Aurichalcite - An SEM and Raman Spectroscopic Study

Raman spectroscopy complimented with supplementary infrared spectroscopy has been used to characterise the vibrational spectrum of aurichalcite a zinc/copper hydroxy carbonate, (Zn,Cu2+)5(CO3)2(OH)6. XRD patterns of all specimans show high orientation and indicate the presence of some impurities such as rosasite and hydrozincite. However, the diffraction patterns for all samples are well correlated to the standard reference patterns. SEM images show highly crystalline and ordered structures in the form of micron long fibres and plates. EDAX analyses indicate variations in chemical composition of Cu/Zn ratios ranging from 1/1.06 to 1/2.87. The symmetry of the carbonate anion in aurichalcite is Cs and is composition dependent. This symmetry reduction results in multiple bands in both the symmetric stretching and bending regions. The intense band at 1072 cm-1 is assigned to the ν1 (CO3)2- symmetric stretching mode. Three Raman bands assigned to the ν3 (CO3)2- antisymmetric stretching modes are observed for aurichalcite at 1506, 1485 and 1337 cm-1. Multiple Raman bands are observed in 800 to 850 cm-1 and 720 to 750 cm-1 regions and are attributed to ν2 and ν4 bending modes confirming the reduction of the carboante anion symmetry in the aurichalcite structure. An intense Raman band at 1060 cm-1 is attributed to the δ OH deformation mode.