The Raman spectrum of calcite and its interpretation

Abstract

The paper reports the results of a study of the Raman spectrum of calcite recorded with prolonged exposures. (a) The principal Raman shift of 1086 cm.^-1 is accompanied by much weaker satellites whose frequency shifts are 1067, 1072 and 1075 cm.^-1 The o ctave of the principal mode is recorded as a sharp line with a frequency shift of 2173 cm.^-1 with a diffuse companion of comparable intensity which is a superposition of the octaves of the three satellites. The 1436 cm.^-1 Raman shift likewise has satellites at 1399, 1412 and 1418 cm.^-1 and its octave is feebly recorded as a sharp line, while the octaves of the satellites are also observed in its vicinity. The origin of the satellite radiations is discussed and it is shown that they represent the superlattice vibration frequencies of the calcite structure which are feebly excited. (b) The appearance of a sharp Raman line with a frequency shift of ± 221 cm.^-1 is confirmed and it is shown to arise from a normal mode usually considered as inactive. (c) The two intense Raman shifts of 155 and 282 cm.^-1 exhibit a highly unsymmetrical broadening, as also wings of low intensity spread out over the whole range of frequency shifts between -300 cm.^-1 to + 600 cm.^-1 This spectrum is explicable as arising from a very feeble excitation of the whole group of forty lattice and superlattice frequencies of the calcite structure and their overtones and combinations. (d) Sixteen combinational frequency shifts have been recorded. Seven of them are explained as combinations with the 1086 cm.^-1 Raman line and the the nine others as combinations with 875 cm.^-1 Some of the observed combinations violate the selection rules