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| Home > Publications database > An experimental study of the system $\mathrm{FeO-Fe_{2}O_{3}-SiO_{2}}$ at high pressures and temperatures : Garnet, perovskite and post-perovskite phases |
| Home > Publications database > An experimental study of the system $\mathrm{FeO-Fe_{2}O_{3}-SiO_{2}}$ at high pressures and temperatures : Garnet, perovskite and post-perovskite phases |
| Dissertation / PhD Thesis | PUBDB-2016-06058 |
;
2016
Bayreuth
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Please use a persistent id in citations: doi:10.3204/PUBDB-2016-06058
Abstract: Garnets are important minerals in the Earth’s upper-mantle and transition zone, and materials with the garnet structure are essential for many industrial applications. At lower mantle conditions, garnets transform into silicate perovskite (bridgmanite) and eventually into post-perovskite (CaIrO3-structured) silicate. Incorporation of iron into the structures of these phases can strongly affect chemical and physical properties of the Earth’s mantle, as well as its dynamics and evolution. In order to determine these effects, experimental measurements at relevant pressure-temperature conditions are needed. The results that comprise this thesis provide new insights into the chemistry and elasticity of mantle minerals. Single crystals of skiagite-rich garnet were synthesized at a pressure of 9.5 GPa and temperature of 1100 °C using a multi-anvil apparatus. The crystal structure was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mössbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral and octahedral sites, respectively, as expected for the garnet structure. The structural formula Fe3(Fe2+0.234(2)Fe3+1.532(2)Si4+0.234(2))(SiO4)3, obtained from single-crystal X-ray diffraction data and electron microprobe analyses shows that the skiagite-rich garnet synthesized in our experiment contains excess of Si and Fe2+ entering the octahedral site. The occurrence of Si and Fe2+ in the octahedral site means that this synthetic garnet is a skiagite – Fe-majorite solid solution: the end-member skiagite contains ~23 mol% of the Fe-majorite end-member (Fe23+(Fe2+Si4+)Si3O12) component. Systematic synthesis of skiagite-majorite garnets at different pressures and temperatures shows that skiagite can accommodate up to 76% of the Fe-majorite end-member. The substitution of Fe2+ and Si4+ for Fe3+ in the octahedral site decreases the unit-cell volume of garnets at ambient conditions. Equations of states of garnets with different compositions were measured using the diamond anvil cell technique. The analysis of single-crystal X-ray diffraction data collected upon compression up to 90 GPa reveals that with increasing majorite component, the bulk modulus slightly increases from 164(3) to 169(3) GPa. Our experimental results and the analysis of the literature data unambiguously demonstrate a considerable influence of the total iron content and the Fe3+/Fe2+ ratio in (Mg,Fe)-majorites on their elasticity. At pressures between 50 and 60 GPa, a significant deviation from a monotonic dependence of the molar volumes of skiagite-Fe-majorite garnets on pressure was observed, and in the small pressure interval (just 10 GPa) the volume drop was approximately 3%. By combining single crystal diffraction and high-pressure synchrotron Mössbauer spectroscopy results, we found that such changes in the compressional behaviour were associated with changes in the electronic state of Fe in the octahedral site. High-pressure high-temperature experiments in multi-anvil apparatus up to 25 GPa show that skiagite-majorite is stable up to 12.5 GPa and then decomposes to a mixture of Fe oxides and SiO2. Laser heating of skiagite-majorite garnets in the diamond-anvil cell at pressures up to about 40 GPa and temperatures between 1500 and 2300 K resulted in decomposition to Fe4O5 and stishovite (SiO2), but above 45 GPa, heating of the garnet resulted in the formation of a high-pressure form of Fe3O4 and Fe-bridgmanite. Ferric iron stabilizes Fe-rich bridgmanite to such an extent that it is possible to synthesize pure iron silicate perovskite at pressures between ~45 GPa and 110 GPa with the resulting composition (Fe2+0.64(2)Fe3+0.24(2))Si1.00(3)O3 (within uncertainty of measurements independent of conditions of synthesis). The crystal chemistry of ferric iron-bearing bridgmanite is very unusual. First, all iron is located in a large distorted prism (A site) with no evidence of iron in the octahedral (B) site. Second, bridgmanite contains a significant amount of vacancies (~12%) at the A site. Moreover, the compressibility of iron bridgmanite (K300=190(4) GPa, K´=4, V0=178.98(6) Å3/unit cell) is anomalously high compared to any known Fe- or/and Al-rich bridgmanites. At pressures of 127(1) GPa and 146(1) GPa, pure iron silicate post-perovskite was synthesized using laser-heated diamond anvil cell from skiagite-majorite garnet and Fe bridgmanite. Synchrotron X-ray diffraction revealed that the incorporation of iron leads to increase of the unit-cell volume relative to magnesium post-perovskite. The change in unit cell parameter b in the post-perovskite structure is the most significant contribution to volume expansion; it increases in length by 2.39% and increases total volume by 3.5% in comparison to Fe-free post-perovskite. The density of studied post-perovskite (6.670 g/cm3) is 3% lower than the density estimated at the same conditions for pure FeSiO3 (6.928 g/cm3). Results from synthesis of pure iron post-perovskite support the idea that iron enrichment may be one of the reasons for formation of ultra-low velocity zones at the base of the lower mantle.
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