Raman and infrared microscopy study of zunyite, a natural Al 13 silicate

Abstract

Zunyites, Al 13Si 5O 20(OH,F) 18Cl, have been studied by Raman and infrared microscopy. The infrared spectrum (700–4000 cm −1) is characterised by a band near 720 cm −1 and a quintet of bands at 890, 992, 1062, 1130 and 1176 cm −1. The first two bands are assigned to Al–OH deformation modes with the 992 cm −1 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 −1 with a shoulder at ±3200 cm −1 and a low intensity band at ±3623 cm −1. The Raman hydroxyl-stretching region reveals seven bands corresponding with Al–OH modes at ±3634, 3431, 3374, 3354, 3337, 3320 and 3306 cm −1. The corresponding Al–OH deformation modes are observed at 930, 950 and 994 cm −1. In the Raman spectral region of 1000–1300 cm −1, Si–O–Si symmetric stretching modes are observed at 1067, 1101 and 1126 cm −1, 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 −1 are assigned to (i) Al–O symmetric stretch modes (3x) from the Al octahedra and (ii) the AlO 4 tetrahedron in the Al 13 unit and (iii) Al 3+ substituting Si 4+ in the Si 5O 16 unit. The low frequency Raman region is characterised by the symmetric bending modes of Si–O–Si at 202 cm −1 and Si–O–Al at 213 cm −1 in the Si 5O 16 unit. The third symmetric stretching mode at 271 cm −1 is assigned to Al–O–Al in the Al 13 unit. The five bands between 275 and 425 cm −1 are assigned to Si–O bending modes and the bands at 444 and 467 cm −1 to Al–O bending modes. The broad bands at 612 and 701 cm −1 are assigned to the Si–O–Al deformation modes, the latter of which is infrared inactive.