Raman and infrared microscopy study of zunyite, a natural Al13 silicate

Zunyites, AlSiO(OH,F)Cl, have been studied by Raman and infrared microscopy. The infrared spectrum (700-4000 cm) is characterised by a band near 720 cm and a quintet of bands at 890, 992, 1062, 1130 and 1176 cm. The first two bands are assigned to Al-OH deformation modes with the 992 cm mode being infrared inactive and the latter three bands as Si-O symmetric stretching modes. The IR hydroxyl-stretching region shows three Al-OH stretching modes at 3313 cm with a shoulder at ± 3200 cm and a low intensity band at ± 3623 cm. The Raman hydroxyl-stretching region reveals seven bands corresponding with Al-OH modes at ± 3634, 3431, 3374, 3354, 3337, 3320 and 3306 cm. The corresponding Al-OH deformation modes are observed at 930, 950 and 994 cm. In the Raman spectral region of 1000-1300 cm, Si-O-Si symmetric stretching modes are observed at 1067, 1101 and 1126 cm, from which the middle band is infrared inactive and therefore assigned to the 180° intertetrahedral Si-O-Si bond. The remaining bands at 1141, 1176, 1207, 1239 and 1295 cm are assigned to (i) Al-O symmetric stretch modes (3x) from the Al octahedra and (ii) the AlO tetrahedron in the Al unit and (iii) Al substituting Si in the SiO unit. The low frequency Raman region is characterised by the symmetric bending modes of Si-O-Si at 202 cm and Si-O-Al at 213 cm in the SiO unit. The third symmetric stretching mode at 271 cm is assigned to Al-O-Al in the Al unit. The five bands between 275 and 425 cm are assigned to Si-O bending modes and the bands at 444 and 467 cm to Al-O bending modes. The broad bands at 612 and 701 cm are assigned to the Si-O-Al deformation modes, the latter of which is infrared inactive