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

High-pressure electronic absorption spectra at room temperature and at pressures 10^?4<PGPa]<8 were measured in the spectral range 380<λnm] <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_Al6], obtained by HP-XRD on ruby and pyrope, are interpreted by lattice strain induced by Cr³⁺,?Al3+?1]^6] substitution.