Abstract
Optical absorption spectra of synthetic clinopyroxenes of NaVSi2O6, LiVSi2O6 and CaVAlSiO6 composition consist of a high-energy absorption edge and two broad absorption bands ν1 and ν2 at around 15,000 cm−1 and 22,000 cm−1 caused by spin-allowed electronic transitions 3T1g (3F) → 3T2g (3F) and 3T1g (3F) → 3T1g (3P) of V3+ in octahedral M1-site of the clinopyroxene structure. In polarized spectra of NaVSi2O6 and LiVSi2O6 pyroxenes the relative intensities of the polarized components of ν1- and ν2-bands are rather different, revealing differences in the distortion of VO6-octahedra in the two structures. In spite of a formal similarity to the absorption spectra of Cr3+ in Cr-bearing clinopyroxenes, the pressure-induced behaviour of the spin-allowed bands of V3+ in NaVSi2O6-pyroxene is noticeably different: although they also shift to higher energies as do the bands of Cr3+, the integral intensity of the ν1-band remains nearly constant within uncertainty whereas that of the ν2-band somewhat decreases, in contrast to Cr3+ for which the spin-allowed bands significantly decrease. Comparing with koctloc-value of CrO6-octahedron in NaCrSi2O6 kosmochlor, ~196 GPa, the VO6-octahedron in NaVSi2O6 clinopyroxene structure is considerably softer, with koctloc ≈ 163 GPa. The <V – O>distances derived from the optical absorption spectra can fairly well be approximated by a linear dependence. The Racah parameter B of V3+ evidently decreases with increasing pressure from ~554 cm−1 at 10−4 GPa to 508 cm−1 at 10.7 GPa, thus indicating that V – O bonds in the VO6-octahedra become more covalent.
