Gamma resonance of 57Fe (Mössbauer effect) in natural grunerite was studied at six different pressures up to 8.5 GPa. The isomer shifts δ and quadrupole splittings ΔEQ of the two resolved paramagnetic Fe2+ doublets change significantly: ΔEQ of the M1, M2, and M3 positions decreases by 2.9%, and ΔEQ of M4 increases by 21% with increasing pressure; δ of M1, M2, and M3 decreases and δ at M4 increases with increasing pressure, indicating a crossover at about 10 GPa. Linear relationships between δ, ΔEQ, and pressure and between δ and ΔEQ were found at 293 K. At pressures smaller than about 1 GPa, the electronic structure of Fe2+ in M positions is practically independent of the compression of the unit cell.

Fe2+ remains in high-spin configuration up to at least 8.5 GPa. The nuclear electron density of Fe2+ in M1, M2, and M3 increases, whereas that in M4 decreases. The changes of δ(M1,M2,M3) and ΔEQ(M1,M2,M3) are mainly attributed to electronic changes in the Fe2+ valence shell and overlap with the ligands. The increase of δ(M4) is ascribed to 4s, 3d electron transfer. For the change of ΔEQ(M4) the lattice contribution is predominant. The electronic states of Fe2+ in the M octahedra become more similar and the M4 octahedra less distorted at high pressures.

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