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

Abstract Azurite Cu 2+ 3 (CO 3 ) 2 (OH) 2 and malachite Cu 2+ 2 (CO 3 )(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, CO 3 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 (ν 1 ), 837 and 817 cm −1 (ν 2 ), 1490 and 1415 cm −1 (ν 3 ) and 769 and 747 cm −1 (ν 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 ν 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 ν 1 + ν 3 , ν 1 + ν 3 , ν 1 + ν 4 and 2ν 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 (ν 1 ), 834 and 816 cm −1 (ν 2 ), 1430 and 1419 cm −1 (ν 3 ) and 764 and 739 cm −1 (ν 4 ). Combination of the CO 3 vibrational modes ν 1 + ν 4 is observed at 1860 cm −1 . IR bands for Cu O stretching modes are observed at 580, 570 and 505 cm −1 . Copyright © 2002 John Wiley & Sons, Ltd.