Raman spectroscopy of CaCoSi2O6–Co2Si2O6 clinopyroxenes

Abstract

Raman spectra were collected on a set of synthetic clinopyroxenes along the series CaCoSi₂O₆–Co₂Si₂O₆. Changes in peak position and peak width show: (1) evidence of a phase transition from C2/c to P2₁/c, at Ca_0.4Co_1.6Si₂O₆, in agreement with previous X-ray observations; (2) peak broadening for intermediate compositions, with sharper peaks close to the end members. The phase transition is revealed by a decrease or inversion in the slope of the peak position versus composition and by peak splitting of the peaks at 660 and 1,000 cm⁻¹, related to Si–O bending and stretching modes within the tetrahedral chains, respectively. The observed changes with composition depend more on variation in bond lengths due to structural rearrangement with cation substitution, rather than by changes of the M2 cation mass. A comparison with the structurally analogous CaMgSi₂O₆–Mg₂Si₂O₆ (Diopside-Estatite, Ca-Mg) series shows that one of the two splitted peaks is fainter than the Ca–Co pyroxenes. Therefore the frequency of the peak at about 1,000 cm⁻¹ does not change for Ca–Mg substitution, whereas it shifts by as much as 20 cm⁻¹ between CaCoSi₂O₆ and Co₂Si₂O₆. Despite the mechanism of cation substitution is qualitatively similar in the two series, the effect of structural changes and polyhedral deformation on the Raman spectra appeared different. Peak broadening in samples with intermediate compositions could be interpreted as arising by compositional disorder, due to coexistence of local Ca-rich and Co-rich configurations which affect the short range interactions and therefore the Raman frequencies.