ausblenden:
Schlagwörter:
Goethite,
ε-FeO(OH),
Groutite,
Manganite,
High pressure,
Compressibility
In situ XRD measurements
Zusammenfassung:
The P-V-T behavior of FeO(OH) and MnO(OH) has been determined under high pressure and high temperature up to 7.5 GPa and 500 °C using a MAX 80 cubic anvil high-pressure apparatus. The samples, synthetic goethite, α-FeO(OH), respectively, a natural groutite/manganite, α-MnO(OH)/γ-MnO(OH), specimen were mixed with Vaseline to ensure hydrostatic pressure-transmitting conditions, and NaCl served as an internal standard for pressure calibration. Energy-dispersive diffraction patterns were collected at a fixed 2θ angle (θ ≈ 4.52°). At pressures >7.1 GPa and temperatures >310 °C, respectively, P > 6.3 GPa and T > 350 °C, the transformation goethite ↔ ε-FeO(OH) was observed. Between 400 and 450 °C, the sample dehydrated to magnetite due to the reducing conditions caused by the graphite-tube furnace. By fitting a Birch–Murnaghan equation of state to the data, the bulk modulus of goethite was determined as (112.26 ± 2.26) GPa, (K′ = 4), VT,0 = (138.79 ± 0.10) Å3·exp [∫(0.497 ± 0.103) × 10−4 dT], (∂KT/∂T)P = (–0.033 ± 0.020) GPa K−1. For ε-FeO(OH), the values K = (142.8 ± 15.1) GPa, V0 = (66.18 ± 0.16) Å3, (K′ = 4), were obtained. Groutite and manganite are more compressible than their Fe analogues. K(groutite) = (84.0 ± 2.9) GPa, V0 = (139.92 ± 0.13) Å3, (K′ = 4). K(manganite) = (82.2 ± 3.0) GPa, V0 = (135.37 ± 0.15) Å3, (K′ = 4). Groutite disappeared at P ≈ 5.5 GPa and T = 300 °C, only manganite remained. At T > 400 °C, the sample dehydrated first to Mn3O4 [II] and then to manganosite (MnO) again pointing to reducing conditions.
