C. Michael B. Henderson

Direct determination of cation site occupancies in natural ferrite spinels by L2,3 X-ray absorption spectroscopy and X-ray magnetic circular dichroism

Abstract Cation distributions in natural ferrite spinels, some containing large amounts of Mg, Ti, Mn, and Zn, have been investigated using the element-, site-, and symmetry-selective spectroscopic techniques of L2,3 X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). By comparing XMCD data with calculated spectra, the site occupancies of the Fe cations have been determined. From the analysis of natural ferrite spinels with formulae very close to that of pure magnetite (Fe3O4), a standard XMCD spectrum for natural magnetite is proposed. Magnetites with small numbers of cation vacancies due to oxidation (solid solutions with maghemite, γ-Fe2O3) show that all the vacancies occur in octahedral sites. Ti L2,3 XAS of oxidized Ti-bearing magnetites (hereafter referred to titanomagnetites) shows that Ti is tetravalent occurring on the octahedral site with 10Dq ~2eV; Fe L2,3 XMCD spectra indicate that the vacancies occur in both tetrahedral and octahedral sites. Mn L2,3 XAS of the Mn-rich ferrite spinels shows that Mn is predominantly ordered onto the tetrahedral site with an Mn2+:Mn3+ ratio of 0.85:0.15. Mn- and Zn-rich ferrite spinels have an excess of cations over 3.0 per 4-oxygen formula unit. The sign of the XMCD for Mn corresponds to a parallel alignment of the Mn moments with the Fe3+ moments in the tetrahedral sublattice. This work demonstrates clearly that combined XAS and XMCD provides direct information on the distribution of multivalent cations in chemically complex magnetic spinels.

Fe site occupancy in magnetite-ulvöspinel solid solutions: A new approach using X-ray magnetic circular dichroism

Abstract Ordering of Fe3+ and Fe2+ cations between octahedral and tetrahedral sites in synthetic members of the magnetite-ulvöspinel (Fe3O4-Fe2TiO4) solid-solution series was determined using Fe L2,3-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L2,3-edge and Fe K-edge X-ray absorption spectroscopy (XAS), and unit-cell parameters. Microprobe analyses, cell edges, and chemical FeO determinations showed that bulk compositions were stoichiometric magnetite-ulvöspinel solid solutions. XMCD showed that the surface was sensitive to redox conditions, and samples required re-equilibration with solid-solid buffers. Detailed site-occupancy analysis gave Fe2+/Fe3+ XMCD-intensity ratios close to stoichiometric values. L2,3-edge XAS confirmed that Ti4+ was restricted to octahedral sites. XMCD showed that significant Fe2+ only entered the tetrahedral sites when Ti content was >0.40 atoms per formula unit (apfu), whereas Fe2+ in octahedral sites increased from 1 apfu in magnetite to a maximum of ~1.4 apfu when Ti content was 0.45 apfu. As Ti content increased, a steady increase in Fe2+ in tetrahedral sites was observable in the XMCD spectra, concurrent with a slow decrease in Fe2+ in octahedral sites. Calculated magnetic moments decreased rapidly from magnetite (4.06 μB) to USP45 (1.5 μB), then more slowly toward ulvöspinel (0 μB). Two synthesized samples were maghemitized by re-equilibrating with an oxidizing buffer. XMCD showed that Fe2+ oxidation, with concomitant vacancy formation, was restricted to octahedral sites. Through the direct measurement of Fe oxidation states, XMCD results can be used to rationalize the magnetic properties of titanomagnetites, along with oxidized titanomaghemitized analogs, in Earth’s crustal rocks.

In-situ synchrotron study of the kinetics, thermodynamics, and reaction mechanisms of the hydrothermal crystallization of gyrolite, Ca16Si24O60(OH)8·14H2O

Abstract The hydrothermal crystallization of gyrolite was studied dynamically at 190-240 °C using synchrotron- based in situ Energy Dispersive Powder Diffraction (EDPD). The reaction mechanism involves the initial crystallization of a calcium silicate hydrate (C-S-H) gel, which has a sheet structure with well ordered Ca(O,OH) layers and disordered silicate layers. This is followed by the intermediate formation of Z-phase which finally transforms to gyrolite. This process involves ordering of the silicate layers and an increase in the order along c. Kinetics data for all stages of the crystallization process were determined by analyzing the growth and decline of various diffraction peaks with time. The activation energy (Ea) (nucleation) for Zphase is ~39 kJ/mol while that for gyrolite is ~56 kJ/mol. Ea (crystallization) of gyrolite is higher at ~80 kJ/mol. The reaction occurs via a two-dimensional, diffusion-controlled mechanism and is a continuous process that suggests that Z-phase is an unstable, transient phase.

Composition and temperature dependence of cation ordering in Ni-Mg olivine solid solutions: a time-of-flight neutron powder diffraction and EXAFS study

Abstract The non-convergent ordering of Mg and Ni over the M1 and M2 sites of synthetic olivines has been studied using “time of night” neutron powder diffraction and X-ray absorption spectroscopy (EXAFS). The compositional dependence of order/disorder at room temperature was established for solid solutions of general formula (Mg1-xNix)2SiO4. where X= 0.15, 0.2, 0.25,0.3, 0.5, and 0.8 atoms Ni (XNi; i.e.. mole fraction of Ni-olivine end-member). Ni orders into M1 with KD = (Ni/Mg in M1)/ (Ni/Mg in M2) reaching a maximum of 9.5 at a composition of Mg1.0Ni0.4SiO4. The temperature dependence of order/disorder at up to 1100 °C was determined for two samples (XNi = 0.2 and 0.5). Between about 600 and 750 °C the samples show an increase in order due to kinetic effects, while above 750 °C the samples show a progressive decrease in order and describe an equilibrium disordering path. Equilibrium data define a Ni-Mg, M1-M2 intersite exchange energy of 21.5 ± 1.9 kJ/ mol. On cooling, the blocking temperature for cation exchange is about 800 °C. The kinetics of disordering behavior were analyzed using a Ginzburg-Landau model giving activation energies for Mg-Ni exchange between M1 and M2 for samples of composition Mg1.6Ni0.4SiO4 and Mg1.0Ni1.0SiO4 of 145 ± 5 and 160 ± 5 kJ/mol. respectively. The model also shows that the characteristic time scale for re-equilibration of M1-M2 order decreases from around 2.5 s at 1000° to 0.03 s at 1300 °C. This points to the inapplicability of intracrystalline Ni-Mg partitioning for obtaining geothermometry and geospeedometry information for magmatic conditions. Ni X-edge EXAFS data show that samples with XNi = 0.15.0.2,0.25 and 0.3 all show Ni clustering on adjacent M1 sites, indicating the presence of domains of Ni-rich and Mg-rich regions on a nanolength scale of < 10 A. These “precipitates” are at least an order of magnitude too small to be detectable by neutron powder diffraction. We suggest that the elastic strain at the interfaces between the Ni-rich precipitates and the Mg-rich matrix is responsible for the plateau or possible maximum in the b unit-cell parameter as a function of composition across the solid solution, which is observed at a composition of Mg1.6Ni0.4SiO4 at room temperature. Comparison of our data with earlier studies at high P and T on Mg1.0Ni1.0SiO4 olivine suggests that the effect of P is to increase the degree of order of Ni into M1 and to slow down the kinetics of intersite exchange with a ΔVdisorder of 0.039 J/bar.

Cation ordering in MgTi2O5 (karrooite): Probing temperature dependent effects with neutrons

Abstract MgTi2O5 (karrooite) exhibits cation exchange between the two non-equivalent octahedral M1 and M2 sites. The temperature dependence of Ti-Mg disorder has been determined using in situ timeof- fight powder neutron diffraction to establish the cation population of each site. The equilibrium Ti-Mg exchange commences above 700.800 °C, and continues up to 1300 °C. At ~1350 °C MgTi2O5 appears to undergo a reversible, displacive phase transition, although the structure can still be refined in space group 63. This transition shows discontinuities in the degree of order, the c cell parameter, and in the M-O bond lengths, quadratic elongation, and bond angle variance for the M1 octahedron, which becomes pseudo-tetrahedral. By contrast, the M2 octahedron shows no significant change. An Arrhenius plot exhibits linear behavior from 750.1300 °C, gives an exchange enthalpy of ordering of 33.6 kJ/mol, and a value of 10.7 kJ/mol for the entropy of disordering. A value of 5.92 cm3/mol is deduced for the ΔV of the high-temperature reaction of geikielite (MgTiO3) + rutile (TiO2) to form fully disordered MgTi2O5 at an extrapolated temperature of 1860 °C.

Structural basis for the anomalous low-temperature thermal expansion behaviour of the gillespite-structured phase Ba0.5Sr0.5CuSi4O10

The crystal structure of the gillespite-structured phase Ba0.5Sr0.5Cu4SiO10 has been refined from powder neutron time-of-flight data at eight temperatures between 5K and 300K. No structural phase transitions were observed on cooling to the lowest temperature. The temperature dependence of the crystal structure in the a - b plane has been found to be dominated by low energy lattice modes involving cooperative anti-phase rotations of the square-planar CuO4 group with the Si4O10 polyhedral unit which results in non-Groneisen behaviour for the a lattice parameter and negative volume thermal expansion to 100K. Concomitant with these rotations is an associated tilting of SiO4 tetrahedra about the bridging anions resulting in an increasing aplanarity of the Cu site with temperature. The behaviour of the c-axis with temperature is conventional and appears to be related to the tilting of SiO4 tetrahedra and an increase in the Ba/Sr bond distances resulting in a decrease of the overbonded bond valence sum at the Ba/Sr site.

Monoclinic-orthorhombic phase transition in the K 2 MgSi 5 O 12 leucite analog

Abstract The high-temperature cell parameters of K2MgSi5O12have been measured by X-ray powder diffraction between room temperature and 842 K. A first-order ferroelastic phase transition between the room-temperature P21/c structure and a high-temperature Pbca structure occurs at 622 K. The spontaneous strain behaves as the order parameter for the transition, following a simple Landau model. Simple lattice dynamics simulation of the magnesiosilicate framework reveals that acoustic mode softening drives the elastic instability. The first-order character of the transition is attributed to a large excess volume, which arises from collapse of the framework around the alkali site. At higher temperatures the orthorhombic cell edges converge toward a metrically cubic structure, but this transformation requires Mg-Si disorder, which was not observed on the time scale of these experiments.

Structural variations in the wesselsite–effenbergerite (Sr1−xBaxCuSi4O10) solid solution

The crystal structures of eight tetragonal, gillespite-structured phases in the effenbergerite(BaCuSi4O10)–wesselsite (SrCuSi4O10) solid solution (Sr1− x Ba x CuSi4O10, where x is the mol fraction of the Ba end-member), have been refined from powder, neutron time-of-flight, diffraction data. The accommodation of the larger, more electropositive Ba2+ cation within the crystal structure of SrCuSi4O10 is achieved by increasing the magnitude of the rotation of the square-planar CuO4 group about the c axis, coupled with an anti-phase rotation, and concomitant tilting, of the Si4O10 polyhedral unit. To an excellent approximation, these structural changes are equivalent to a rigid sphere substitution, the radius of which is given by the compositionally averaged ionic radii of Sr2+ and Ba2+. The compositional-dependence of the lattice parameter c is significantly larger than that for a at low values of x , and is particularly well parameterised in terms of the variations of the calculated ionic radius of the alkaline-earth site and the observed tilt of the SiO4 tetrahedron. The lattice parameter a exhibits a negative deviation from Vegard’s rule resulting from the more complex, coupled structural response to the change in the effective ionic radius at the Sr/Ba site.

A simple predictive model for the thermal expansion of AlSi3 feldspars

Abstract We have investigated the thermal expansion of several synthetic feldspars, including Li-feldspar, rubicline (Rb-microcline), Rb-sanidine, and buddingtonite (NH4-feldspar). When analyzed in conjunction with earlier data on both ordered and disordered Na- and K-feldspars, it is clear that the coefficient of thermal expansion (α) decreases dramatically, and linearly, with increasing room-temperature volume. For “AlSi3” feldspars, then, chemical expansion limits thermal expansion. The relationship between α and room-temperature volume provides a useful predictive tool based simply on the volume of a feldspar at room temperature. This relationship also reveals that volumes of K-Na mixing in naturally occurring alkali feldspars decrease with increasing temperature.

An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl₂O₄ – Fe₃O₄ (spinel – magnetite) solid solution series; implications for magnetic properties.

Abstract Fe L2,3-edge XAS and XMCD studies have been used to unravel structural trends in the MgAl2O4–Fe3O4 solid solution where thermodynamic modeling has presented a challenge due to the complex ordering arrangements of the end-members. Partitioning of Fe3+ and Fe2+ between tetrahedral (Td) and octahedral (Oh) sites has been established. In the most Fe-rich samples, despite rapid quenching from a disordered state, FeTd2+