Barbara Lavina

Siderite at lower mantle conditions and the effects of the pressure-induced spin-pairing transition

evidenced by a sharp volume collapse of about 10%. The initially colorless crystals assume an intense green color after the transition, which progressively turns to red above 60 GPa. We present clear evidence for the instability of an intermediate spin state in siderite at ambient temperature. At the transition pressure, domains of high and low spin siderite coexist. The unit cell volume difference between magnesite and siderite is significantly decreased by the spin transition, enhancing the solubility between the two calcite-type minerals. A siderite component in magnesite at lower mantle pressure would significantly increase its density and slightly increase the carbonate bulk modulus. Citation: Lavina, B., P. Dera, R. T. Downs, V. Prakapenka, M. Rivers, S. Sutton, and M. Nicol (2009), Siderite at lower mantle conditions and the effects of the pressure-induced spinpairing transition, Geophys. Res. Lett., 36, L23306, doi:10.1029/ 2009GL039652.

An experimental study of the oxidation state of vanadium in spinel and basaltic melt with implications for the origin of planetary basalt

Abstract The distribution of V in magmatic rocks is controlled primarily by spinel stability. Extensive previous experimental work at oxidized conditions on doped (V-rich) compositions has led to the recognition of the importance of temperature, oxygen fugacity, and spinel composition, but also left ambiguity with respect to the relative importance of these variables in controlling DVspinel/melt. One major uncertainty has been the valence of V in the spinel and glass. Spinel-melt pairs were equilibrated at low and variable oxygen fugacities, with a range of V and Ti contents. XANES spectra were measured on the spinel and glass products, and pre-edge peaks measured and calibrated against valence with the use of glass and oxide standards. The valence of V is always greater in the glass than in the spinels. In spinel, V is dominantly 3+ at oxygen fugacities near the FMQ (fayalite magnetite quartz) buffer, but we find evidence for mixed 3+, 4+, and 5+ at oxidized conditions (FMQ to air), and 2+ and 3+ at very reduced conditions [FMQ to IW-1 (1 log fO₂ unit below the iron wüstite buffer)]. Increased V contents in spinels are correlated with increased DVspinel-melt, at constant temperature and oxygen fugacity. However, increased Ti content causes only a slight decrease in DVspinel-melt and a shift to more reduced V (smaller pre-edge peak), which may be related to Fe-V exchange equilibria. Using the new partition coefficients, together with published results and valence information, expressions have been derived to predict DVspinel/melt for basaltic systems. Application of these expressions to natural suites illustrate their utility and also the great range of DVspinel/melt values relevant to natural systems. Calculation of V depletions in planetary mantles from basalt suites must take silicate, oxide, and metal fractionation into account, as is demonstrated using terrestrial, lunar, martian, and eucritic samples.

Discovery of the recoverable high-pressure iron oxide Fe4O5

Phases of the iron–oxygen binary system are significant to most scientific disciplines, directly affecting planetary evolution, life, and technology. Iron oxides have unique electronic properties and strongly interact with the environment, particularly through redox reactions. The iron–oxygen phase diagram therefore has been among the most thoroughly investigated, yet it still holds striking findings. Here, we report the discovery of an iron oxide with formula Fe4O5, synthesized at high pressure and temperature. The previously undescribed phase, stable from 5 to at least 30 GPa, is recoverable to ambient conditions. First-principles calculations confirm that the iron oxide here described is energetically more stable than FeO + Fe3O4 at pressure greater than 10 GPa. The calculated lattice constants, equation of states, and atomic coordinates are in excellent agreement with experimental data, confirming the synthesis of Fe4O5. Given the conditions of stability and its composition, Fe4O5 is a plausible accessory mineral of the Earth’s upper mantle. The phase has strong ferrimagnetic character comparable to magnetite. The ability to synthesize the material at accessible conditions and recover it at ambient conditions, along with its physical properties, suggests a potential interest in Fe4O5 for technological applications.

Single-crystal X-ray diffraction of spinels from the San Carlos Volcanic Field, Arizona: Spinel as a geothermometer

Abstract Fourteen spinels from two types of mantle xenoliths from the San Carlos Volcanic Field in Arizona were characterized using single-crystal X-ray diffraction and electron microprobe analysis. The dominant feature seen in the chemistry of spinels from the Group I xenoliths is the extensive substitution of Cr for Al (Cr0.20Al1.76 to Cr0.83Al1.10) correlated with Mg for Fe2+ (Mg0.69Fe2+ 0.31 to Mg0.80Fe2+0.19). Although Group II spinels display consistently low Cr values, they also show a well-correlated substitution of Mg for Fe2+ (Mg0.63Fe2+ 0.37 to Mg0.69Fe2+0.31). Unit-cell parameters for spinels from the Group I xenoliths range from 8.1259 to 8.2167 Å, while those from the Group II xenoliths range from 8.1247 to 8.1569 Å. The cell parameters are linearly correlated with Fe2+ and Cr contents. Cation distributions were determined from experimental bond lengths and refined site occupancies using the algorithm of Lavina et al. (2002). The San Carlos spinels display variable degrees of order, with inversion parameters ranging from 0.10 to 0.16 for Group I and from 0.17 to 0.22 for Group II. Closure temperatures were computed with the Princivalle equation, giving averages of 808(37) °C for spinels from Group I xenoliths and 822(62) °C for samples from Group II xenoliths. We show that these results are reasonable, and thus extend the use of the Princivalle equation, or at least its functional form, to samples with significant Cr and Fe2+ contents. This study demonstrates that, in spite of the extensive chemical variability of the San Carlos spinels, and given that the origins of the two groups of xenoliths are different, the oxygen coordinates remain fixed, suggesting that the oxygen coordinate is a function of thermal history.

Unraveling the complexity of iron oxides at high pressure and temperature: Synthesis of Fe5O6

A novel iron oxide obtained at mantle’s conditions could play a critical role in processes shaping planets from their interiors. The iron-oxygen system is the most important reference of rocks’ redox state. Even as minor components, iron oxides can play a critical role in redox equilibria, which affect the speciation of the fluid phases chemical differentiation, melting, and physical properties. Until our recent finding of Fe4O5, iron oxides were assumed to comprise only the polymorphs of FeO, Fe3O4, and Fe2O3. Combining synthesis at high pressure and temperature with microdiffraction mapping, we have identified yet another distinct iron oxide, Fe5O6. The new compound, which has an orthorhombic structure, was obtained in the pressure range from 10 to 20 GPa upon laser heating mixtures of iron and hematite at ~2000 K, and is recoverable to ambient conditions. The high-pressure orthorhombic iron oxides Fe5O6, Fe4O5, and h-Fe3O4 display similar iron coordination geometries and structural arrangements, and indeed exhibit coherent systematic behavior of crystallographic parameters and compressibility. Fe5O6, along with FeO and Fe4O5, is a candidate key minor phase of planetary interiors; as such, it is of major petrological and geochemical importance. We are revealing an unforeseen complexity in the Fe-O system with four different compounds—FeO, Fe5O6, Fe4O5, and h-Fe3O4—in a narrow compositional range (0.75 < Fe/O < 1.0). New, finely spaced oxygen buffers at conditions of the Earth’s mantle can be defined.

Effect of dilution on the spin pairing transition in rhombohedral carbonates

The compressibility of an iron-bearing magnesite was determined by means of single crystal diffraction up to 64 GPa. Up to 49 GPa the pressure-evolution of the unit cell volume of the solid solution with 12% of Fe2+ can be described by a third-order Birch–Murnaghan equation of state with parameters V 0=281.0(5) Å3, K 0=102.8(3) GPa, K . The spin pairing of the Fe2+ d-electrons occurs between 49 and 52 GPa, as evidenced by a discontinuous volume change. The transition pressure is increased by about 5 GPa compared with the iron end-member; an effect consistent with a cooperative contribution of adjacent clusters to the spin transition. The trend is, however, opposite in the periclase–wüstite solid solution. Differences among the two structures, in particular in the Fe–Fe interactions, that might explain the different behavior are discussed.

High-Pressure Structural, Elastic, and Thermodynamic Properties of Zircon-Type Hopo4 and Tmpo4

Zircon-type holmium phosphate (HoPO4) and thulium phosphate (TmPO4) have been studied by single-crystal x-ray diffraction and ab initio calculations. We report on the influence of pressure on the crystal structure, and on the elastic and thermodynamic properties. The equation of state for both compounds is accurately determined. We have also obtained information on the polyhedral compressibility which is used to explain the anisotropic axial compressibility and the bulk compressibility. Both compounds are ductile and more resistive to volume compression than to shear deformation at all pressures. Furthermore, the elastic anisotropy is enhanced upon compression. Finally, the calculations indicate that the possible causes that make the zircon structure unstable are mechanical instabilities and the softening of a silent B 1u mode.

Chemical Composition, Crystal Structure, and Their Relationships with the Intrinsic Properties of Spinel-Type Crystals Based on Bond Valences

Spinel-type crystals may possess complex and versatile chemical composition and crystal structure, which leads to difficulty in constructing relationships among the chemical composition, crystal structure, and intrinsic properties. In this work, we develop new empirical methods based on bond valences to estimate the intrinsic properties, namely, compressibility and thermal expansion of complex spinel-type crystals. The composition-weighted average of bond force constants in tetrahedral and octahedral coordination polyhedra is derived as a function of the composition-weighted average of bond valences, which can be calculated according to the experimental chemical composition and crystal structural parameters. We discuss the coupled effects of tetrahedral and octahedral frameworks on the aforementioned intrinsic properties. The bulk modulus could be quantitatively calculated from the composition-weighted average of bond force constants in tetrahedral and octahedral coordination polyhedra. In contrast, a quantitative estimation of the thermal expansion coefficient could be obtained from the composition-weighted average of bond force constants in octahedral coordination polyhedra. These empirical methods have been validated by the results obtained for a new complex quaternary spinel-type oxynitride Mg0.268Al2.577O3.733N0.267 as well as MgAl2O4 and Al2.85O3.45N0.55 from the literature. Further, these empirical methods have the potential to be extensively applied in other types of complex crystals.

Cation distribution and cooling rates of Cr-substituted Mg-Al spinel from the Olkhon metamorphic complex, Russia

A crystal chemical study was carried out on seven Mg-Al rich and Cr-bearing spinels from a calciphyre of the Olkhon metamorphic complex (western Lake Baikal, Russia) in order to: a) verify the structural effects related to the progressive substitution of Cr 3+ , the main substituent cation in this suite pertaining to the MgAl 2 O 4 - MgCr 2 O 4 binary join; b) estimate the closure temperature of the exchange reaction involving Mg and Al in tetrahedral (T) and octahedral (M) sites. This will be a first step towards evaluation of the cooling rate of the host rock. Chemical composition is Mg (Al 2-p , Cr p ) O 4 with 0.03 ≤ p ≤ 0.23 afu. With increasing p, the cell edge a increases from 8.090 to 8.117 A, whereas the oxygen positional parameter u is almost constant. As u is related to the (M-O)/(T-O) ratio, Cr substituting for Al in the M site causes an increase not only of the M-O bond distance, but also that of the tetrahedral T-O. The observed T-O increase is due to Mg ordering into the T site, with increasing Cr content. Cation distributions where used to estimate the closure temperature of the exchange reaction applying two available models. Both estimates are rather consistent for very low Cr content, but substantial disagreement is observed for Cr-rich samples. The inversion trend of the studied spinel suite extrapolated to Cr= 0, enables us to calculate the closure temperature of the MgAl 2 O 4 end-member with the same cooling rate. This suggests that kinetic results from cooling experiments on MgAl 2 O 4 may be applied to cation distributions measured in samples along the MgAl 2 O 4 - MgCr 2 O 4 binary join.

Closure temperatures of intracrystalline ordering in anatectic and metamorphic hercynite, Fe2+Al2O4

Abstract The closure temperature, TC, of the intracrystalline ordering of Mg-hercynite is estimated with a comparative crystal-chemical approach. The single crystals were selected from two distinct geological environments that represent extremely different cooling rates. The fast cooled setting refers to anatectic metapelitic enclaves that occur in the high-K calc-alkaline lavas of the Neogene Volcanic Province of SE Spain. The slow cooled setting refers to metabauxite from the Anga metamorphic complex, Lake Baikal. Parameters sensitive to TC include the oxygen fractional coordinate (u) and the inversion parameter (i). Experimental equilibration data on the spinel and hercynite end-members and on their solid solution are fitted to equations where TC is given as a function of the hercynite content (Hc) of the solid solution and of u or i. The unavoidable simplifications made in this empirical approach are discussed. A reasonable value for TC, ~400 °C, was obtained for the slow cooled metamorphic hercynite from the oxygen fractional coordinates. In contrast, an unreasonably high value of TC, ~600 °C, was obtained from the inversion parameters. In the case of the fast cooled anatectic samples, TC calculated from the two structural parameters are comparable; the five crystals show a range in the calculated values for TC over ~250 °C, from ~700 to ~950 °C, which is reasonable considering the known diversity of cooling rates exhibited by their volcanic host-rocks.