| Popis: |
Optical absorption spectra of synthetic clinopyroxenes of NaVSi 2 O 6 , LiVSi 2 O 6 and CaVAlSiO 6 composition consist of a high-energy absorption edge and two broad absorption bands ν 1 and ν 2 at around 15,000 cm −1 and 22,000 cm −1 caused by spin-allowed electronic transitions 3 T 1 g ( 3 F ) → 3 T 2 g ( 3 F ) and 3 T 1 g ( 3 F ) → 3 T 1 g ( 3 P ) of V 3+ in octahedral M1-site of the clinopyroxene structure. In polarized spectra of NaVSi 2 O 6 and LiVSi 2 O 6 pyroxenes the relative intensities of the polarized components of ν 1 - and ν 2 -bands are rather different, revealing differences in the distortion of VO 6 -octahedra in the two structures. In spite of a formal similarity to the absorption spectra of Cr 3+ in Cr-bearing clinopyroxenes, the pressure-induced behaviour of the spin-allowed bands of V 3+ in NaVSi 2 O 6 -pyroxene is noticeably different: although they also shift to higher energies as do the bands of Cr 3+ , the integral intensity of the ν 1 -band remains nearly constant within uncertainty whereas that of the ν 2 -band somewhat decreases, in contrast to Cr 3+ for which the spin-allowed bands significantly decrease. Comparing with k oct loc -value of CrO 6 -octahedron in NaCrSi 2 O 6 kosmochlor, ~196 GPa, the VO 6 -octahedron in NaVSi 2 O 6 clinopyroxene structure is considerably softer, with k oct loc ≈ 163 GPa. The distances derived from the optical absorption spectra can fairly well be approximated by a linear dependence. The Racah parameter B of V 3+ 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 VO 6 -octahedra become more covalent. |
