Factors Controlling the Gallium Preference in High-Al Chromitites

Abstract

Gallium (Ga) belongs to the group of critical metals and is of noticeable research interest. Although Ga3+ is highly compatible in high-Al spinels a convincing explanation of the positive Ga3+–Al3+ correlation has not yet been proposed. In the present study, spinel-chemistry and geochemical data of high-Al and high-Cr chromitites from Greece, Bulgaria and the Kempirsai Massif (Urals) reveals a strong negative correlation (R ranges from −0.95 to −0.98) between Cr/(Cr + Al) ratio and Ga in large chromite deposits, suggesting that Ga hasn’t been affected by re-equilibration processes. In contrast, chromite occurrences of Pindos and Rhodope massifs show depletion in Ga and Al and elevated Mn, Co, Zn and Fe contents, resulting in changes (sub-solidus reactions), during the evolution of ophiolites. Application of literature experimental data shows an abrupt increase of the inversion parameter (x) of spinels at high temperature, in which the highest values correspond to low-Cr3+ samples. Therefore, key factors controlling the preference of Ga3+ in high-Al chromitites may be the composition of the parent magma, temperature, redox conditions, the disorder degree of spinels and the ability of Al3+ to occupy both octahedral and tetrahedral sites. In contrast, the competing Cr3+ can occupy only octahedral sites (due to its electronic configuration) and the Ga3+ shows a strong preference on tetrahedral sites.

Mineralogy of Ga- and Ge-bearing metallurgical slags from Tsumeb, Namibia

“…A recent study compared the Ga binding preferences in spinels from high-Al and high-Cr chromitites and found a similar phenomenon (Eliopoulos and Eliopoulos, 2019). Gallium concentrations were substantially higher in Al-rich spinels than in Cr-bearing spinels, because at magmatic conditions, Ga 3+ shows a strong preference on tetrahedral sites where it can substitute for Al 3+ , which can occupy both octahedral and tetrahedral sites in contrast to Cr 3+ , which only occupies octahedral sites in the spinel structure (Eliopoulos and Eliopoulos, 2019). There is quite limited information about Ga content in phases produced from pyrometallurgy in the available literature.…”

“…3). A recent study compared the Ga binding preferences in spinels from high-Al and high-Cr chromitites and found a similar phenomenon (Eliopoulos and Eliopoulos, 2019). Gallium concentrations were substantially higher in Al-rich spinels than in Cr-bearing spinels, because at magmatic conditions, Ga 3+…”

“…shows a strong preference on tetrahedral sites where it can substitute for Al 3+ , which can occupy both octahedral and tetrahedral sites in contrast to Cr 3+ , which only occupies octahedral sites in the spinel structure (Eliopoulos and Eliopoulos, 2019).…”

Trace Element Distribution in Magnetite Separates of Varying Origin: Genetic and Exploration Significance

“…The average Ga content for karst bauxite deposits is 58 ppm with a range from less than 10 to 812 ppm, because the close geochemical affinity of Ga to Al enables Ga to substitute easily in rock-forming alumino-silicates [77]. Such a preference Ga with high-Al chromitites, as well and the structural incorporation of Ga into chromite lattice, is evidenced by the progressive and linear increase of Al or decreasing Cr/(Cr + Al) atomic ratio [78,79].…”