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 baye rite, were generated during the synthesis, but gibbsite was dominant in mos t samples and commonly the only phase present. At < 10 degreesC, hydrogen-i sotope equilibrium between the synthetic Al-hydroxides and water was not ac hieved 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 be tween 9 and 51 degreesC after 10 years of synthesis. Based on these results plus data for naturally occurring gibbsite from bauxite deposits, an (alph a (H)(gibbsite-water) value of 0.995 +/- 0.003 was obtained for surficial t emperatures. Using this value and oxygen-isotope results, the following "gi bbsite line" is proposed for its formation from meteoric water at 20 degree sC: deltaD = 7.84 delta O-18 - 114.2. Most naturally occurring gibbsite sam ples 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 thei r isotopic composition from precursor kaolinite. Copyright (C) 2001 Elsevie r Science Ltd.