Compression moduli of Cr³⁺-centered octahedra in a variety of oxygen-based rock-forming minerals

Abstract

 High-pressure electronic absorption spectra at room temperature and at pressures 10⁻⁴<P[GPa]<8 were measured in the spectral range 380<λ[nm] <780(26218>ν˜[cm⁻¹]>12820) on analysed single crystal slabs, about 20 μm thick, of Cr³⁺-bearing spinel (I), kyanite (II), corundum (III), pyrope (IV) and uvarovite (V) using DAC-cell techniques in combination with single-beam microscopespectrometry. Ligand field theoretical evaluation of the spectra yielded following results:

(i) the octahedral crystal field parameter, 10Dq_Cr3+[6], linearly shifts on increasing pressure to higher energies with slopes, (δ10Dq_Cr3+[6]/δP), of 103.1 (I), 99.5 (II), 104.0 (III), 111.7 (IV) and 110.3 [cm⁻¹/GPa] (V) (reliability parameters r≥0.92),

(ii) The Racah-parameter B_Cr3+[6], reflecting the covalency of the Cr–O bonds, does not significantly change with pressure up to 8 GPa, in those cases where it could be evaluated from the spectra (III, IV, V). This result is contrary to the behaviour of B_Cr3+[6] with increasing temperature (Taran et al. 1994) and shows that P and T are not inversely correlated parameters with respect to B_Cr3+[6], a decrease of which reflects an increase in covalency.

(iii) This result enabled to extract octahedral compression moduli, k_Cr3+[6], from the pressure slopes of 10Dq_Cr3+[6]: 312+48 (I), 297+70 (II), 298+44 (III), 275+35 (IV), 257+32 GPa (V). Quotients k_cr3+[6]/k_bulk,phase are nearly the same (ca. 1.6) for I, II, IV and V but significantly lower (ca. 1.1) for III. Deviations between spectroscopically determined k_Cr3+[6] and published k_Al[6], obtained by HP-XRD on ruby and pyrope, are interpreted by lattice strain induced by [Cr³⁺, Al3+−1]^[6] substitution.