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

Abstract

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 Sr2+ cations occupy cavities between pairs of octahedral sheets and are surrounded by six oxygen atoms from the Al3+O6 octahedra.The very intense sharp band at 983cm−1 is assigned to the ν1PO43- 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 1029cm−1 and 1037cm−1 are assigned to the ν3PO43- antisymmetric stretching vibrations. Two Raman bands at 895 and 927cm−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 612cm−1 are assigned to the ν4PO43- bending modes, suggesting a reduction of symmetry of phosphate units. Two sharp Raman bands at 3609 and 3631cm−1 are attributed to OH stretching vibrations from the goyazite hydroxyl units. Broad Raman bands at 2924, 3043, 3210, 3429 and 3511cm−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.