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 degreesC. 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 degreesC, 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 alpha (H)(gibbsite-water) value of 0.998 +/- 0.006 was obtained between 9 and 51 degreesC after 10 years of synthesis. Based on these results plus data for naturally occurring gibbsite from bauxite deposits, an (alpha (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 degreesC: deltaD = 7.84 delta O-18 - 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. Copyright (C) 2001 Elsevier Science Ltd.