Leyla Ismailova

High-pressure spectroscopic study of siderite (FeCO3) with a focus on spin crossover

Abstract Fe-bearing carbonates have been proposed as possible candidate host minerals for carbon inside the Earth’s interior and hence their spectroscopic properties can provide constraints on the deep carbon cycle. Here we investigate high-pressure spin crossover in synthetic FeCO3 (siderite) using a combination of Mössbauer, Raman, and X-ray absorption near edge structure spectroscopy in diamond-anvil cells. These techniques sensitive to the short-range atomic environment show that at room temperature and under quasi-hydrostatic conditions, spin crossover in siderite takes place over a broad pressure range, between 40 and 47 GPa, in contrast to previous X-ray diffraction data that described the transition as a sharp volume collapse at approximately 43 GPa. Based on these observations we consider electron spin pairing in siderite to be a dynamic process, where Fe atoms can be either high spin or low spin in the crossover region. Mode Grüneisen parameters extracted from Raman spectra collected at pressures below and above spin crossover show a drastic change in stiffness of the Fe-O octahedra after the transition, where they become more compact and hence less compressible. Mössbauer experiments performed on siderite single crystals as well as powder samples demonstrate the effect of differential stress on the local structure of siderite Fe atoms in a diamond-anvil cell. Differences in quadrupole splitting values between powder and single crystals show that local distortions of the Fe site in powder samples cause spin crossover to start at higher pressure and broaden the spin crossover pressure range.

Stability of iron-bearing carbonates in the deep Earth’s interior

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO3 partially dissociates to form various iron oxides. At higher pressures FeCO3 forms two new structures—tetrairon(III) orthocarbonate Fe43+C3O12, and diiron(II) diiron(III) tetracarbonate Fe22+Fe23+C4O13, both phases containing CO4 tetrahedra. Fe4C4O13 is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle.

Stability of Fe,Al-bearing bridgmanite in the lower mantle and synthesis of pure Fe-bridgmanite

A study of Fe,Al-bearing bridgmanite in Earth‘s mantle and synthesis of pure Fe-bridgmanite with anomalously low compressibility. The physical and chemical properties of Earth’s mantle, as well as its dynamics and evolution, heavily depend on the phase composition of the region. On the basis of experiments in laser-heated diamond anvil cells, we demonstrate that Fe,Al-bearing bridgmanite (magnesium silicate perovskite) is stable to pressures over 120 GPa and temperatures above 3000 K. Ferric iron stabilizes Fe-rich bridgmanite such that we were able to synthesize pure iron bridgmanite at pressures between ~45 and 110 GPa. The compressibility of ferric iron–bearing bridgmanite is significantly different from any known bridgmanite, which has direct implications for the interpretation of seismic tomography data.

Discovery of Fe7O9: a new iron oxide with a complex monoclinic structure.

Iron oxides are fundamentally important compounds for basic and applied sciences as well as in numerous industrial applications. In this work we report the synthesis and investigation of a new binary iron oxide with the hitherto unknown stoichiometry of Fe7O9. This new oxide was synthesized at high-pressure high-temperature (HP-HT) conditions, and its black single crystals were successfully recovered at ambient conditions. By means of single crystal X-ray diffraction we determined that Fe7O9 adopts a monoclinic C2/m lattice with the most distorted crystal structure among the binary iron oxides known to date. The synthesis of Fe7O9 opens a new portal to exotic iron-rich (M,Fe)7O9 oxides with unusual stoichiometry and distorted crystal structures. Moreover, the crystal structure and phase relations of such new iron oxide groups may provide new insight into the cycling of volatiles in the Earth’s interior.

A new high-pressure phase transition in clinoferrosilite: In situ single-crystal X-ray diffraction study

Abstract Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on synthetic pure clinoferrosilite, Fe2Si2O6, at room temperature to a maximum pressure of 45 GPa. In addition to the previously described P21/c → C2/c phase transition between 1.48 and 1.75 GPa (Hugh-Jones et al. 1994), we observe further transition between 30 and 36 GPa into the high-pressure P21/c phase (HP-P21/c). The C2/c → HP-P21/c transition is induced by rearrangement of half of the layers of corner-sharing SiO4 tetrahedra into layers of edge-sharing SiO6 octahedra. The new configuration of VISi layers suggests a possibility of a progressive transformation of the pyroxene into an ilmenite-type structure. The persistence of metastable pyroxene up to pressures higher than expected and its feasible direct transformation to ilmenite are of special interest for understanding the dynamics of cold-subducting slabs. We report on structural and compressibility features of both high-pressure phases as well as address thermal stability of HP-P21/c.

High-pressure synthesis of skiagite-majorite garnet and investigation of its crystal structure

Abstract Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia3̅d, a = 11.7511(2) Å, V = 1622.69(5) Å3, Dcalc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X‑ray diffraction. Synchrotron Mössbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X‑ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X‑ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.

Effect of composition on compressibility of skiagite-Fe-majorite garnet

Abstract Skiagite-Fe-majorite garnets were synthesized using a multianvil apparatus at 7.5–9.5 GPa and 1400–1600 K. Single-crystal X-ray diffraction at ambient conditions revealed that synthesized garnets contain 23 to 76% of an Fe-majorite component. We found that the substitution of Fe2+ and Si4+ for Fe3+ in the octahedral site decreases the unit-cell volume of garnet at ambient conditions. Analysis of single-crystal X-ray diffraction data collected on compression up to 90 GPa of garnets with different compositions reveals that with increasing majorite component the bulk modulus increases from 159(1) to 172(1) GPa. Our results and literature data unambiguously demonstrate that the total iron content and the Fe3+/Fe2+ ratio in (Mg, Fe)-majorites have a large influence on their elasticity. At pressures between 50 and 60 GPa we observed a significant deviation from a monotonic dependence of the molar volumes of skiagite-Fe-majorite garnet with pressure, and over a small pressure interval the volume dropped by about 3%. By combining results from single-crystal X-ray diffraction and high-pressure synchrotron Mössbauer source spectroscopy we demonstrate that these changes in the compressional behavior are associated with changes of the electronic state of Fe in the octahedral site.

Diamond-forming efficiency of chloride-silicate-carbonate melts

According to the carbonatite model of the diamond formation [Litvin, 2007] based on the large volume of mineralogical and physicochemical experimental information, carbon-silicate-carbonate (carbonatitic) melts with widely variable compositions are the growth medium for most mantle diamonds and inclusions in them. In addition to the major completely miscible carbonate and silicate components (minerals of peridotite and eclogite assemblages), such melts contain minor soluble components (oxides, phosphates, chlorides, C-O-H-N fluids, and others), as well as minor completely immiscible and insoluble solid and melt phases (sulfides, metals). The diamond-forming efficiency of silicate-carbonate melts clearly corresponds to the important criterion of syngenesis of diamond with its silicate and carbonate inclusions. High-pressure experimental associations not only comprise the whole set of minerals typical for inclusions in diamonds of peridotitic (olivine, garnet, clino-, and orthopyroxenes) and eclogitic (garnet and clinopyroxene) type, but demonstrate the characteristic features of minerals of diamond paragenesis. These comprise significant admixtures of Na in garnets and K in clinopyroxenes, which are the reliable indicators of crystallization of these minerals from alkaline silicate-carbonate melts. Experimental investigations of multicomponent peridotite-carbonatite and eclogite-carbonatite systems at a standardized pressure of 8.5 GPa within the narrow pressure range of 1760–1820°C demonstrated that the diamond-forming efficiency of their carbon-bearing melts had concentration limitations [Bobrov and Litvin, 2009]. The compositions effective for the diamond nucleation comprise only significantly carbonatite parts of the systems and are limited by the concentration barriers of diamond nucleation (CBDN) in the cases of K-Na-Ca-Mg-Fe-carbonatite, Ca-Mg-, and Kcarbonate compositions (at the concentrations of 30, 25, and 30 wt % of peridotite components and 35, 30, and 45 wt % of eclogite component, respectively). This means that the inhibitory influence of peridotite and eclogite components dissolved in carbon-bearing carbonatite melts on diamond nucleation is observed only at their relatively low concentrations. We established the diamond-forming efficiency of chloride [Litvin, 2003] and chloride