Carboxylate ions and nordstrandite growth: AFM surface observations

The aluminum hydroxide minerals are important in the production of aluminium, the regulation of aluminum in natural waters and the adsorption of ions. The polymorphs of Al(OH)3 (gibbsite, nordstrandite, doyleite) differ in terms of the stacking of their unit layers. Nordstrandite occurs in many soils and has industrial uses. The presence of carboxylate ions in solution during crystal growth causes nordstrandite crystals to be less well-formed in overall appearance, compared to when grown without the presence of the carboxylate ions. In order to gain more understanding of the effects of carboxylate ions on Al(OH)3 growth, atomic force microscopy (AFM) was used to observe the nanometer scale surface morphology of nordstrandite crystals synthesized from Al metal in ethylenediamine solutions in the presence of malonate, aspartate, oxalate and citrate ions. The AFM images showed that crystal faces were smoother overall for the nordstrandite crystals synthesized without carboxylate ions. For crystals synthesized in solutions with the largest concentrations of carboxylate ions, the nanometer scale images showed that the surfaces were very irregular and were characterized by elongated islands. Our AFM observations were the same for nordstrandite samples grown with large differences in the amount of stirring of the crystallizing solutions, indicating that the effect of bulk diffusion alone could not have been responsible for the effect of the carboxylate ions on crystal growth. Moreover, only a weak correlation existed between the concentration of carboxylate ions in the crystallizing solution and the amount of nordstrandite produced. On the {110} basal faces of the nordstrandite crystals, which are oriented parallel to the stacking of the unit layers, steps having a height of one nordstrandite unit layer were observed. 2dimensional growth was primarily observed on these faces. For the nordstrandite crystals synthesized without carboxylate ions, the steps were oriented along crystallographic directions, and extended all or most of the way across the crystal faces. Many of these steps were parallel to each other. The nucleation of new terraces occurred primarily at the edges of the crystal faces, and few if any islands nucleated in the centres of the faces. The crystals synthesized with the highest concentrations of carboxylate ions had steps on the basal surfaces that were more rounded and conformed less to the crystallographic directions. The carboxylate ions also promoted the nucleation of small monolayer islands in the interiors of the crystal faces. Melt-rock reaction in Canadian Cordillera mantle xenoliths A.H. PESLIER, A.D.BRANDON, D.FRANCIS AND J. LUDDEN Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA (apeslier@mail.uh.edu) NASA-JSC, Mail Code SR, Bdg. 31, Room 114, Houston, TX 77058, USA (alan.d.brandon1@jsc.nasa.gov) Dpt. of Earth and Planetary Sciences, McGill University, Montreal, QC H3A 2A7, Canada (don_f@eps.mcgill.ca) Centre de Recherches Petrographiques et Geochimiques, B.P.20, 54501 Vandoeuvre-les-Nancy, France (ludden@crpg.cnrs-nancy.fr)