Oxygen-isotope data were obtained for synthetic aluminum-hydroxide phases precipitated over 65-125 mo and have been compared to results from similar experiments conducted for 3-56 mo. The AI(OH)3 polymorphs, gibbsite, nordstrandite, and bayerite, were synthesized, but gibbsite was dominant in most samples, and commonly the only phase present. Using pure gibbsite samples, the following oxygen-isotope fractionation factors, c%jbbsite_n2o, were obtained: 1.0167 • 0.0003 (9 -+ 1~ 1.0147 • 0.0007

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... -+ 2~ 1.0120 + 0.0003 (51 • 2~ These values, and the associated equation for an oxygenisotope geothermometer for the interval 0-60~ 1031n 0~gibbsi,e.H20 = 2.04 • 106/T 2 --3.61 • 10317 + 3.65 (T in K), are not significantly different from those obtained from experiments of much shorter duration. These results, and the good agreement with agibbsite_n2 o values obtained for well-constrained natural systems, suggest that the experimentally determined fractionation factors describe equilibrium conditions for gibbsite that has precipitated directly from solution. As also proposed by others using a modified-increment calculation, our synthesis experiments suggest that e~A~OH), _H2 o is polymorph-dependent at low temperatures and that a significant temperature-dependent trend exists in the values of eLAI~Om3_H2 o. However, previously calculated fractionation factors obtained using the modified-increment method are higher than those obtained from the experiments, with this discrepancy becoming larger as temperature decreases.