High-pressure crystal-held spectra of single-crystal clinoferrosilite

Electronic absorption spectra of a single crystal of clinoferrosilite, FeSiO(3), 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 (5)A(1) --> (5)A(1) and (5)A(1) --> (5)B(1) transitions of Fe(2+) on the M2 site, and two peaks near 10,100 cm(-1) and 8,000 cm(-1) are assigned to the (5)B(2g) --> (5)A(1g) and (5)B(2g) --> (5)B(1g) transitions of Fe(2+) on the M1 site. At ambient conditions, crystal-field splitting (Delta(o)) and crystal-field stabilization energy (CFSE) of M1 are estimated to be 8,681 cm(-1) and 3,972 cm(-1), respectively, and Delta(o) 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 cm(-1) 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 P2(1)/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 FeSiO(3) is predicted to gain additional stabilization energy relative to the P2(1)/c polymorph at high pressure and room temperature by increased crystal-field effects of Fete in the octahedral sites. The best estimates for CFSE of Fe(2+) on the hll and M2 sites of the C2/c structure at 1.65 GPa are similar to 4,200 cm(-1) and similar to 4,000 cm(-1) respectively, compared with similar to 4,000 and similar to 3,770 cm(-1) for the M1 and M2 sites of the P2(1)/c structure at 1.4 GPa. The lowering of the transition pressure from MgSiO(3) to FeSiO(3) can be explained by increased crystal-field stabilization energy of Fe(2+) in the octahedral sites of the high-density phase, combined with a smaller contribution from the increase in cation size of Fe(2+) substituting for Mg(2+).