Laurence A. J. Garvie

Ratios of ferrous to ferric iron from nanometre-sized areas in minerals

Minerals with mixed valence states are widespread and form in many different rock types. They can contain, for example, Fe2+–Fe3+ and Mn2+–Mn3+–Mn4+, with the ratios of oxidation states reflecting the redox conditions under which the host materials crystallized. The distribution of the ratio of iron (III) to total iron content (Fe3+/ΣFe) in minerals reflects the oxidation states of their host rocks and is therefore important for answering fundamental questions about the Earths evolution and structure. Iron is the most sensitive and abundant indicator of oxidation state, but many mineral samples are too fine-grained and heterogeneous to be studied by standard methods such as Mössbauer spectroscopy, electron microprobe, and wet chemistry. Here we report on the use of electron energy-loss spectroscopy with a transmission electron microscope to determine Fe3+/ΣFe in minerals at the nanometre scale. This procedure is efficient for determining Fe3+/ΣFe ratios of minor and major amounts of iron on a scale heretofore impossible and allows information to be obtained not only from ultra-fine grains but also, for example, at reaction fronts in minerals.

Icosahedral packing of B12 icosahedra in boron suboxide (B6O)

Objects with icosahedral symmetry (Ih) bear a special fascination; natural examples are rare, but include radiolaria and virus particles (virions). The discovery of C60, a molecule in the shape of a truncated icosahedron with Ih symmetry, has aroused widespread interest. In 1962, Mackay described a radiating packing of spheres in Ih symmetry, in which the centres of successive shells of spheres lie on the surfaces of icosahedra. There has been extensive investigation of the conditions under which such packing might be realized in assemblies of atoms or of molecules such as C60 (ref. 5). Here we report the preparation, at high temperatures and pressures, of boron suboxide (B6O) in which the preferred form of the material is as macroscopic, near-perfect, regular icosahedra, similar to the multiply-twinned particles observed in some cubic materials. A major difference is that B6O has a rhombohedral structure that nearly exactly fits the geometrical requirements needed to obtain icosahedral twins. These icosahedral particles have a structure that can be described as a Mackay packing of icosahedral B12 units, and thus has long-ranged order without translational symmetry.

Determination of Ce4+/Ce3+ in electron-beam-damaged CeO2 by electron energy-loss spectroscopy

Abstract We followed the reduction of Ce 4+ in CeO 2 by observing changes in the shape of the Ce M 4,5 edge by parallel electron energy-loss spectroscopy with a transmission electron microscope. The energy-loss near-edge structure of the beam-damaged CeO 2 exhibits Ce M 4,5 and O K-edge shapes that are consistent with reduction to a Ce 3+ oxide. During the damage process the spectrum of CeO 2 changes as follows: (a) decreases in energies of the M 5 and M 4 maxima; (b) changes in shape of the near-edge structure; (c) inversion of the M 5 to M 4 branching ratio; and (d) increase in the M 5 to M 4 area ratio. We simulated M 4,5 edges of damaged CeO 2 with a linear combination of Ce 4+ and Ce 3+ spectra and observed no intermediate oxidation states.

Complex heterogeneous precipitation in titanium-niobium microalloyed Al-killed HSLA steels - I. (Ti,Nb)(C,N) particles

Abstract Precipitation in Ti–Nb Al-killed microalloyed HSLA steels (Ti/N weight ratio from 4.4 to 1) was investigated in both the as-rolled and the normalised conditions using analytical electron microscopy including parallel electron energy loss spectroscopy (PEELS). An extensive formation of heterogeneously nucleated complex (Ti,Nb)(C,N) particles down to 10 nm in size was observed. The core of such a complex particle is based on TiN and has a spherical, cubic or cruciform shape. The N/(Ti+Nb) atomic ratio in the core is similar to the average value in the steel whereas the Nb/Ti ratio is much smaller than the average value and not proportional to it. Many of the cores have caps in the form of epitaxial overgrowths based on NbC. Their composition changes from Nb(C,N) to (Nb,Ti)C as the N/Ti ratio decreases. The formation of these complex particles and their detailed morphology are controlled by the processing conditions as well as the overall composition.

Bonding in silicates; investigation of the Si L (sub 2,3) edge by parallel electron energy-loss spectroscopy

For two series of aluminosilicate glasses on the SiO2-NaAlO2 and SiO2-CaAl2O4 joins, Si magicangle-spinning (MAS) NMR spectra were measured. Systematic variations in peak positions and widths with composition are closely related to the extent of ordering of Si and Al cations. A statistical thermodynamic model based on the quasi-chemical approximation was formulated to calculate the proportions of SiO 4 groups with varying numbers of Al neighbors and thus to quantify the extent of ordering. Multiple spectra in each compositional series were fitted simultaneously with several peaks representing each of these structural species and with area constraints generated by the model. The extent of aluminum avoidance (Q), which was defined using the relative lattice energy differences among the linkages Si-O-Si, Si-O-Al, and Al-O-Al, was optimized for each series. For the calcium aluminosilicates, the best fit is with 0.8 ≤ Q ≤ 0.875, where Q = 1 represents perfect Al-avoidance. For the sodium series, Q was found to be larger (0.93 ≤ Q ≤ 0.99), as expected from energetic considerations and from known variations in ordering in minerals. The contributions to the overall configurational entropy and heat capacity from Si-Al disorder can be calculated, and are significant fractions of experimentally estimated values. However, major contributions must also come from other sources of disorder, such as “topological” disorder of bond angles and length.

Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material

Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroidal impacts. It is thought to play a central role during the graphite-to-diamond transformation, and calculations suggest that it possesses mechanical properties superior to diamond. However, despite extensive efforts, lonsdaleite has never been produced or described as a separate, pure material. Here we show that defects in cubic diamond provide an explanation for the characteristic d-spacings and reflections reported for lonsdaleite. Ultrahigh-resolution electron microscope images demonstrate that samples displaying features attributed to lonsdaleite consist of cubic diamond dominated by extensive {113} twins and {111} stacking faults. These defects give rise to nanometre-scale structural complexity. Our findings question the existence of lonsdaleite and point to the need for re-evaluating the interpretations of many lonsdaleite-related fundamental and applied studies.

Decay-induced biomineralization of the saguaro cactus (Carnegiea gigantea)

Abstract The saguaro, Carnegiea gigantea (Englemann), is a columnar cactus that grows to 15 m tall and weighs up to several tons, of which 85 to 90% of the mass is water. Roughly 18% of the dry mass consists of the biomineral weddellite (CaC2O4·2H2O). The C in the weddellite derives from atmospheric CO2 via photosynthesis. A mature saguaro can contain on the order of 1 × 105 g of weddellite. The weddellite crystals occur as aggregates up to 1 mm wide. After the death of the saguaro, a series of minerals crystallize in the rotting flesh. These minerals form from elements released from the decay of the cactus by microorganisms and thus is a type of biologically induced mineralization. During the initial stages of decay, authigenic Mg- and Ca-bearing minerals crystallize from elements released by the putrefying flesh and include lansfordite (MgCO3·5H2O), nesquehonite (MgCO3·3H2O), several polymorphs of MgC2O4·2H2O including glushinskite, monohydrocalcite (CaCO3·H2O), calcite, vaterite, and several unidentified Mg-bearing phases. As the saguaro decays, the soft, water-rich pith shrinks, but the ribs and skin remain intact, producing warm, moist pockets within the dead saguaro. Abundant, glassy lansfordite crystals to 1 mm in diameter grow in these pockets during the cooler winter months. Further decay leaves a dried hollow shell covered by the saguaro skin, inside of which nesquehonite and monohydrocalcite crystallize. Lansfordite and nesquehonite are unstable in the desert and rapidly amorphize after exposure to the atmosphere. Magnesium oxalates are locally abundant in the decayed flesh and occur as crystals up to 1.5 mm in length. The common occurrence of fungal hyphae on the glushinskite suggests that it forms as a result of the reaction between oxalic acid released by fungi and the Mg-rich solutions of the rotting saguaro. During the final stages of decay, the pith consists of a pale-brown to tan-colored sand of weddellite and its transformation product monohydrocalcite. This sand lithifies to porous spongelike masses during the final stages of saguaro decay. This monohydrocalcite further alters to calcite. The δ13CVPDB of the monohydrocalcite and calcite after weddellite range from -1.65 to + 0.76‰. The calcite is subsequently solubilized and remobilized, precipitating as caliche in the desert soil, or redistributed by wind. In arid environments, the desert fauna metabolize the atmospheric C bound in the organic matter to CO2. In contrast, decay of the saguaro adds atmospheric C to the soil as inorganic C via the transformation of the biomineral weddellite to calcite. In areas with high saguaro density, it is estimated that up to 2.4 g/m2/yr of calcite can be added to the desert from the decayed cacti. This inorganic C has geologically long soil residence times, thus effectively sequestering the atmospheric C.

Review of methods for calculating near edge structure

Electron energy loss inner shell near edge structures within about 30 eV of threshold can give information on charge redistribution, conduction band changes, coordination and structure changes on a local scale. A theoretical understanding is necessary to go beyond empirical rules based on fingerprinting. All approaches to near edge structure calculation in solids are derived from various band theory methods with varying degrees of approximation. There is a place for theories based on simple physical ideas as well as large sophisticated calculations. In many oxides estimates of the number of peaks and their positions can be based on an extension of single scattering EXAFS theory to include strong second order intrashell scattering. These effects will be illustrated in MgO and NiO. In other cases it is necessary to use band structure calculations to give either projected densities of states or wave functions from which matrix elements can be calculated directly. Examples of this approach will show results of calculations for Si, diamond and SiC using the Cambridge plane wave pseudopotential code (CASTEP).

Crystal structure of kanemite, NaHSi2O5•3H2O, from the Aris phonolite, Namibia

Abstract Kanemite was studied by single-crystal X-ray diffraction. The mineral, ideally NaHSi2O5·3H2O, is orthorhombic (space group Pbcn); unit-cell parameters are a = 4.946(3), b = 20.502(15), c = 7.275(3) Å, with Z = 4. The structure is solved and refined to an R value of 0.058 for 825 independent reflections. The arrangement of atoms consists of alternating (010) sheets of corrugated [Si2O4OH]nn- and hydrated Na. The silicate sheets contain six-membered rings of HOSiO3-SiO4 units. Sodium atoms coordinate to six water molecules, forming layers of distorted octahedra. Residual electron densities were located that give reasonable positions for four H atoms. One H is part of a silanol group, and the other three H atoms are associated with water bonded to Na. Bonding between the silicate and Na sheets is through hydrogen bonding from H of the Na layer to O of the silicate sheet.

Parallel electron energy-loss spectroscopy (PEELS) study of B in minerals; the electron energy-loss near-edge structure (ELNES) of the B K edge

Abstract The B K-edge spectra of a wide variety of minerals have been measured using the technique of parallel electron energy-loss spectroscopy (PEELS) conducted in a scanning transmission electron microscope (STEM) from sample areas of nanometer dimensions. The B K edges of the minerals exhibit electron energy-loss near-edge structure (ELNES) characteristic of B coordination. For threefold-coordinated B ([3]B), the spectra are dominated by a sharp peak at ca. 194 eV because of transitions to unoccupied states of π* character, followed by a broader peak at ca. 203 eV attributed to states of σ* character. The ELNES on the B K edge (B K ELNES) of fourfold-coordinated B ([4]B) consists of a sharp rise in intensity with a maximum at ca. 199 eV followed by several weaker structures. For [4]B, the ELNES is interpreted as transitions to states of antibonding σ* character. Minerals that possess both [3]B and [4]B exhibit an edge shape that is composed of B K edges from the respective BO3 and BO4 units, and we demonstrate the feasibility of quantification of relative site occupancies in minerals containing a mixture of B coordinations. The origins of the B K ELNES are discussed in terms of both molecular orbital (MO) and multiple scattering (MS) theory. We also present the B K-edge spectra of selected nonminerals and show how differences in edge shapes and energy onsets allow these nonminerals to be readily distinguished from borates and borosilicates.