Raman and infrared spectroscopic study of the mineral goyazite SrAl3(PO4)2(OH)5·H2O.

We have studied the mineral goyazite using Raman and infrared spectroscopy. Goyazite is a member of the crandallite subgroup of the alunite supergroup. The crystal structure is of the alunite-type and consists of sheets of corner-sharing AlO6 octahedra parallel to (0001). The octahedrally coordinated Sr(2+) cations occupy cavities between pairs of octahedral sheets and are surrounded by six oxygen atoms from the Al(3+)O6 octahedra. The very intense sharp band at 983 cm(-1) is assigned to the ν1PO4(3-) symmetric stretching mode. The observation of a single band supports the concept that all the phosphate units are equivalent in the structure of goyazite. Raman bands observed at 1029 cm(-1) and 1037 cm(-1) are assigned to the ν3PO4(3-) antisymmetric stretching vibrations. Two Raman bands at 895 and 927 cm(-1) are attributed to the stretching vibrations of H2PO4; thus indicating some hydrogen phosphate units in the structure of goyazite. Raman bands at 556, 581, 596 and 612 cm(-1) are assigned to the ν4PO4(3-) bending modes, suggesting a reduction of symmetry of phosphate units. Two sharp Raman bands at 3609 and 3631 cm(-1) are attributed to OH stretching vibrations from the goyazite hydroxyl units. Broad Raman bands at 2924, 3043, 3210, 3429 and 3511 cm(-1) are assigned to water stretching vibrations. This research shows that from a vibrational spectroscopic point of view, the formula SrAl3(H PO4,PO4)2(OH)6 · H2O is a better formulation for the mineral goyazite. Vibrational spectroscopy enables subtle details of the molecular structure of goyazite to be determined.