The aqueous geochemistry of gallium, germanium, indium and scandium

Abstract

Relatively little information is available in the literature regarding the speciation and solubility of Ga, Ge, In and Sc in aqueous solutions, especially at elevated temperatures and pressures. In this paper we critically review stability constants for relevant aqueous complexes of these metals and solubility products for relevant solid phases. Most of the available data refer to standard conditions of temperature and pressure (25 °C and 1 bar), although for Ga and Ge, experimentally derived data are available for some geologically relevant species and phases up to 200–300 °C. The stable oxidation states of the four metals in aqueous solution are Ga(III), Ge(IV), In(III) and Sc(III). The ions of each of these metals are relatively hard in the Pearson [Pearson, R.G. 1963. Hard and soft acids and bases. Journal of the American Chemical Society 85, 3533–3539] sense, forming the strongest complexes with hard ligands such as hydroxide, fluoride, sulfate and phosphate, and weaker complexes with soft ligands such as chloride and bisulfide. The main exception to this rule is In(III), which forms reasonably stable chloride and bisulfide complexes. The hydrated Ga 3+, In 3+ and Sc 3+ ions are all octahedrally coordinated by water molecules, but there is some evidence, e.g., for GaCl 4 − and InCl 4 −, that a conversion to tetrahedral coordination may occur upon replacement of water by a sufficient number of other ligands. In most hydrothermal solutions, we predict that hydroxide complexes will be the most important forms of transport of Ga and Sc, although fluoride complexes will be important in environments where fluoride activities are relatively high (e.g., during greisen formation). In analogy with Si, the most important species for Ge is germanic acid, H 4GeO 4 0, but again fluoride complexes may be important at high fluoride activities. Chloride complexes are not expected to play a significant role in the transport of Ga, Ge or Sc at temperatures below approximately 300 °C. The behavior of In is expected to be the most variable, with, depending on conditions, hydroxide, chloride, fluoride or bisulfide complexes all contributing to its transport. Sulfate and phosphate complexes of Ga, In and Sc may play limited roles in the hydrothermal mass transfer of these elements, but only under special conditions; normally these complexes will be less important than hydroxide or fluoride complexes. Solubility calculations for 25 °C indicate that In-sulfide and Sc-phosphate are less soluble (i.e., more stable) than the corresponding oxyhydroxides, even when In-bisulfide and Sc–phosphate complexes are taken into account. However, the phases GaPO 4(s) and InPO 4(s) are generally more soluble than the corresponding oxyhydroxides. Solubilities of α-GaOOH(s) and GeO 2(tetragonal) have been estimated up to 300 °C and both increase with increasing temperature. The solubility of pure α-GaOOH(s) at 25 °C (< 10 − 6 m) is quite low from pH 3 to pH 8, consistent with the general immobility of Ga in the weathering environment and its concentration in bauxites.