Raman spectroscopic study of azurite and malachite at 298 and 77 K

Azurite [Cu-3(2+)(CO3)(2)(OH)(2)] and malachite [Cu-2(2+)(CO3)(OH)(2)] are both monoclinic hydroxy carbonates of copper. The Raman spectra of these two minerals were collected at both 298 and 77 K together with the single crystal Raman spectra of azurite at 298 K. The spectra of both azurite and malachite contain modes of three separate vibrational groups: OH, CO3 and Cu-O. Accordingly, for azurite the bands at 3453 and 3427 cm(-1) have been assigned as the O-H stretching mode with the O-H bending modes found at 1035 and 952 cm(-1). Malachite displays two hydroxyl stretching bands at 3474 and 3404 cm(-1) at 298 K which shift to 3470 and 3400 cm(-1) at 77 K while O-H out-of-plane bending modes are found at 1045 and 875 cm(-1). For the carbonate group, bands are observed at 1090 cm(-1) (nu(1)), 837 and 817 cm(-1) (nu(2)), 1490 and 1415 cm(-1) (nu(3)) and 769 and 747 cm(-1) (nu(4)). The effect of collecting the spectra at 77 K results in considerable band narrowing. The Raman spectra of the single crystals of azurite show the orientation dependence of the vibrational modes. A mechanism for nu(2) splitting is proposed whereby vertical carbonate ions couple to form an in-phase and out-of-phase bending modes. Combination of these vibrational modes such as nu(1) + nu(3), nu(1) + nu(3), nu(1) + nu(4) and 2nu(2) are found at 2530, 2414,1860, and 1670 cm(-1), respectively. IR bands for the Cu-O stretching modes are observed at 495 and 400 cm(-1), while Cu-O bending modes occur at 455 and 345 cm(-1). Bands at 305 and 240 cm(-1) are assigned to the O-Cu-OH bending modes. The Raman out-of-plane bending modes are found at 194 and 170 cm(-1). For the carbonate group, infrared bands are observed at 1095 cm(-1) (nu(1)), 834 and 816 cm(-1) (nu(2)), 1430 and 1419 cm(-1) (nu(3)) and 764 and 739 cm(-1) (nu(4)). Combination of the C03 vibrational modes nu(1) + nu(4) is observed at 1860 cm(-1). IR bands for Cu-O stretching modes are observed at 580, 570 and 505 cm(-1). Copyright (C) 2002 John Wiley Sons, Ltd.