The ϵ-Al 13 Keggin aluminum hydroxide clusters are essential models in establishing molecular pathways for geochemical reactions. Enthalpies of formation are reported for two salts of aluminum centered ϵ-Keggin clusters, Al 13 The formation of both ε-Al 13 Keggin cluster compounds is exothermic from oxide-based components but energetically unfavorable with respect to a gibbsite-based assemblage. To understand the relative affinity of the ϵ-Keggin clusters for selenate and sulfate, the enthalpy associated with two S-Se exchange reactions was calculated. In the solid state, selenium is favored in the Al 13 compound relative to the binary chalcogenate, while in 5 N HCl, sulfur is energetically favored in the cluster compound compared to the aqueous solution. This contribution represents the first thermodynamic study of ε-Al 13 cluster compounds and establishes a method for other such molecules, including the substituted versions that have been created for kinetic studies. Underscoring the importance of ε-Al 13 clusters in natural and anthropogenic systems, these data provide conclusive thermodynamic evidence that the Al 13 Keggin cluster is a crucial intermediate species in the formation pathway from aqueous aluminum monomers to aluminum hydroxide precipitates.aluminum hydrolysis | polyoxocations F ew subjects are more central to geochemistry and materials science than aluminum hydrolysis chemistry. From a geochemist's perspective, aluminum hydrolysis products are key to mineral paragenesis and watershed chemistry. In addition, nanometer-size aluminum hydroxide molecules, including Al 13 ϵ-Keggin clusters, are critical to both experiment and simulation to detail possible reaction pathways at the solid-water interface (1). Aluminum clusters form complexes with a variety of reactive components including clay particles, natural organic matter, viruses, bacteria, and spores and thus are of interest to the environmental community for applications in water treatment and contaminant remediation technologies (2). Such clusters have also been used in a wide variety of industrial and consumer applications (3) and, because of their unique size and reactivity, they are effective as clay pillars (4) and catalysts (5).At near-neutral and higher pH, the dominant multimeric aluminum complex in aqueous systems is the large polycation, ½Al 12 ðAlO 4 ÞðOHÞ 24 ðH 2 OÞ 12 7þ , which forms quickly via Al 3þ hydrolysis and is a precursor to solid flocs (6). It consists of a central tetrahedrally coordinated AlO 4 moiety, enveloped by a sheath of 12 edge-shared AlO 6 units in four sets of three linked trimers. The most familiar form of this molecule, referred to as ε-Al 13 , has a structure identical to the ϵ-isomer of the BakerFiggis-Keggin structures (7-9). This isomer has edge-shared trimeric capping groups organized around a central tetrahedral AlO 4 site that is inert. Under favorable conditions, other isomers and larger clusters can be synthesized by hydrothermal reaction or by slow aging at room temperature (10).Because high-purit...