Identification orAl hydroxides by laser

Gibbsite and bayerite may be distinguished by Raman spectroscopy (Huneke et al., 1980), with the laser Raman microprobe providing a non-destructive method of analysing small quantities particularly where the habit and/or fine grain size defy other techniques (e.g. Crestin-Desjobert et al., 1987). The Raman spectra of other polymorphs of aluminium hydroxide have not been recorded. This study determines those of nordstrandite and doyleite, and assesses some of the potentials and limitations of the method as a routine determinative procedure for such minerals. Gibbsite, bayerite and nordstrandite are well-defined, natural crystalline forms of aluminium hydroxide. A triclinic modification of gibbsite was recognized by Saalfield (1960) among single crystals from the Urals. Chao et al. (1985) identified a further polymorph, doyleite, from two localities in Quebec, although reservations as to its status have been expressed by Wefers & Misra (1987). Published infrared (IR) spectral data for these species vary (e.g. Fredrickson, 1954; Vivien et al., 1973; Chao et al., 1987; Hsu, 1989). Twenty two samples from the collections of the Natural History Museum, London, of the Australian Museum, and of private individuals, were examined. A brief description of those found to contain at least one polymorph is given in the Appendix. The sole example available of "gibbsite" from the Urals proved to be largely hydrotalcite and no triclinic gibbsite was identified. S A M P L E P R E P A R A T I O N The preparation of mineral samples for Raman microprobe analysis depends on the purpose of the individual analysis and the amount and nature of the sample available. As with X-ray powder diffraction, a single method of preparation and presentation of sample to the radiation and detectors is desirable for comparisons of results within a set (cf. Moore & 9 1993 The Mineralogical Society