INTRACRYSTALLINE FRACTIONATION OF OXYGEN ISOTOPES BETWEEN HYDROXYL AND NON-HYDROXYL SITES IN KAOLINITE MEASURED BY THERMAL DEHYDROXYLATION AND PARTIAL FLUORINATION

Thermal dehydroxylation and partial fluorination techniques were used to measure intracrystalline fractionation of oxygen isotopes between h ydroxyl and non-hydroxyl sites in kaolinite. Several aliquots of a wel l characterized, fine-grained (<1 mu m) kaolinite from Macon, GA, were dehydroxylated in vacuo using a variety of heating procedures, heatin g rates, and target temperatures. Measured delta(18)O values of both t he liberated water and the dehydroxylated residue are consistent over a wide range of temperatures (550-850 degrees C) when dehydroxylation is performed in a single-step fashion at a rapid heating rate (>50 deg rees C/min.). Similar dehydroxylation experiments indicate that brucit e dehydroxylation occurs without any significant isotopic fractionatio n of the oxygen isotopes. By extrapolation we postulate that no signif icant fractionation occurs during single-step thermal dehydroxylation of fine-grained kaolinite, provided that dehydroxylation is performed under well controlled conditions. In contrast, gibbsite dehydroxylatio n is accompanied by substantial isotopic fractionation. This is probab ly the result of the complex, multi-pathway dehydroxylation reaction o f this mineral. Similarly, thermal dehydroxylation of coarse-grained ( >1 mu m) kaolinites and dickites of weathering and hydrothermal origin yield results that are dependent on the temperature of dehydroxylatio n. We suggest that this effect may be caused by isotopic exchange duri ng diffusion of water molecules through coarse particles. Partial fluo rination of fine-grained kaolinite in the presence of excess F-2 at lo w temperatures (<185 degrees C) yields unreproducible delta(18)O value s. When performed at high temperatures (220-240 degrees C) in the pres ence of insufficient F-2, delta(18)O values are systematically lower t han the bulk delta(18)O value and increase linearly with the percent s toichiometric yield. They are consistent with a greater rate of reacti on of hydroxyl oxygen than of non-hydroxyl oxygen, but examination of the isotopic data as well as XRD and IR analyses of the residues after partial fluorination indicates that the separation between the two ty pes of oxygen is not complete. The results, therefore, do not yield a reliable delta(18)O value of the hydroxyl oxygen. The results of this study suggest that the thermal dehydroxylation technique may be approp riate for analysis of OH groups in fine-grained kaolinite. The partial fluorination approach appears less suitable. Intracrystalline fractio nation of oxygen isotopes between hydroxyl and non-hydroxyl sites in k aolinite may be larger than previously reported in experimental studie s. Based on the results obtained for the finer-than 1 mu m Macon kaoli nite a value of 1.0272 is proposed for the intracrystalline fractionat ion factor, alpha(non-OH/OH), Of kaolinite at 20 +/- 10 degrees C.