High-pressure crystal-field spectra of single-crystal clinoferrosilite
Ross, Nancy L.; Sowerby, John R.
European Journal of Mineralogy Volume 11 Number 5 (1999), p. 791 - 802
published: Sep 30, 1999
manuscript accepted: Mar 31, 1999
manuscript received: Sep 1, 1998
ArtNo. ESP147051105001, Price: 29.00 €
Abstract Electronic absorption spectra of a single crystal of clinoferrosilite, FeSiO3, have been collected between 4,000 and 15,000 cm-1 from room pressure to 4.8 GPa. Four bands are observed in all of the spectra: two peaks near 11,000 cm-1 and 5,100 cm-1 are attributed to the 5A1 → 5A1 and 5A → 5B1 transitions of Fe2+ on the M2 site, and two peaks near 10,100 cm-1 and 8,000 cm-1 are assigned to the 5B2g → 5Al1g and 5B2g → 5B1g transitions of Fe2+ on the Ml site. At ambient conditions, crystal-field splitting (Δ0) and crystal-field stabilization energy (CFSE) of Ml are estimated to be 8,681 cm-1 and 3,972 cm-1, respectively, and Δ0, and CFSE of M2 are estimated to be 7,262 cm-1 and 3,705 cm-1, respectively. Between room pressure and 1.40 GPa, the crystal-field bands all shift to higher energies at rates between 118 and 287 cm1 GPa-1. At approximately 1.65 GPa, discontinuities in the peak positions between 270 to 600 cm-1 are observed that coincide with the transition from the P21/c structure to the C2/c structure. Above the transition, the bands shift to higher energies at rates between 150 and 270 cm-1 GPa-1. The C2/c polymorph of FeSiO3 is predicted to gain additional stabilization energy relative to the P21/c polymorph at high pressure and room temperature by increased crystal-field effects of Fe2+ in the octahedral sites. The best estimates for CFSE of Fe2+ on the Ml and M2 sites of the C2/c structure at 1.65 GPa are ~ 4,200 cm-1 and ~ 4,000 cm-1 respectively, compared with ~ 4,000 and ~ 3, 770 cm-1 for the Ml and M2 sites of the P21/c structure at 1.4 GPa. The lowering of the transition pressure from MgSiO3 to FeSiO3 can be explained by increased crystal-field stabilization energy of Fe2+ in the octahedral sites of the high-density phase, combined with a smaller contribution from the increase in cation size of Fe2+ substituting for Mg2+.