Optical absorption spectroscopy study of three synthetic V (super 3+) -bearing clinopyroxenes
Optical absorption spectroscopy study of three synthetic V (super 3+) -bearing clinopyroxenes (in The 7th European conference on Mineralogy and spectroscopy, Monika Koch-mueller (editor), Michael Fechtelkord (editor), Vladimir Khomenko (editor) and Sergio Speziale (editor))
European Journal of Mineralogy (September 2012) 24 (5): 823-829
Optical absorption spectra of synthetic clinopyroxenes of NaVSi (sub 2) O (sub 6) , LiVSi (sub 2) O (sub 6) and CaVAlSiO (sub 6) composition consist of a high-energy absorption edge and two broad absorption bands nu (sub 1) and nu (sub 2) at around 15,000 cm (super -1) and 22,000 cm (super -1) caused by spin-allowed electronic transitions (super 3) T (sub 1) (sub g) ( (super 3) F) --> (super 3) T (sub 2) (sub g) ( (super 3) F) and (super 3) T (sub 1) (sub g) ( (super 3) F) --> (super 3) T (sub 1) (sub g) ( (super 3) P) of V (super 3+) in octahedral M1-site of the clinopyroxene structure. In polarized spectra of NaVSi (sub 2) O (sub 6) and LiVSi (sub 2) O (sub 6) pyroxenes the relative intensities of the polarized components of nu (sub 1) - and nu (sub 2) -bands are rather different, revealing differences in the distortion of VO (sub 6) -octahedra in the two structures. In spite of a formal similarity to the absorption spectra of Cr (super 3+) in Cr-bearing clinopyroxenes, the pressure-induced behaviour of the spin-allowed bands of V (super 3+) in NaVSi (sub 2) O (sub 6) -pyroxene is noticeably different: although they also shift to higher energies as do the bands of Cr (super 3+) , the integral intensity of the nu (sub 1) -band remains nearly constant within uncertainty whereas that of the nu (sub 2) -band somewhat decreases, in contrast to Cr (super 3+) for which the spin-allowed bands significantly decrease. Comparing with k (sub oct) (super loc) -value of CrO (sub 6) -octahedron in NaCrSi (sub 2) O (sub 6) kosmochlor, approximately 196 GPa, the VO (sub 6) -octahedron in NaVSi (sub 2) O (sub 6) clinopyroxene structure is considerably softer, with k (sub oct) (super loc) nearly equal 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 V (super 3+) evidently decreases with increasing pressure from approximately 554 cm (super -1) at 10 (super -4) GPa to 508 cm (super -1) at 10.7 GPa, thus indicating that V - O bonds in the VO (sub 6) -octahedra become more covalent.
