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Surface Enthalpy of Boehmite

Published online by Cambridge University Press:  28 February 2024

Juraj Majzlan ,
Alexandra Navrotsky  and
William H. Casey
Juraj Majzlan*
Affiliation:
Thermochemistry Facility, Department of Geology, University of California at Davis, Davis, California 95616, USA
Alexandra Navrotsky
Affiliation:
Thermochemistry Facility, Department of Geology, University of California at Davis, Davis, California 95616, USA
William H. Casey
Affiliation:
Land, Water, and Air Resources, University of California at Davis, Davis, California 95616, USA
*
E-mail of corresponding author: jmajzlan@ucdavis.edu
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Abstract

The persistence of many seemingly metastable mineral assemblages in sediments and soils is commonly attributed to their sluggish transformation to the stable-phase assemblage. Although undoubtedly kinetics plays a major role, this study shows that thermodynamic factors, particularly surface energy, significantly influence the free energy. Enthalpies of formation of boehmite samples with variable surface area were derived using high-temperature oxide-melt calorimetry. The average surface enthalpy for all faces terminating boehmite particles was calculated at +0.52 ± 0.12 J/m2. This value represents the surface enthalpy for surfaces exposed to vacuum assuming that H2O adsorbed on the surface of boehmite is loosely bound. These results show that the enthalpy of formation of boehmite may vary by ≤8 kJ/mol as a function of particle size. An overview of published values of surface energies of gibbsite, γ-Al2O3, corundum, and the results here indicates that the hydrated phases (boehmite, gibbsite) have lower surface energies than the anhydrous phases (corundum, γ-Al2O3). Lower surface energies allow the hydrated phases to maintain high surface area, i.e., small particle size. Similar surface energies of boehmite and gibbsite suggest kinetic control favoring the crystallization of boehmite or gibbsite from aqueous solution. The enthalpy of formation of bulk boehmite from the elements was calculated at −994.0 ±1.1 kJ/mol. Combining this result with the data in existing thermodynamic databases, we confirm that bulk boehmite is metastable with respect to bulk diaspore at ambient conditions.

Keywords

BoehmiteEnthalpy of FormationSurface Enthalpy
Type
Research Article
Information
Clays and Clay Minerals , Volume 48 , Issue 6 , December 2000 , pp. 699 - 707
Copyright
Copyright © 2000, The Clay Minerals Society

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