ALBERT

Description: 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 Ia d , a = 11.7511(2) Å, V = 1622.69(5) Å 3 , D calc = 4.4931 g/cm 3 ] was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mössbauer source spectroscopy revealed that Fe 2+ and Fe 3+ 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 Fe 3 2+ (Fe 2+ 0.234(2) Fe 3+ 1.532(1) Si 4+ 0.234(2) )(SiO 4 ) 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 Fe 2+ 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 Fe 3 2+ (Fe 2+ Si 4+ )(SiO 4 ) 3 .

Description: 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 FeCO 3 (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.

Description: 〈span〉〈div〉Abstract〈/div〉A new (Mg〈sub〉0.5〈/sub〉Fe0.53+)(Si〈sub〉0.5〈/sub〉Al0.53+)O〈sub〉3〈/sub〉 LiNbO〈sub〉3〈/sub〉-type phase was synthesized at 27 GPa and 2000 K under highly oxidized conditions using an advanced multi-anvil apparatus. Single crystals for this phase are 0.2–0.3 mm in dimension and maroon in color. They crystallize in a noncentrosymmetric structure with space group 〈span〉R〈/span〉3〈span〉c〈/span〉 and lattice parameters of 〈span〉a〈/span〉 = 〈span〉b〈/span〉 = 4.8720(6) Å, 〈span〉c〈/span〉 = 12.898(2) Å, and 〈span〉V〈/span〉 = 265.14(8) Å〈sup〉3〈/sup〉. Fe〈sup〉3+〈/sup〉 and Al〈sup〉3+〈/sup〉 cations substitute into 〈span〉A〈/span〉 (Mg〈sup〉2+〈/sup〉) and 〈span〉B〈/span〉 (Si〈sup〉4+〈/sup〉) sites through charge-coupled substitution mechanism, respectively. The distortion of 〈span〉B〈/span〉O〈sub〉6〈/sub〉 (〈span〉B〈/span〉 = Si〈sub〉0.5〈/sub〉Al0.53+) octahedra is 1.6 times higher than that of AO〈sub〉6〈/sub〉 (A = Mg〈sub〉0.5〈/sub〉Fe0.53+) octahedra. This phase is probably recovered from bridgmanite at lower-mantle conditions by a diffusionless transition because of the displacement of 〈span〉A〈/span〉 cations and distortion of 〈span〉B〈/span〉O〈sub〉6〈/sub〉 octahedra on releasing pressure. Bridgmanite can thus contain the FeAlO〈sub〉3〈/sub〉 component (50 mol%) beyond previously reported solubility limit (37 mol%). The present study shows that the Earth's most abundant elements form a new Fe〈sup〉3+〈/sup〉- and Al〈sup〉3+〈/sup〉-rich LiNbO〈sub〉3〈/sub〉-type compound from bridgmanite at lower mantle conditions. This new compound provides a new insight into the complicated crystal chemistry of LiNbO〈sub〉3〈/sub〉-type phase/bridgmanite and constrains the pressure and temperature conditions for shocked meteorites.〈/span〉

Description: The structural behavior of Cr 2 O 3 was investigated up to ~70 GPa using single-crystal X-ray diffraction under a quasi-hydrostatic pressure (neon pressure medium) at room temperature. The crystal structure remains rhombohedral with the space group Rc (No. 167) and upon compression the oxygen atoms approach an ideal hexagonal close-packing arrangement. An isothermal bulk modulus of Cr 2 O 3 and its pressure derivative were found to be 245(4) GPa and 3.6(2), respectively, based on a third-order Birch-Murnaghan equation of state and V 0 = 288.73 Å 3 . An analysis of the crystal strains suggest that the non-hydrostatic stresses can be considered as negligible even at the highest pressure reached.

Description: The infrared spectra of hydroxyl in synthetic hydrous wadsleyite (β-Mg 2 SiO 4 ) and ringwoodite (-Mg 2 SiO 4 ) were measured at room temperature up to ~18.8 GPa for wadsleyite and up to ~21.5 GPa for ringwoodite. High-temperature spectra were measured in an externally heated diamond-anvil cell up to 650 °C at ~14.2 GPa for wadsleyite and up to 900 °C at ~18.4 GPa for ringwoodite. The synthetic samples reproduce nearly all the important OH bands previously observed at ambient conditions. Only subtle changes were observed in the infrared spectra of both minerals, both upon compression at room temperature and upon heating at high pressure. For wadsleyite, upon compression to ~18.8 GPa, the frequencies of the bands at ~3600 cm –1 remain almost unchanged, while the main bands at 3200–3400 cm –1 shift to lower frequencies. During heating at 14.2 GPa to 650 °C the bands at 3200–3400 cm –1 broaden and shift to slightly lower frequencies. For ringwoodite, upon compression to ~21.5 GPa, the main bands at 3115 cm –1 progressively shift to lower frequencies. During heating at 18.4 GPa to 900 °C, no frequency shift was observed for the band at ~3700 cm –1 , but the band initially at ~3115 cm –1 shifts very slightly to higher frequencies, which should yield almost the same band positions at 1300–1400 °C as those measured at ambient conditions. Our data suggest that water speciation in hydrous wadsleyite and ringwoodite at ambient conditions may be comparable to that under mantle conditions, except perhaps for subtle changes in hydrogen bonding. The low OH-stretching frequencies in wadsleyite and ringwoodite under transition zone conditions imply a large H/D fractionation during degassing of the deep mantle. This may explain the apparent disequilibrium between the hydrogen isotopic composition of the upper mantle and the ocean.

Description: High-pressure behavior of coesite was studied on single crystals using diamond-anvil cells with neon as the pressure-transmitting medium by means of in situ Raman spectroscopy up to pressures of ~51 GPa. The experimental observations were complemented with theoretical computations of the Raman spectra under similar pressure conditions. We find that coesite undergoes two phase transitions and does not become amorphous at least up to ~51 GPa. The first phase transition (coesite I to coesite II) is reversible and occurs around 23 GPa. The second transition (coesite II to coesite III) at about 35 GPa is also reversible but involves a large hysteresis. Samples recovered from the highest pressure achieved, ~51 GPa, show Raman spectra of the initial coesite. The ab initio calculations gave insight into the initiation mechanism of the first phase transition, implying, from the analysis of unstable phonon modes, that it is probably a displacive phase transition due to shearing of the four-membered rings of SiO 4 tetrahedra upon compression. The transition to the lowest-symmetry phase, coesite III, is possibly a first-order phase transition that leads to a very distinct structure. None of the metastable high-pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure-induced amorphization at significantly lower pressures (30 GPa). The study of the high-pressure behavior of coesite is important to better constrain the metastable phase diagram of silica. Further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms. Crystalline or amorphous metastable phases derived from coesite under high-pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures (stress) reached in processes such as impacts or faulting.

Description: We report results from high-pressure single-crystal X-ray diffraction and Mössbauer absorption experiments on magnetite. Based on high-quality diffraction data, we have obtained accurate information on the crystal structure of magnetite below 25 GPa, which enables an unambiguous interpretation of the Mössbauer data using constrained area ratios and a full transmission integral fit that avoids area distortion due to thickness effects. Based on our analysis, all aspects of the electronic and magnetic properties of magnetite reported previously below 25 GPa at ambient temperature can be explained solely by the enhanced delocalization of 3d electrons of iron atoms. For instance, we present evidence that the compression-induced metallization changes the sign of the charge carrier spin polarization at 15 GPa.

Description: Members of the edgrewite Ca 9 (SiO 4 ) 4 F 2 -hydroxyledgrewite Ca 9 (SiO 4 ) 4 (OH) 2 series, structural analogues of clinohumite-hydroxylclinohumite series, Mg 9 (SiO 4 ) 4 (F,OH) 2 , were discovered in xenoliths of carbonate-silicate rock altered to skarn within ignimbrites of the Upper Chegem volcanic structure, Kabardino-Balkaria, Northern Caucasus, Russia. The new minerals occur sparingly in zones containing bultfonteinite, hillebrandite, jennite, and chegemite, as well as rare relics of larnite and rondorfite enclosed in a matrix of hydroxylellestadite. Edgrewite and hydroxyledgrewite are largely altered to jennite in places with admixed zeophyllite and trabzonite, and are preserved as elongate relics mostly 0.1–0.4 mm long in the central part of atoll-like pseudomorphs. The new minerals form a solid-solution series Ca 9 (SiO 4 ) 4 (F,OH) 2 , in which the content of the edgrewite end-member Ca 9 (SiO 4 ) 4 F 2 ranges from 74% (F = 3.64 wt%) to 31% (F = 1.52 wt%). Structure refinement of crystals containing 51% and 37% of the edgrewite end-member gave, respectively, R 1 = 3.03%, space group P 2 1 / b 11 (no. 14), Z = 2, a = 5.06870(10), b = 11.35790(10), c = 15.4004(2) Å, α = 100.5980(10)°, V = 871.47(3) Å 3 ; and R 1 = 1.61%, space group P 2 1 / b 11 (no. 14), Z = 2, a = 5.06720(10), b = 11.35450(10), c = 15.3941(2) Å, α = 100.5870(10)°, and V = 870.63(2) Å 3 . Minerals of the edgrewite-hydroxyledgrewite series are colorless, optically biaxial (+), 2 V meas = 80(5)°; 2 V calc = 78.7°; dispersion r 〉 v , medium; orientation: Z = a , X ^ c = 12(2)°; edgrewite: α = 1.621(2), β = 1.625(2), = 1.631(2); hydroxyledgrewite: α = 1.625(2), β = 1.629(2), = 1.635(2) (589 nm). The micro-hardness VHN 50 = 352–366 kg/mm 2 corresponds to the Mohs scale of 5.5–6. 5. FTIR spectra of edgrewite and hydroxyledgrewite show resolved bands at (edgrewite/hydroxyledgrewite, cm –1 ): 3558 and 3551 and 3543/3554, absent/3486, 1075/1075, 996/996, 980/982, 934/933, 917/918, 904/903, 890/884, 864/864, 842/842, 818/820. Raman spectra are characterized by the following bands (edgrewite/hydroxyledgrewite, cm –1 ) at: 921/923, 889/890, 839/840, and 815/814 (SiO 4 stretching), at: 556/559, 527/527, 423/419, 406/404, and 394/394 (SiO 4 bending), 309/295, 269/256, and 163/166 (CaO 6 ). In the OH stretching region three bands are noted at 3554, 3547, and 3540 cm –1 for edgrewite and two – 3550 and 3475 cm –1 for hydroxyledgrewite confirming the corresponding IR spectra. The major difference in Raman and IR spectra of edgrewite and hydroxyledgrewite is the presence of two resolved peaks in the OH stretching region at ca. 3550 and 3480 cm –1 for hydroxyledgrewite.

Description: Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on synthetic pure clinoferrosilite, Fe 2 Si 2 O 6 , at room temperature to a maximum pressure of 45 GPa. In addition to the previously described P 2 1 / c -〉 C 2/ 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 P 2 1 / c phase (HP- P 2 1 / c ). The C 2/ c -〉 HP- P 2 1 / c transition is induced by rearrangement of half of the layers of corner-sharing SiO 4 tetrahedra into layers of edge-sharing SiO 6 octahedra. The new configuration of VI Si 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- P 2 1 / c .

Description: 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 Fe 2+ and Si 4+ for Fe 3+ 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 Fe 3+ /Fe 2+ 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.