 0 mol/km; diamonds--[Ca] = 0.01 mol/kg, [Si] > 0 mol/kg, [All = 0 mol/kg; downward pointing triangles--[Ca] = 0.1 mol/kg, [Sil = [All = 0 mol/kg; and upward pointing triangles--[Ca] = [Si] = [All = 0 mol/kg.
,
-11
g
e = 16.4 kJ -11.2
2.7
2.8
2.9 1000
3
3.1
3.2
/ (T/K °)
FIG. 8. Logarithms of anorthite steady-state dissolution rates for pH = 2.6, depicted as a function of reciprocal temperature. The symbols designate the interpolations of experimental data depicted in Fig. 7, and the solid line is consistent with a constant activation energy of 18.4 kJ/mol. The error bars were determined from the positive and negative deviations between the data points and corresponding lines in Fig. 7.
E. H. Oelkers and J. Schott
5048
Table 5. Logarithms of anorthite steady state dissolution rates (in mol/cmVsec) as a function of temperature and pH calculated using Eqn. (9) -- see text. Temperature, C 25 50 75 100 125 150 200
pH 1.00 -8.82 -8.57 -8.36 -8.18 -8.01 -7.87 -7.63
1.50 -9.57 -9.32 -9.11 -8.93 -8.76 -8.62 -8.38
and K-feldspar dissolution rates as a function of solution pH and A I / S i ratio, Oelkers et al. (1994) and Gautier et al. (1994) proposed that alkali feldspar hydrolysis consists of the three step mechanism outlined in Fig. 9. This mechanism consists of (1) the relatively rapid exchange of hydrogen and
®
®
(b)
2.00 -10.32 -10.07 -9.86 -9.68 -9.51 -9.37 -9.13
2.50 -11.07 -10.82 -10.61 -10.43 -10.26 -10.12 -9.88
3.50 -12.57 -12.32 -12.11 -11.93 -11.76 -11.62 -11.38
4.00 -13.32 -13.07 -12.86 -12.68 -12.51 -12.37 -12.13
alkali ions near the mineral surface, (2) an exchange reaction between three hydrogen and one Al in the mineral structure resulting in the breaking of AI-O bonds coupled to the formation of Si rich precursor complexes, and (3) the hydrolysis of Si-O bonds releasing the precursor complexes into solution. Rates are proportional to the concentration of precursor complex at or near the surface, which can be related to solution composition through the law of mass action for the precursor forming reaction. Because, in accord with second step of this mechanism, three Si-rich rate controlling precursor complexes are formed by an exchange reaction between an A1 at or near the surface with three aqueous hydrogens, the far from equilibrium dissolution rates of the alkali feldspars are observed to be proportional to the ratio (aH~/aAl+~) ~/3 over wide ranges of AI concentration and pH. The fact that the precursor complex for alkali feldspar hydrolysis consists of partially liberated silica tetrehedras that require only the breaking of Si-O bonds to complete hydrolysis is also apparently consistent with the fact that the activation energies of alkali feldspar dissolution is similar to that of quartz (Schott and Oelkers, 1995). The pH dependence of alkali feldspar dissolution, however, differs significantly from that of quartz and amorphous silica. Because the precursor complex for alkali feldspar dissolution is formed by an A1 for hydrogen exchange, this dependence is proportional to the variation of (aH~la A~) with pH. As the precursor complex for quartz and amorphous silica dissolution is most likely formed by a reaction that does not contain A1, a different pH dependence is exhibited. The major difference between the structure of alkali feldspars and anorthite is the A1/Si ratio of their aluminosilicate frameworks. As emphasized by Blum (1994), in the alkali feldspars, with an A1/Si ratio of 3, the preferential attack and removal of A1 will leave a fragmented but partially linked tetrahedral Si framework. The destruction of this fragmented Si framework still requires the hydrolysis of Si-O bonds (see Fig. 9). In contrast, in anorthite, where the A1/Si ratio is equal to one, the removal of A1 from the feldspar structure will lead to completely detached Si tetrahedra, and thus the hydrolysis of the relatively slow reacting Si-O bond is not required for anorthite dissolution. It thus follows that, in contrast to the alkali feldspar mechanism outlined in Fig. 9, anorthite dissolution is not controlled by the relatively slow destruction of a Si-O-Si linked, Si-rich precursor complex. The fact that the hydrolysis of the anorthite aluminosilicate framework requires the destruction of only AI-O bonds is apparently manI/~
FIG. 9. Schematic illustration depicting the three major steps in the dissolution of an alkali feldspar: (a) The exchange of hydrogen with alkali ions in the feldspar structure. (b) An exchange reaction among aqueous hydrogen ions and AI in the feldspar framework leading to the formation of Al-deficient surface precursor complexes. (c) The irreversible detachment of the precursor complex.
3.00 -11.82 -11.57 -11.36 -11.18 -11.01 -10.87 -10.63
•
Dissolution of anorthite ifested in observations that ( 1 ) anorthite rates are independent of aqueous A1 concentrations at far from equilibrium conditions (see above), (2) the dissolution rates of anorthite-rich plagioclases are faster than corresponding alkali feldspar rates (Holdren and Speyer, 1987; Casey et al., 1991; Blum, 1994; Oxburgh et al., 1994), and (3) the activation energy in acidic solutions of anorthite dissolution (18.4 kJ/mol, see above) is substantially lower than those of the alkali feldspars, which have been reported to range from --70 kJ/mol (Rose, 1991 ) to ~ 9 0 kJ/mol (Hellmann, 1994a). Further insight into the mechanism of anorthite dissolution can be deduced by considering the available plagioclase dissolution rate data as a function of anorthite content. The compositional dependence of plagioclase dissolution rates in acidic solutions at 25°C has been determined experimentally by a number of investigators (Holdren and Speyer, 1987; Casey et al., 1991; Oxburgh et al., 1994). These studies reveal a dramatic increase in plagioclase dissolution rates with increasing anorthite content. At pH = 2 and 25°C, this increase was reported to be greater than three orders of magnitude (Casey et al., 1991 ). Blum (1994) noted that this increase is near-exponential on anorthite content till ~An80. A discontinuity appears at ~Ans0, where a stronger dependence becomes apparent. These studies also indicate that the pH dependence of plagioclase dissolution rates increases with increasing anorthite content. Helgeson et al. (1984) and Knauss and Wolery (1986) proposed that albite dissolution rates are proportional to a~ +, but Chou and Wollast (1984) suggested that these rates are proportional to a 0.5 n +. In contrast, reported values for the dependence of anorthite dissolution rates on hydrogen values have range from ah+ (Sverdrup, 1990) to a3+ (Amrhein and Suarez, 1988, for Grass Valley Anorthite, An93). Experimental results reported by Oxburgh et al. (1994) indicate that there is a negligible change in the pH dependence of plagioclase dissolution over the An0 to ~An70 compositional range; the increase in this dependence is apparently limited to the ~AnT0 to An]0o compositional range (Blum, 1994). The observations summarized above suggest strongly that there is a transition between two distinct dissolution rate mechanisms controlling plagioclase dissolution rates, one which dominates at anorthite contents of less than ~AnT0 (albite-rich plagioclases) and the other which dominates at anorthite contents greater than ~AnT0 (anorthite-rich plagioclases). It seems reasonable to assume that the mechanism outlined above for alkali feldspar dissolution controls the dissolution of all albite-rich plagioclases. This concept is strongly supported by the fact that the pH dependence of these reaction rates are independent of plagioclase content to ~AnT0. Within the hydrolysis mechanism outlined in Fig. 9, the pH dependence is governed by the fact that the exchange of three hydrogens for one A1 creates three Si-rich precursor complexes. In accord with the A1 avoidance rule (Lowenstein, 1954), the number of silica atoms surrounding each A1 atom in the plagioclase structure is independent of composition. It follows that for all albite-rich plagioclases, the number of hydrogens used to form each silica-rich precursor complex is constant, and thus the pH dependence of these rates is also independent of composition. Because the coordination of A1 in the structure of the anorthite-rich plagioclases is identical
5049
to that of the albite-rich plagioclases (all A1 atoms are tetrehedreally linked to oxygen atoms that are themselves linked to Si atoms), it seems reasonable to postulate that anorthite hydrolysis at acidic conditions proceeds by a similar attack of an A1 by three hydrogen ions. This attack eventually leads to an exchange of these hydrogens for the AI. Because of the ordering imposed by the avoidance rule, this exchange of A1 for hydrogen has the effect of leaving completely detached Si tetrehedra and thus destroying the anorthite framework. This reaction mechanism is schematically summarized in Fig. 10. As for the case of the alkali feldspars, hydrolysis begins via the relatively rapid exchange of hydrogen with the large intrastructural Ca +2 cation. This exchange of Ca for hydrogen has been demonstrated to produce a Ca-depleted surface layer of up to 1000 ,~ in depth at acidic conditions (Casey et al., 1988). This step is followed by the adsorption of three hydrogens on sites adjoining an A1 ion at the surface. This A1 surrounded by three adsorbed hydrogens represents one or more precursor complexes. A reaction forming a single precursor complex can be expressed by
Ibl
®
®
'°'
FiG. 10. Schematic illustration depicting the three major steps in the dissolution of anorthite: (a) An exchange of hydrogen with calcium in the feldspar structure. (b) The adsorption of hydrogen ions leading to the formation surface precursor complexes. (c) The irreversible detachment of the precursor complex.
5050 3
-
E. H• Oelkers and J, Schott
H + + H~/.AII/.Sij/.O4/. = (H4/.AII/.SII/nO4/.,) +3/,, - , -
(9)
6. E X P E R I M E N T A L A N D C O M P U T A T I O N A L UNCERTAINTIES
n
where n designates the number of precursor complexes formed by the adsorption of three hydrogens, H~/.AI~/. Si ~/.O4/. represents a hydrogenated anorthite surface site, and • +3In • (H4/~All/nSIj/~O4/.) stands for a single precursor complex. The exchange of these hydrogens for AI leads directly to the release of both silica and A1 into solution and is thus the limiting step in anorthite-rich plagioclase hydrolysis. A rate expression consistent with this mechanism can be deduced by first considering the law of mass action for reaction 9 that can be expressed by K" =
•
÷M,
(10)
a(HnlnAll/"SII/nO4t"~ 3/n aHi/,~Alr/.Sil/oO4/na
'
H +
where a~ refers to the activity of the subscripted species and K" designates the equilibrium constant for reaction 10. Assuming that all of the surface sites contain either HI/,AI~/o • +3In • Sil/nO4/n o r (H41nAll/nSll/nO4/n) one can write XHi/nAIiinSil/nO4/n + X(H4/,,AIIInSil/~04/31~ ~ ~- 1,
(11)
where X~ denotes the mole fraction of the subscripted surface species. C o m b i n i n g Eqn. 10 with Eqn. 11 and the assumption that the activities of Ht~nAll/.Si~/.O4/. and (FL/nAI~/.Si i/.O4~i~/n)* are equal to their mole fraction* yields •
X ( H 4 / n A l l l n S n l l .n .O.4.1.n l
"
-
-
~/n
K ah, 1 + K.a~/~
+
,
(12)
which can be further combined with Eqn. 5 to yield r+ = k+
3In all÷ •
1 +Kai~.
"¢/n '
(13)
where k÷ = k p . K ' .
(14)
It follows from Eqn. 13 that when K ' a ~ / ¢ ' ~ 1 this equation reduces to r+ = k+a~/~
(15)
which implies that, in accord with the mechanism described above, anorthite far from equilibrium steady-state dissolution rates are a function of only aqueous hydrogen activity. Ideally the value o f n in Eqns. 9 - 1 3 and 14 can be determined from measurement of the pH dependence of anorthite dissolution rates. However, there is no current consensus on this dependence. The values that have been reported a~+ (Sverdrup, 1990), a~iS+ (present study), and a~+ (Amrhein and Suarez, 1988) yield values of n equal to 3, 2, and 1, respectively. Clearly, further experiments are essential to decipher these discrepancies. * Note the activities of surface sites may be complex functions of solute concentration at certain conditions (Wollast and Chou, 1985; Stumm, 1992; Devidal, 1994).
Uncertainties associated with the steady-state rate constants generated above stem from a number of sources, including the measurement of aqueous solution concentrations and mineral surface areas. The uncertainties in the measured values of the total fluid concentrations of A1 and Si are on the order of ± 2 % . Computational and experimental uncertainty in the pH of these solutions are on the order of ±0.04 pH units• Uncertainties associated with the measurement of the surface area of the solid powder is ± 10%. Moreover, as the total mass of mineral powder changed over the duration of the experiment, so too did the mineral surface area. To assess the temporal effects of changing mineral surface areas on the resulting steady state dissolution rates, one of the final fluid flow rates for several of the mineral samples of a single fluid composition was set approximately equal to the first. The difference in the resulting fluid concentrations were --20% or less. Because the uncertainties associated with the resulting steady state mineral dissolution rates are directly proportional to the uncertainties in the fluid concentrations and the mineral surface areas, the overall uncertainties in these rates are on the order of -- ±25%. The uncertainties in the computed chemical affinities reported in Tables 3 and 4 are difficult to assess owing to the large number of equilibrium constants upon which they depend. In addition, the likely precipitation of secondary phases makes it impossible to perform the definitive reversed equilibrium experiments to completely constrain the effective equilibrium constant for reaction 1 for the natural mineral sample used in the present study at temperatures from 45 to 95°C. Nevertheless, as emphasized by Burch et al. (1993), changes in the equilibrium constants will shift only the origin (A = 0) without affecting the functional dependence of rate on chemical affinity. For example, if the value for the equilibrium constant of reaction 1 was taken to be an order of magnitude more negative, each chemical affinity listed in Tables 3 and 4 would increase by ~ 5 . 3 kJ/mol at 45°C and ~6.1 kJ/mol at 95°C. 7.
CONCLUSIONS
Values of the steady-state dissolution rates of natural anorthite have been measured in an open-system mixed-flow reactor as a function of solution composition over a temperature range from 45 to 95°C and a pH range from 2.4 to 3.2. These rates were found to be independent of aqueous aluminum, silica, and calcium concentration but proportional to a~iS+ over this pH range. In contrast, the dissolution rates of albite (Oelkers et al., 1994; E. H. Oelkers and J. Schott, unpubl. data), K-feldspar (Gautier et al., 1994), kaolinite (Devidal et al., 1992; Devidal, 1994), kyanite (Oelkers and Schott, 1994, unpubl, data), and analcime (Murphy et al., 1995 ) are strong inverse functions of aqueous aluminum concentration. The fact that far-from-equilibrium anorthite dissolution rates do not depend on aqueous aluminum concentration is consistent with the fact that the breaking of its aluminosilicate framework involves the breaking of only one type of bond. Far-from-equilibrium dissolution rates of minerals such as anorthite and quartz, that require the breaking
Dissolution of anorthite o f only one type o f bond, are independent o f aqueous silica and aluminum concentration. In contrast, the far-from-equilibrium dissolution rates o f minerals such as kaolinite, kyanite, and the alkali feldspars, whose hydrolysis mechanisms require the breaking o f more than one type o f bond, have been found to depend on aqueous aluminum concentration. It follows that the dependence o f mineral dissolution rates on solution composition depends on the number o f different types o f bonds that need to be broken and their relative reactability. This observation, together with further experimental results on minerals o f different structures will likely lead to the general understanding o f the dissolution behavior o f natural solids. Acknowledgments--This manuscript is contribution number 732 of the Dynamique et Bilan de la Terre (DBT) program of the CNRS. We would like to thank Jean-Louis Dandurand, Robert Gout, Gilles Berger, Eric CadorG Christophe Monnin, Sigurdur Gfslason, Gleb Pokrovski, Igor Diakonov, Jean Luc Devidal, and Jean-Made Gautier for helpful discussions during the course of this study. We are indebted to Jocelyne Escalier, Jean Claude Harrichoury, Claude Lurde, and Michel Thibaut for technical assistance. This manuscript benefited from thoughtful reviews provided by Kevin Knauss, Susan Carroll, and Alex Blum. Support from Centre National de la Recherche Scientifique is gratefully acknowledged. Editorial handling: E.J. Reardon
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