Hydrogen-isotope fractionation in aluminum hydroxides: Synthesis products versus natural samples from bauxites

Hydrogen-isotope data have been gathered for synthetic aluminum hydroxides precipitated over 3 to 121 months at temperatures varying between 8 and 51°C. All three Al(OH)₃ polymorphs, gibbsite, nordstrandite, and bayerite, were generated during the synthesis, but gibbsite was dominant in most samples and commonly the only phase present. At <10°C, hydrogen-isotope equilibrium between the synthetic Al-hydroxides and water was not achieved until more than 2 years had elapsed. Using pure gibbsite samples, an average α^H"gibbsite-water" value of 0.998 ± 0.006 was obtained between 9 and 51 °C after 10 years of synthesis. Based on these results plus data for naturally occurring gibbsite from bauxite deposits, an α^H"gibbsite-water" value of 0.995 ± 0.003 was obtained for surficial temperatures. Using this value and oxygen-isotope results, the following "gibbsite line" is proposed for its formation from meteoric water at 20°C: δD = 7.84 δ¹⁸O - 114.2. Most naturally occurring gibbsite samples plot about this line. which indicates their direct precipitation from solution. However, a few samples of gibbsite, especially those from Hawaii, plot to the right of the "gibbsite line" and likely inherited part of their isotopic composition from precursor kaolinite.