Relative Solubility of Corundum, Gibbsite, Boehmite, and Diaspore at Standard State Conditions1

F. J. Peryea2 and J. A. Kittrick
Department of Agronomy and Soils, Washington State University Pullman, Washington 99164
1 Scientific Paper 7888, Project 0747, College of Agriculture and Home Economics Research Center, Washington State University, Pullman, Washington 99164.
2 Present address: Washington State University, Tree Fruit Research Center, 1100 North Western Avenue, Wenatchee, Washington 98801.

Abstract: The solubilities of HCl-treated samples of corundum, gibbsite, boehmite, and diaspore in aqueous solutions at 298 K and one atmosphere pressure were determined from undersaturated and supersaturated initial conditions. Solution characteristics at apparent equilibrium were measured and used to calculate equilibrium constants (Kr) for mineral dissolution reactions: Al 2 O 3  (corundum) + 3 H + = Al 3+  + 1 .5H 2 O, Al(OH) 3  (gibbsite) + 3 H + = Al 3+ + 3 H 2 O, and AlOOH (boehmite or diaspore) + 3 H + =  Al 3+ + 2 H 2 O. Assuming solid phase and water activities of unity, log Kr = 3pH - pAl3+ for all three equations. The calculated log Kr values were: 9.01 ± 0.05 (corundum), 7.76 ± 0.14 (gibbsite), 7.49 ± 0.09 (boehmite), and 6.75 ± 0.24 (diaspore), indicating that the relative thermodynamic stabilities under the experimental conditions were: corundum < gibbsite < boehmite < diaspore. The gibbsite value agreed well with that determined independently by another research group using acid-treated subsamples of the same source mineral (7.70 ± 0.02). The calculated Gibbs free energies of formation (kJ/mole) were: -1587.4 ± 2.1 (corundum), -1156.7 ± 1.6 (gibbsite), -921.0 ± 1.5 (boehmite), and -925.1 ± 2.0 (diaspore). The Δ G f o values of the acid-treated minerals were 1.8 to 12.4 kJ/mole more negative than values recently compiled for untreated mineral samples. This result is attributed to the removal of reactive surface coatings that isolated the bulk mineral phases from the solution phase.

Key Words: Acid solubility • Aluminum • Boehmite • Corundum • Diaspore • Gibbsite

Clays and Clay Minerals; October 1988 v. 36; no. 5; p. 391-396; DOI: 10.1346/CCMN.1988.0360502
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)