High-pressure electronic absorption spectroscopy of natural and synthetic Cr3+-bearing clinopyroxenes

Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi 2 O 6 and LiCrSi 2 O 6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all cases Cr 3+ enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe 2+ (M1, M2) and intervalence Fe 2+ /Fe 3+ charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr 3+ in the crystal structures. The positions of the spin-allowed bands of Cr 3+ , especially of the low energy one caused by the electronic 4 A 2g → 2 T 1g transition, are found to be in accordance with mean M1–O distances. The local relaxation parameter ε calculated for lim Cr 3+ → 0 from the spectra and interatomic and distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the “guest” ion, Cr 3+ , deviates greatly from the “diffraction” value, ε = 0, than in any other known Cr 3+ -bearing systems studied so far. Under pressure the spin-allowed bands of Cr 3+ shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr 3+ reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO 6 octahedral modulus, , derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi 2 O 6 kosmochlor are very close, ~203 and ~196 GPa, respectively, being, however, nearly twice higher than that of MgO 6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs ( 2008 ). Such a strong stiffening of the structural octahedron, i.e. twice higher value of comparing with that of , may be caused by simultaneous substitution of Ca 2+ by larger Na + in the neighboring M2 sites at so-called jadeite-coupled substitution Mg 2+ + Ca 2+ → Cr 3+ + Na + . It is also remarkable that the values of CrO 6 octahedral modulus of NaCrSi 2 O 6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressu