Abstract
Raman spectroscopic data were obtained for (Mg,Fe)2SiO4 samples during compression to 57 GPa. Single crystals of San Carlos olivine compressed hydrostatically above 41 GPa showed appearance of a new “defect” peak in the 820–840 cm−1 region associated with SiOSi linkages appearing between adjacent Embedded Image tetrahedra to result in five- or sixfold-coordinated silicate species. Appearance of this feature is accompanied by a broad amorphous background. The changes occur at lower pressure than metastable crystalline transitions of end-member Mg2SiO4 forsterite (Fo-I) into Fo-II and Fo-III phases described recently. We complemented our experimental study using density functional theory (DFT) calculations and anisotropic ion molecular dynamics (AIMD) simulations to study the Raman spectra and vibrational density of states (VDOS) of metastably compressed Mg2SiO4 olivine, Fo-II and Fo-III, and quenched melts at high and low pressures. By 54 GPa all sharp crystalline peaks disappeared from observed Raman spectra indicating complete pressure-induced amorphization (PIA). The amorphous (Mg,Fe)2SiO4 spectrum contains Si-O stretching bands at lower wavenumber than expected for Embedded Image indicating high coordination of the silicate units. The amorphous spectrum persisted on decompression to ambient conditions but with evidence for reappearance of tetrahedrally coordinated units. Non-hydrostatic compression of polycrystalline olivine samples showed similar appearance of the defect feature and broad amorphous features between 43–44 GPa. Both increased in intensity as the sample was left at pressure overnight but they disappeared during decompression below 17 GPa with recovery of the starting olivine Raman signature. A hydrated San Carlos olivine sample containing 75–150 ppm OH was also studied. Significant broadening of the Embedded Image stretching peaks was observed above 43 GPa but without immediate appearance of the defect or broad amorphous features. However, both of these characteristics emerged after leaving the sample at 47 GPa overnight followed by complete amorphization that occurred upon subsequent pressurization to 54 GPa. During decompression the high-density amorphous spectrum was retained to 3 GPa but on final pressure release a spectrum similar to thermally quenched low-pressure olivine glass containing isolated Embedded Image groups was obtained. Leaving this sample overnight resulted in recrystallization of olivine. Our experimental data provide new insights into the metastable structural transformations and relaxation behavior of olivine samples including material recovered from meteorites and laboratory shock experiments.
Highlights
Olivine [a-(Mg,Fe)2SiO4] is the dominant mineral of the Earth’s upper mantle and is present within chondritic meteorites as well as a wide range of other extra-terrestrial environments [see Finkelstein et al (2014) for a summary of literature]
Equilibrium phase relations within the Mg2SiO4-Fe2SiO4 system are well established from high pressure–high temperature (P-T) experiments. a-Mg2SiO4 transforms first into spinelloid b-Mg2SiO4 and spinel-structured g-Mg2SiO4 phases at between P = 12–22 GPa depending upon the temperature, before chemically dissociating into MgSiO3 perovskite + MgO above ~25 GPa
36 are Raman active: GRaman = 11 Ag + 11 B1g + 7 B2g + 7 B3g that have been assigned to their respective atomic displacement patterns from polarized single-crystal studies and isotopic exchange experiments along with empirical force field, ionic model and ab initio density functional theory (DFT) calculations (Servoin and Piriou 1973; Pâques-Ledent and Tarte 1973; Iishi 1978; Price et al 1987; Chopelas 1991; Kolesov and Geiger 2004; Noel et al 2006)
Summary
Olivine [a-(Mg,Fe)2SiO4] is the dominant mineral of the Earth’s upper mantle and is present within chondritic meteorites as well as a wide range of other extra-terrestrial environments [see Finkelstein et al (2014) for a summary of literature]. Equilibrium phase relations within the Mg2SiO4-Fe2SiO4 system are well established from high pressure–high temperature (P-T) experiments. A-Mg2SiO4 (forsterite; Fo) transforms first into spinelloid b-Mg2SiO4 (wadsleyite) and spinel-structured g-Mg2SiO4 (ringwoodite) phases at between P = 12–22 GPa depending upon the temperature, before chemically dissociating into MgSiO3 perovskite (bridgmanite) + MgO above ~25 GPa. The a-Fe2SiO4 end-member (fayalite, Fa). High P-T transformations of Fo-Fa olivine solid solutions give rise to the wadsleyite and ringwoodite phases found among chondritic meteorite samples
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