Structural and electronic properties of Fe₂SiO₄‐fayalite at ultrahigh pressures: Amorphization and gap closure

X ray diffraction and infrared spectroscopic measurements to pressures of 45–50 GPa, electrical resistivity and optical absorption to 70–80 GPa, and reflectance measurements to 225 GPa are presented for Fe 2 SiO 4 fayalite at 300 K. Diffraction results document that Fe 2 SiO 4 fayalite becomes amorphous on static loading to pressures in excess of 39 (±3) GPa, a pressure identical to that at which the “mixed phase” regime of fayalite commences under shock compression. As with more polymerized silicates, the high‐pressure amorphization of metastable fayalite is associated with the SiO 4 tetrahedron becoming unstable relative to higher coordinations of silicon. Infrared absorption spectroscopy reveals the pressure‐induced change in coordination through a decline in intensity of the tetrahedral asymmetric stretching vibration, accompanied by an increase in amplitude at frequencies characteristic of SiO 6 vibrations. The coordination change is not quenchable to zero pressure, and infrared spectra of amorphous fayalite quenched from pressure document that the local structure of the sample is similar to that of crystalline Fe 2 SiO 4 . This represents the first example of the static synthesis of a glass without fusion in a silicate containing isolated tetrahedra. The electrical resistivity of initially crystalline Fe 2 SiO 4 drops from approximately 2×10 5 ohm m at zero pressure to 10 −3 ohm m at 79 GPa, with the pressure dependence of the resistivity decreasing at approximately the pressure of the coordination change (and amorphization). The electrical properties are quantitatively consistent with previous measurements, both under static and under shock wave loading. Optical transmission experiments demonstrate that in accord with the rapidly changing electron‐transport properties of Fe 2 SiO 4 , a strong absorption edge decreases in energy from the ultraviolet to infrared energies of about 3500 cm −1 (0.4 eV) on compression to 70 GPa. However, the reflectance of Fe 2 SiO 4 is less than 3.1% between 780 and 2100 nm at all pressures to 225 GPa, indicating that amorphous fayalite does not metallize to at least this pressure. We interpret our resistivity and optical results in terms of increased interactions between iron ions with increasing pressure in Fe 2 SiO 4 and an approach toward a Fe 3+ +Fe 1+ metallic configuration at high pressures. When combined with previous observations of amorphization at low temperatures and high pressures, our results suggest that transition to an amorphous phase is likely to be a general phenomenon in metastable silicates when the distortion of SiO 4 tetrahedra becomes a major mechanism of compression.