Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi
2O
6 and LiCrSi
2O
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
á
Cr -
O
ñ \left\langle {Cr - O} \right\rangle and
á
Mg -
O
ñ \left\langle {Mg - O} \right\rangle 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,
k\textpoly\textloc k_{\text{poly}}^{\text{loc}} , derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi
2O
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
k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} comparing with that of
k\textMg2 + \textloc k_{{{\text{Mg}}^{2 + } }}^{\text{loc}} , 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
2O
6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement
by Origlieri et al. (
2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility
of M1- and M2-sites, our
k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} -value, ~196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa.
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