Re. Blake et Lm. Walter, EFFECTS OF ORGANIC-ACIDS ON THE DISSOLUTION OF ORTHOCLASE AT 80-DEGREES-C AND PH 6, Chemical geology, 132(1-4), 1996, pp. 91-102
The dissolution of K-rich feldspar (orthoclase), quartz and Al(OH)(3)
was investigated at 80 degrees C and pH 6 in buffered solutions of org
anic acids. Previous studies of the effects of organic acids (OA) on f
eldspar dissolution have typically been conducted in acidic, low-ionic
strength solutions often under conditions which preclude isolation of
effects of pH from those due to organic acids. Our experiments were c
onducted at constant pH, temperature, ionic strength, and buffer compo
sition to allow direct comparison of experiments with and without OA.
The dissolution experiments were conducted under closed-system conditi
ons to: (1) determine the magnitude of mineral solubility enhancement
by OA; (2) examine changes in reaction stoichiometry as equilibrium is
approached; and (3) investigate the effects of OA on secondary minera
l precipitation. The carboxylic acid species, oxalate and citrate, sig
nificantly enhanced the dissolution of orthoclase at pH 6. The concent
rations of Si and Al in 10 mM oxalate and citrate solutions were nearl
y 3 times that in solutions of acetate buffer without oxalate or citra
te. Aluminum was below the limit of detection (< 0.007 mM) in the acet
ate buffer alone. Citrate increased the release of Si and Al from orth
oclase more than did oxalate at the same concentration. Equilibrium mo
deling indicates that solutions with oxalate and citrate attained supe
rsaturation with respect to gibbsite, kaolinite, and smectite, and sat
uration with respect to quartz. Nevertheless, orthoclase dissolution r
emained congruent with respect to Si and Al release. Separate experime
nts using pure quartz and Al(OH)(3) suggest that a mechanism other tha
n formation of Si-organic complexes may be involved in the OA-enhanced
release of Si from orthoclase. The increase in dissolved silica was m
odest in 10 mM oxalate and 10 mM citrate solutions reacted with quartz
, and did not increase with increasing OA concentration. The solubilit
y of quartz was similar in solutions of oxalate and citrate. In contra
st, Al(OH)(3) dissolution was 50% greater in citrate than in oxalate,
similar to the behavior of orthoclase. Citrate may be more effective i
n dissolving orthoclase than oxalate due to a stronger interaction bet
ween citrate and Al, rather than due to a synergistic effect of Al-cit
rate and Si-citrate complexes. These results have implications for the
mass transport of Si and Al in diagenetic environments.