Kinetics of the alteration of K-feldspar in buffered solutions at low temperature

Qemhimka et Ckmmchlmica Acta1967. Vol.31,pp.635to648.Pergamon Press Ltd. Printed in Northem Ireland

Kinetia

ofthe alteration

of K-feldsgarin bufked

8olutiouiat low temperah

R. WOIXAST Universit6 Libre de Bruxelles, Department of Solid State Chemistry, Belgium (Received 15 Jammy

1966)

Ah&r&-A study has been made of the release of Si and Al to solution from the alteration of a potassia feldspar in solutions buffered at pH values between 4 and 10. Release of both Si and Al is consistent with diffusion from an altered layer, presumably formed by rapid initial hydration and exchange of H+ for K+. In a limited volume of solution diffusion ceases when the Al concentration in the external solution reaches a fixed value at each pH; this value is

reasonably eon&tent with the solubility of AQOH),. The Si concentration tends to reach a maximum at eat& PH. The interpretationis made that the maximum in Si concentrationcorrespondsto a balance between Si Fusion into the solution, and Si removed from the solution by reaction with Al(OH), to form a hydrated silicate. The calculated equilibrium value for the reaction AI( + SiOl, = Al-silicate is 5 ppm SiO,. INTRODUCTION

IN TWOearlier studies of the weathering of feldspars (WOUST, 1961,1963), emphasis was placed upon determining the conditions controlling the composition of the reaction products and particularly in differentiating the processes that lead to formation of aluminum oxide hydrates from those that lead to kaolinite. As a continuation of this program, the present study is directed toward an analysis of the effects of various experimental conditions on the rate of alteration of potassic feldspar. The experiments were carried out at room temperature and pressure, so that the rates of reaction observed would resemble nature more closely than any preceding investigations at elevated temperatures and pressures. CORRENS and VON ENGE LHARDT (1938, 1940) demonstrated decomposition of feldspar at low temperature by continually eliminating the ions placed in solution by means of a dialysis cell and ultrafllters. The results show that the various constituents of feldspars go into solution at different rates, leaving a slightly soluble residual layer at the surface. The composition of the layer was shown to be dependent on pH. In all cases the ratio of SiO, to A&O, in the residual layer was greater than that of kaolin&e (>2/1). Consequently, they concluded that kaolinite could form only by reactions among dissolved substances. The rate of the alteration reaction was controlled by the rate of diffusion of ions through the residual layer. The diffusion rate was found to be very slow, approximately lo-la cm*/sec (10-l* cm2/day). On the other hand, LAGACHE, WYART and SABATIER (1961a, 196lb) observed that at 200°C and 5 bars pressure the dissolution rate of feldspar was not controlled by a residual layer. Instead the feldspar dissolved continuously to yield precipitates and dissolved materials; but the reaction rate depended on the concentration of silica and alumina in the solution. In this work an attempt is made to explain the markedly different behavior found by these two sets of investigators. 036

636

R.

WOLLAST

EXPERIMENTAL Procedure

The concentrations of silica and alumina liberated at room temperature from suspensions of finely ground feldspar were determined in solutions buffered at constant pH, in the range pH 4-10. * The suspensions were continuously agitated to keep the solid in suspension and to homogenize the liquid phase. At given intervals, a few cm3 of the suspension were removed, centrifuged, and alumina and silica were determined in the clear liquid by calorimetric analysis (SHAPIROand BRANNOCK,1956). The feldspar used was an orthoclase containing about 5% of quartz as an impurity. No analysis was made for the Na/K ratio of the feldspar. Rem&s

Experimental results are given in Table 1 (a) and (b), which shows silica in solution as functions of time, pH, and wt.% suspended feldspar. Results are also shown graphically: Figs 1 and 2 indicate the release of silica as a function of time for various pH and Fig. 3 the alumina dissolved at pH 4 and 5. At the higher pH values, the amount of alumina in solution is always less than 1 mg/l. and it is difficult to measure the change of this constituent during a given time interval. Table 2 gives the results obtained in a particular experiment during which concentrations of Al were measured with fair accuracy. Table 3 summarizes the results obtained after 25 days indicating the maximum concentrations realized during the alteration of the feldspar. These experimental results show that when a potassium feldspar is ground and placed in buffered aqueous solutions between pH values of 4 and 10, the alumina content of the solution quickly reaches a low and constant value in all but strong acid solutions. The silica content of the solution rises over a much longer period of time, and reaches a maximum that is nearly independent of the weight ratio of feldspar to solution. Nevertheless, it must be emphasized that the rate of dissolution is greatly affected by the wt.% of feldspar in suspension as shown by Table l(b). Finally, the dissolution of feldspar is incongruent whatever the pH value. Interpretation,

It has already been pointed out, in the work of CORRENS,that K+ is quickly released during alteration of K feldspar. The appearance of K+ in solution is in * The

buffers used had the following compositions:

0.1 M K PH 4 5 6 8 Y 10

biphthalate

w 50 50 50 -._ (adjusted to 100 ml)

0.1

M NaOH WI 0.40 23.85 46.46 3.97 21.30 43.90

0.1 M H,BO, (ml)

50 60 50

637

Kinetics of the alteration of K-feldspar Table l(a). Silica oonoentration(mg/l.) in solution at various pH (6% solid in suspension) PH 4 C (mgll.)

(& 0.26 2 6 11 24 48 72 95 144 168 200 320 576

PH 6 C @g/l.) 0.625 1.55 1.65 2.17 3.46 3.70 5.50 7.31 7.61 9.12 10.4 11.5

2.9 7.75 9.1 10.0 12.4 14.2 16.6 17.7 20.4 21.8 24.4 20.8 31.1

pH 10 C (mg/U

pH8 C (mgk)

-

0.65 -

0.06 0.78 1.03 3.44 5.50 6.75 6.66 7.09 7.91 8.12 -

0.95 1.04 3.18 3.97 4.56 4.86 5.38 6.09 6.18 5.99 6.30

-

Table l(b). Mice conaentration (mg/l.) in solution as a function of time and O/Osuspended solids, at pH 4 Time wt.% suspension 3.15 6.9 13.0 20.1 38.0

2 5 10 25 50

24 hr C (mgk)

48 hr C (mg/l.)

3.63 8.3 16.0 33.0 66.0

4.72 9.7 19.1 36.6 69.3

Table 2. Alumina conoentrationin solution at pH 4.2 (10% solid in suspension) t W)

C (mgll.)

2 4 6 8 24 48

4.36 7.99 9.06 11.8 16.76 21.5

Table 3. Concentrationsof silioa and alumina in the presence of solid feldspar after 26 days PH

SiO, (mgP.)

4 5 6.8 8 9 10

87.8 44.3 33.1 17.9 16.9 22.6

AU% @g/l.) 139 26.4 0.46 0.65 0.10 I.6

Fig. 1. Change in silica concentrationwith time at pH 4 and 0 in a ‘rweath&ng” solution containing 5 y0 feldspar.

Fig. 2. Changesin silica concentretionwith time at pH 8 and 10 in a “weathering” solution containing 5 yOfeldspar.

6189

Kinetics of the alteration of K-feldspar

faot accompanied by the disappearance of H+ ions : the pH of a suspension of feldspar in distilled water irmreases during the early stages of the reaction from 6.8 to 94. The first reaction step was analyzed by determining the rate of dissolution of silica and alumina for time intervals between 15 min and 2 hr.

Y O

30’

sd

t Zh

3h

Bh Fig. 3. Changein &mine, conoent~t~onwithtimeat pH 4 and 5 in 8 %e&hering” solutionao&aining10% feldsptw.

Figure 4 represents the logarithm of silica and alumina concentrations expressed in mg/l., found after 30 min in various bufFered solutions. It can be seen that the initial reaction rate is a simple function of PH. In terms of molar concentrations the rate equation can be written vu1 =

~~~20&01 1 WWl,ol at =- i-76 at

=

L,[H-+]l’~

This fractional order for H+ and the increase of pH in distilled water indicates that the first step of the reaction is probably exchange of H+ for K+ with decomposition of the structure into a thin surface layer of amorphous Al(OH), and SiO, or H,SiO,. Loss of alumina and silica from this sheath increases the content of these components in the solution. The solution quickly steak with respect to alumina. As shown in Fig. 5, the maximum concentration of alumina into solution is in good agreement with the solubility of amorphous Al(OH), at low pH and is too high as compared with solubilities of orystallised species like gibbsite or bayerite. Because of the relatively high solubility of H,SiO, (-115 mg/l.), silica continues to di&zse from the sheath but at a diminishing rate because of the inoreasing distance of diffusion of silica from the fresh feldspar to the solution through the aluminaenriched outer portion of the sheath.

R. WOLLAST

Pig. 4. Logaxithm of silica

(filledcircles)and slumin~ (orosses)concentrationsin n&l. aa a fun&ion of pH after 30 min. Solutions contained 6 wt o/ofeldspar.

PH 4

5

6

7

6

9

10

Fig. 6. Maximum concentration of alumina in solution (+) compared with calcul&ed solubilities of amorphous Al(OH), (open circles) and gibbsite.

10

4

6

8

,PH

,

10

Kinetics of the alteration of K-feldspar

041

Furthermore, SiO, reaches a series of maxima at various pH values ; these values are not in accord with the solubility of any known species. This leads to the argument that the silica values represent an increase toward 116 ppm by reason of diffusion and a decrease because of some reaction that tends to remove silica,. The maximum value of silica is consistent with a rate equilibrium between the two processes. As. suggested by R. M. Garrels, the removal reaction may be the formation of an amorphous alumina-silicate, perhaps a hydrated kaolinite-like material HJ.l,Si,Oo~nH,O, by reaction of dissolved silica with the aluminous outer portion of the altered feldspar grains. It will be shown further that the kinetics data are in good agreement with this interpretation. Let us imagine a grain of feldspar placed in a finite volume of pure water. It is well established that the silica, alumina and alkalies that dissolve are not in the proportions found in feldspar, and that around the grain a residual layer of slightly soluble substances is formed. The dissolution rate can depend on the rate of the chemical reaction itself, between the feldspar and the water, the diffusion rate of the reacting substances through the residual layer. In the beginning the fresh feldspar is in contact with pure water and the chemical reaction determines the dissolution rate following vi = k,[H+]1’3 The grain of feldspar is then partially altered and surrounded by a residual layer, If the rate of the chemical reaction is much higher than the diffusion rate of the ions through the residual layer, the rate determining step of the weathering reaction is msss transfer through this layer. One may then consider that at the boundary between fresh feldspar and residual layer, the concentrations of dissolved species are controlled by the equilibrium (stable or metastable) of the chemicsl reaction. To approach quantitstively the phenomenon of diEusion through the residual layer it is necessary to put down simplifying hypotheses, We assume that the solution is perfectly agitated and that the concentrations are uniform throughout. Let us consider a feldspar grain of radius R partially altered and surrounded by a residual layer of thickness 1. If 1 is much smaller than R, one may consider that contact ares IR of feldspar with the solution does not vary and that diffusion takes place only st right angles to this surface. Also in order to simplify the diffusion equations we will consider that at any time, a, quasi-stationary diffusion is realized. Let us analyse the diffusion of H,SiO, formed during the weathering of feldspar. At time t, the thickness of the residual layer is I,. The concentration of silicic acid on the surface of the non-altered feldspar is C,, while in aqueous solution, it is C at time t. The amount of silicic acid brought into solution during the intervsl dt is then given by the diffusion equation

(1): wherein n is the number of particles of feldspar considered. If C, represents the amount of silica present initially in each unit of volume of fresh feldspar (moles 12

642

R. WOLLAST

Si0,/cm8 of feldspar) and Q the amount of silica eliminated at time t, one gets q = c,nzt

(2)

Now, if V represents the total volume of solvent and if C = 0 at t = 0

Substituting (2) and (3) in (l), the result is dC -a-

naDQ2 c

_-

C, -

V’2

O

C (4)

1

or

cat (C, - C)

and integrating

= n2D F2 Co dt

(5)

(6) When C is much smaller than C,, the relation becomes, after expanding the series

Equations ,(0) and (7) only give the amount of silica brought into solution by diffusion from the feldspar. We must now subtract the amount of silica lost from the solution by the formation of an alumino-silicate, to find the actual concentration of siliua. If dissolved silica reacts with amorphous Al(OH,) on the exterior of feldspar grains, the chemical reaction is H&iO, eolution + A(

amorph

=

~t~4~&%~~)amomh

+

S/2

W

The rate of the reaotion might be expected to be proportional to H*SiO, concentration above the equilibrium level ((7,). This concentration represents the maximum solubility of the amorphous-silicate and the reaction takes place only when the concentration of H,SiO, is superior to this value. We have then

ac

dt= -MC - CJ

If C is much smaller than C, (as is borne out by the experimental values except at pH 4), equation (7) holds. After differentiation it becomes

TE= k at

t-112

3

Combining equations (8) and (9), one obtains the material volume of siliaainto solution :

ac

-

at

= k&l’2 -

k,(C -

C,)

This expression is not easily integrated to obtain an algebraic relation, so that graphical integration was employed to obtain the oaleulated curves of Figs. 6-9.

/

.

.

I

I

I

Hours

300 100 200 Fig. 6. Theoretical curve for the change in silica content of a solution at pH 4 in contaet with feldspar compared with the experimental points.

i-

Fig. 7. Theoretical curve for the change in silica content of a solution at pH 0 in contact with feldspar compared with the experimental points,

Fig. 8. Theoretical curve for the change in silica content of a solution at pH 8 in contact with feldspar compared with the experimental points.

Fig. 9. Theoretical curve for the change in silica contents of a solution at pH 10 in contact with feldspar compared with the experimental points.

646

Kinetics of the alteration of K-feldqmr

Their correspondence with observed relations is good. In fitting the experimental relations, it was found that the best value for C,,is about 6 mg/l. as SiO,. The silica concentration for equilibrium between crystalline kaolinite and crystalline gibbsite is l-2 mg/l. (POLZER,1962) so that we can consider our value as a reasonable value for equilibrium between the amorphous equivalents. It is possible to determine an approximate specific rate constant/cm2 of feldspar surface/cma of solution. If the rate is expressed in mg/l-hr the constants are PH

4 6 8 10

k2

k, 3.0 x 10-B 3.8 x 10-G 66 x 10-b 1.5 x 10-h

2.6 6.4 6.6 7.4

x x x x

lo-4

10-s

lCrs 10-S

These values suggest that at the beginning of the reaction the silica content of the solution is only determined by diffusion from the fresh feldspar especially at low pH. Further, the rate of dissolution decreases rapidly and the second term of the rate equation, i.e. the synthesis of alumino-silicate becomes predominant until the equilibrium concentration C, is reached. The order of magnitude of the diffusion coefficient of silicio acid, calculated from the value of k, is lo-l4 cm2/sec, decreasing from low to high pH. As shown before (equation (7)) the rate of diffusion of silica is proportional to the surface area nQ/V; the rate of the counter-reaction also would be expected to be proportional to the surface area, as it takes place on the exterior surface of the feldspar grain. These relations are in accord with the observation that the maximum silica content observed at a given pH is nearly independent of the weight of feldspar (surface area) used in a given volume of solution, since both the addition and removal reaction rates would be changed proportionally. We studied the influence of the amount of feldspar in a suspension in great detail, varying the weight from O-1 to 50%. Let us first consider the experimental results wherein the concentration of silica is negligible, a condition realized for dilute suspensionsand, in the case of concentrated suspensions in the beginning of the weathering reaction. In this case, relation (7) shows that the silica concentration in solution varies proportionately to the amount of feldspar in suspension represented by n/V. This relation is verified over a long period of n/V in Fig. 10 where the log of the concentration of silica liberated after 2 hr is represented as a function of the log of n/V. The experimental points fall well along a straight line with a 46’ slope. We also calculated the concentrations of silicic acid in solution after various times, in feldspar suspensions where the wt.% ranged from 1 to 50% of solid. The theoretical concentrations thus obtained, compared with the experimental concentrations are shown in Table 4. Here too, the agreement is satisfactory. It is interesting to compare the thickness of the residual layer formed around the feldspar grains in these various suspensions. From equations (2) and (3)

646

R.

WOIJAST

I’ 0’ ._ ul 2- $

IFig. 10. Log of conoentration of silica released after two hours as a function of surfme area (n/V).

1-

I

a

-1

II

Table 4. Comparison of the theoretical and experimentel oonoentrationof silica @g/l.) at pH 4 for varions suspensionsand various times Tim0

Shr

y. Solid in suspension 2 5 10 25 50

c CslC 1.8 4.5 8.7 20.8 3’7.2

48 hr

24 hr c ob8

c ca1c

3.15 6.9 13.0 20.1 38.0

3.1 7.8 14.4 33.1 66.2

c ohs

c C&k

c ohs

3.53 8.30 15.0 33.0 66.0

4.0 10.0 19.3 40.0 67.5

4.72 9.7 19.1 36.6 69.3

Table 6. Calculated thioknessof the residual layer formed after 48 hr in a solution at pH 4

o/oSolid in suspension 0.01 0.1 1 10 25 50

ct (mg/l.) 0.022 0.22 2.40 19.1 36.6 69.3

1, (am) l-5 x 10-s 1.5 x 10-e 1.6 x lo-* 1.45 x 10-e 1.16 x 10-e 0.95 x 10-B

647

Kinetioe of the alteration of K-feldspar

Table 6 indicates the thickness of the residual layer evaluated from the experimental results obtained by weathering of various suspensions of feldspar in a solution at pH 4, after 48 hr. The thickness of the layer formed is independent of the concentration of solid as long as the ratio C/C, is inferior to approximately 0.26. For higher values of the ratio, the growing rate of the residual layer is slowed owing to the influence of the substances in solution and the increase of silica into solution is limited by the counter reaction to form silicate. DIscuss10x

The weathering of feldspar under natural conditions can be described by a diffusion mechanism of H,SiO, through a residual layer, constituted by slightly

4 .I

z

10

ECjU. 5

timr 20

LO

60

00

Fig. 11. Theoretid ourve for the change in silioe oontent of a solution in oontaet with feldspar initially saturated with Al(OH),.

soluble Al(OH), and subsequent reaction of these two substances to form a hydrated alumino-silicate. The mechanism proposed shows that there is no contradiction between the results of Correns and Wyart. If the concentration of the substances in the solution bathing the feldspar is weak, the reaction rate is limited by the thickness of the residual layer as postulated by Correns. The reaction rate is only slowed by the presence of dissolved substances, as in the results of Wyart, when their concentration approaches the saturation concentration or when a counter-reaction can occur. It is perhaps worthwhile to speculate briefly on the events that should occur after

648

R. WOLLAST

long time intervals. Figure 11 shows a theoretical curve for the change in silica content of s, very concentrated suspension of feldspar (76% by weight). This curve would correspond to the alteration of feldspar into kaohnite, with amorphous Al(OH), as intermediate product. Continuous kaolinization of the feldspar should take place, but perhaps at it slowly increasing rate as the thickness of the diffusion layer diminishes by the kaolinization reaction and the silica concentration again increases because of the excess of silica in the feldspar over that required to make kaolinite. However: further studies are required to determine, for instance, whether the initial alumino-silicate will be converted to a new species higher in silica (montmorillonite?) ss H,SiO, in solution increases. The theoretical and experimental results emphasize the importance of an open system in the bauxitization of feldspar. Only so long as the concentration of silica in the external solution is maintained below about 5 mg/l. can the counter-reaction to form kaolinite be avoided. The natural conditions suggested are good drainage and rains of high intensity and frequency, but of short duration, to avoid waterlogging. Also, the role of pH is somewhat minimized, because of the demonstrated independence of reaction rates. At pH values less than 5, where the solubility of Al(OH), is high, no protective sheath is formed, because silica and alumina dissolve at very nearly the same rate in solutions undersaturated with both. Under such conditions, the feldspar dissolves, and formation of kaolinite would be through the uninvestigated reactions in homogeneous media : H,O + 2A13++ 2H,SiO, = H,A&Si,O, + (6H+) or 2H+ + 2AlO,- + 2H,SiO, = H,AI,Si,O, + 3H,O However, the present study shows that these reactions must be slow relative to the reaction between H,SiO, in solution and “amorphous” Al(OH), on grain surfaces. Acknowlemnt-The author wishes to thank the CEMUBAC, which sponsoredthis study, and Prof. W. L. DE KEYSER who directed his activities. The author expresses his sincere thanks to Prof. R. M. GARREL~ for his interest in this study, and for his suggestion concerning the formation of an alumina-silicate at the feldspar surfaoe. REFERENCES C. W. and VON ENQELFURDT W. (1938) Neue untersuohungeniiber die Verwitterung des Kalifeldspates. Chem. d. Erde 12, 1. CORRENSC. W. and VON EN~ELEARDTW. (1940) Die ohemisohe Verwitterung der Silicate. Natu97&38en.28, 369. LAUAOHEM., WYART J. and SABATIJ~R G. (1961s) Dissolution des feldspaths &&ins dans l’eau pure ou ahargee de CO, a 200°C. C.R. Ad. Sci. P&S %a, 2019. LAOACJXE M., WYART J. and SABATIERG. (1961b) Meoanisme de la dissolution des feldspaths alc&ns dens I’eau pure ou chargee de CO, s 200°C. C.R. Ad. Sci. Parie 858,2296. POLZERW. (1962) Personal communication. SHAPIROL. and BRANNOCI( W. W. (1956) Rapid anslysis of silicate rooks. U.S. Geol. Surwez, Bull. 1036 C. WOLLAST R. (1961) Aspect ohimique du mode de formation des beuxites dens le Bas-Congo. Bull. Ad. Roy. Sci. d’Outre.Mer (Belgiqw) 7, 468. Wor,r,~r R. (1963)A,spe&chimique du mode de formation des bauxites dens le Baa-Congo, II. B&L Ad. Roy. Sci. d’outre-Mer (Belgiqw) 9, 392. CORRENS