Caroline A. Kirk

Dehydration of Ca-montmorillonite at the crystal scale. Part I: Structure evolution

Abstract The dehydration dynamics of the of the Ca-saturated <1 μm size fraction of SWy-1 (low-charge montmorillonite) were studied at the crystal scale under isothermal conditions using X-ray diffraction with a position-sensitive detector (XRD-PSD) in the 30.170 °C temperature range. A total of 630 XRD patterns were modeled between 30 and 125 °C using a trial-and-error approach based on the direct comparison of experimental and calculated XRD patterns. The proportion of layers with different hydration states (bihydrated, mono-hydrated, and dehydrated) were determined in the temperature-time space as well as small variations of layer thickness within each hydration state. The results showed that dehydration produces complex structures with heterogeneous hydration states, some of which are stable (not transient) and remain at the end of the experiment. The evolution of other structural parameters (interlayer water content, layer thickness fluctuation) was consistent with previous reports of smectite hydration. For bihydrated layers, the amount of water molecules per interlayer cation indicated the presence of water molecules both coordinated and non-coordinated to the interlayer cation. The transition from bi- to mono-hydrated layers produced the maximum structural heterogeneity, with (1) strong interlayer thickness fluctuation (in individual layers), and (2) the presence of several elementary mixed-layer structures. In contrast, the transition from mono-hydrated to dehydrated layers occurs homogeneously within layers. Finally, the decrease in thickness of mono-hydrated layers only implied the removal of some water molecules forming the hydration shell of the interlayer cation.

Dehydration of Ca-montmorillonite at the crystal scale. Part 2. Mechanisms and kinetics

Abstract A kinetic study of Ca-montmorillonite dehydration was performed based on information derived from X-ray diffraction (Ferrage et al. 2007, this issue) and, thus, focusing on interlayer water only. The dehydration was quantified following the two processes that were observed in the X-ray pattern modeling: the transitions between the different hydration states and small thickness decrease observed in the bi- and mono-hydrated layers. The thickness decrease of bihydrated layers with dehydration (activation energy Ea = 16 kJ/mol) was found to be controlled by a mechanism of two-dimensional diffusion of water molecules through the interlayer space, whereas for mono-hydrated layers the variation of thickness (Ea = 18 kJ/mol) occurred as a mechanism of slight local layer collapse and collapse propagation, attributed to a rearrangement of the configuration of the interlayer cation hydration shell. For the transition between the bi- and mono-hydrated state (Ea = 84 kJ/mol), the mechanism of reaction was found to evolve gradually with increasing temperature from local layer collapse and collapse propagation to a two-dimensional diffusion mechanism, as the forced diffusion of water molecules produced by the layer collapse transfers the control of the process to diffusion mechanism. This phenomenon causes the coexistence of two hydration states in a given interlayer. Finally, the transition between mono-hydrated and dehydrated layers (Ea = 132 kJ/mol) indicated the concomitance of water diffusion and local layer collapse and propagation mechanisms, although the structures were found to be homogeneous during this transition. The determination of both mechanisms and the activation energy for these processes were used to establish a model of smectite dehydration at the crystal scale. This model can be used to calculate crystal shrinkage and interlayer water content upon dehydration, and to predict the evolution of the system.

The crystal structure and chemistry of mereheadite

Abstract The crystal structure of mereheadite (monoclinic, Cm, a = 17.372(1), b = 27.9419(19), c = 10.6661(6) Å, β = 93.152(5)°, V = 5169.6(5) Å3) has been solved by direct methods and refined to R1 = 0.058 for 6279 unique observed reflections. The structure consists of alternating Pb-O/OH blocks and Pb-Cl sheets oriented parallel to the (201) plane and belongs to the 1:1 type of lead oxide halides with PbO blocks. It contains 30 symmetrically independent Pb positions, 28 of which belong to the PbO blocks, whilst two positions (Pb12 and Pb16) are located within the tetragonal sheets of the Cl− anions. Mereheadite is thus the first naturally occurring lead oxychloride mineral with inter-layer Pb ions. The coordination configurations of the Pb atoms of the PbO blocks are distorted versions of the square antiprism. In one half of the coordination hemisphere, they are coordinated by hard O2−and OH− anions whose number varies from three to four, whereas the other coordination hemisphere invariably consists of four soft Cl− anions located at the vertices of a distorted square. The Pb12 and Pb16 atoms in between the PbO blocks have an almost planar square coordination of four Cl− anions. These PbCl4 squares are complemented by triangular TO3 groups (T = B, C) so that a sevenfold coordination is achieved. The Pb-O/OH block in mereheadite can be obtained from the ideal PbO block by the following list of procedures: (1) removal of some PbO4 groups that results in the formation of square-shaped vacancies; (2) insertion of TO3 groups into these vacancies; (3) removal of some Pb atoms (that correspond to the Pb1A and Pb2A sites), thus transforming coordination of associated O sites from tetrahedral OPb4 to triangular OHPb3; and (4) replacement of two O2−anions by one OH− anion with twofold coordination; this results in formation of the 1 × 2 elongated rectangular vacancy. The structural formula that can be derived on the basis of the results of single-crystal structure determination is Pb47O24(OH)13Cl25(BO3)2(CO3). Welch et al. (1998) proposed the formula Pb2O(OH)Cl for mereheadite, which assumes that neither borate nor carbonate is an essential constituent of mereheadite and their presence in the mineral is due to disordered replacements of Cl− anions. However, our study demonstrates that this is not the case, as BO3 and CO3 groups have well-defined structural positions confined in the vacancies of the Pb-O/OH blocks and are therefore essential constituents. Our results also show that mereheadite is not a polymorph of blixite, but is in fact related to symesite. Symesite thus becomes the baseline member of a group of structurally-related minerals.

The first environmental science experiments on the new microfocus spectroscopy beamline at Diamond

Abstract Synchrotron based μ-XRF, μ-XAS and μ-XRD have made a major impact in the field of environmental science in the last ten years. One of the first seven ‘day one’ beamlines on the Diamond Light Source is a microfocus spectroscopy beamline, beamline I18. Here the current status of the beamline and the opportunities it presents in the field of environmental science are described, with results from two of the first experiments also included. The first is based on the use of bonemeal to remediate soil. We used Zn K-edge and Pb L3-edge spectroscopy to characterize the speciation of these two elements on a soil after bonemeal treatment. The results are compared with bulk measurements taken on the whole soil and standard materials. The second experiment described here is a study of the speciation and association of Ni in a laterite from Moa Bay, Cuba. Here the differences in the Ni speciation associated with Mn oxides are examined and compared with Fe oxides phases.

The role of earthworm communities in soil mineral weathering: a field experiment

Minerals are an important component of the soil environment (Dixon et al. , 1977). They provide a structural framework which supports plants and contribute to the physical heterogeneity of soil. Minerals also contribute to the fertility of soils. Primary minerals are a source of essential plant nutrients (Harley and Gilkes, 2000). Clay minerals impact on the water-holding capacity of soils, affect the cation exchange capacity of soils (Brown, 1977) and are able to sequester nutrients and contaminants within the soil (Dubbin, 2001). Mineral weathering is an important process in soils and releases nutrients from the mineral structure into a form available for uptake by plants (Harley and Gilkes, 2000). Mineral weathering also increases the number of cation exchange sites, which increases the fertility of soils by improving nutrient retention. The weathering of clay minerals and the expansion of clay layers increases the water-holding capacity of soils (Brown, 1977). Mineral weathering also contributes to the pedological development of soils. Pedologically old soils are highly weathered and have a large proportion of clay minerals and a small proportion of primary minerals (Dubbin, 2001). Highly weathered soils, such as those found in the tropics, tend to be less fertile than less weathered soils (Dubbin, 2001). Earthworms are an important component of the soil ecosystem and have been described as ecosystem engineers because of the major role they play in modifying the soil ecosystem (Lavelle et al. , 1997). They play a key role in modifying the physical structure of soils by creating aggregates (Haynes and Fraser, 1998), creating pores which increases infiltration and drainage (Lamande et al. , 2003) and contributing to the development of soil horizons by the transport of material and incorporation of organic matter (Marhan and Scheu, 2006). Earthworms are also one of the key drivers of …

Rickturnerite, Pb7O4[Mg(OH)4](OH)Cl3, a complex new lead oxychloride mineral

Abstract Rickturnerite, which has the ideal formula Pb7O4[Mg(OH)4](OH)Cl3, is a new mineral from Torr Works (Merehead) quarry, near the village of Cranmore in Somerset, United Kingdom. It occurs as pale emerald green to grey porous aggregates of disordered interwoven minute fibrous crystals with mereheadite, cerussite, calcite, aragonite, mimetite, hydrocerussite, ‘plumbonacrite’ and an uncharacterized lead oxychloride, in cavities inside a manganite and pyrolusite pod. The crystals are typically less than 5 pm wide and 200 pm long, but they can reach 40 × 100 μm in cross-section and over 1 mm in length. The mineral is translucent with a vitreous lustre and each needle is brittle with an indistinct cleavage, breaking with a splintery fracture. The streak is white, the Mohs hardness ~3 and the density calculated using the empirical formula 6.886 g cm-3. Electron microprobe analyses yielded PbO 87.7, MgO 1.79, CuO 0.14, Cl 6.62 wt.%; H2O was calculated on the basis of structural considerations as 2.27 wt.% totalling 97.02 wt.%. A charge-balanced formula, based on 12 anions, is Pb7.16Mg0.81Cu0.03Cl3.40H46.0O8.60. Rickturnerite is orthorhombic Pnma, with a = 5.8024(6), b = 22.717(2), c = 25.879(3) Å, V = 3411.2(6) Å3 and Z = 8. The diffraction pattern contains strong reflections that define a subcell with a = 5.8034(5), b = 11.3574(9), c = 12.939(2) A, V= 852.9(6) Å3 (space group Pmm2 which is related to the real unit cell by the transformation matrix [100/020/002]), and weak reflections that correspond to doubled b and c parameters. Since the difference between the large and small cells is only in a number of split and low-occupancy positions in the disordered region of the structure we provide the description of the subcell structure. The five strongest lines in the X-ray powder diffraction pattern [listed as dobs (Å), Iobs, (hkl)\ are as follows: 6.474, 100, (400); 3.233, 73, (107); 2.867, 57, (705); 5.636, 44, (011); 3.112, 31, (802). The crystal structure was solved by direct methods and refined using 1318 unique reflections to R1 = 0.063. The structure is composed of a fully ordered part consisting of double [O2Pb3]2+ chains of oxocentred [OPb4] tetrahedra extended along the b-axis, which together with Cl- ions form 2-dimensional blocks parallel to (001). In between these blocks, there is a disordered region containing ordered [Mg(OH)6]4- octahedra and low-occupancy Pb and OH sites with a slight degree of ordering; these produce the weak supercell reflections.

Synthesis of Gold Nanoparticles Using the Interface of an Emulsion Droplet

A facile and rapid method for synthesizing single crystal gold spherical or platelet (nonspherical) particles is reported. The reaction takes place at the interface of two immiscible liquids where the reducing agent decamethylferrocene (DmFc) was initially added to hexane and gold chloride (AuCl4-) to an aqueous phase. The reaction is spontaneous at room temperature, leading to the creation of Au nanoparticles (AuNP). A flow focusing microfluidic chip was used to create emulsion droplets, allowing the same reaction to take place within a series of microreactors. The technique allows the number of droplets, their diameter, and even the concentration of reactants in both phases to be controlled. The size and shape of the AuNP are dependent upon the concentration of the reactants and the size of the droplets. By tuning the reaction parameters, the synthesized nanoparticles vary from nanometer to micrometer sized spheres or platelets. The surfactant used to stabilize the emulsion was also shown to influence the particle shape. Finally, the addition of other nanoparticles within the droplet allows for core@shell particles to be readily formed, and we believe this could be a versatile platform for the large scale production of core@shell particles.

Analysis of deposits formed during biomass co-firing on 15Mo3 under different gas and temperature conditions

Abstract This paper reports on the analysis of fly ash and oxides on 15Mo3 after the co-firing of eucalyptus and a Russian coal at various temperatures and gas conditions for 50 hours. The loose deposits present have been characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Results show that under reducing conditions KCl deposition is increased and a variation in oxide scale composition is observed. Complex silicates, often present as spheres are imaged through SEM.

High-resolution synchrotron X-ray diffraction studies of size and strain effects in a complex Al–Fe–Cr–Ti alloy

We present a study of a complex ultra-high-strength Al alloy containing ~40 volume per cent of second-phase particles, ranging in size from nanometres to a few microns. The microstructure has been investigated using scanning electron microscopy and high-resolution synchrotron X-ray diffraction using the I11 beam line at the Diamond Light Source, UK. Powder diffraction was carried out to (i) determine phases present, (ii) quantify the weight per cent of each phase and (iii) quantify size and strain effects in the Al matrix. The high beam quality (i.e. low divergence and wavelength purity) and multi-analysing crystal detectors makes this an ideal instrument to resolve the high peak density and determine the contribution of sample broadening in the complex alloy. Using Pawley and Rietveld full pattern fitting, the intermetallic phases present were determined to be Al 3 Ti, Al 13 Cr 2 and Al 13 Fe 4 . The weight fraction of each phase was calculated from the Rietveld refinements and correlated well with thermodynamic calculations assuming an equilibrium microstructure. Size and strain in the Al matrix was measured from peak broadening using a Double Voigt analysis and showed significant physical broadening due to both size and strain.