Vibrational spectra of synthetic crandallite-type minerals - optical and inelastic neutron scattering spectra

The vibrational spectra of the synthetic minerals crandallite and goyazite MAl3(OH)6(HPO4)(PO4) (M= Caand Sr) were studied by means of vibrational (IR and Raman) and inelastic neutron scattering (INS)spectrometries at room and low temperatures. The results were discussed taking into consideration theeffects of different types of disorder (random distribution of PO4 and HPO4, orientational distributionwith HPO4 and displacement of Ca in crandallite). The consequences of the resulting perturbed symmetryand of the non-equivalence of the bridging mu-OH groups in the Al2OH fragments were pointed out(complexity of the nu(Al2O-H), delta(Al2OH) and gamma(Al2OH) bands). The vibrations of the PO4 and HPO4 groupscould be separated. Some aspects of the vibrational behaviour of the AlO2(OH)4 units and of the layerscomposed therewith were addressed. The translational modes of the interlayer cations Ca2+ and Sr2+ couldbe observed; the findings support the suggested displacement of Ca in crandallite.