B. A. Cressey

The effect of organic ligands on the crystallinity of calcium phosphate

Calcium phosphate phases precipitated under critical supersaturation were identified and studied in detail using X-ray powder diffraction, electron probe microanalysis, infrared spectroscopy (IR) and transmission electron microscopy. These synthetic calcium phosphates formed by spontaneous precipitation at pH 7, 25degreesC and 0.1 M ionic strength (NaCl as the background electrolyte). The combination of several methods allowed detailed characterisation of the calcium phosphates. The purpose of the work was to assess the influence of carboxylate ligands, specifically acetate and citrate, on the quality of the calcium phosphate precipitate. All precipitates were identified as non-stoichiometric, calcium-deficient hydroxylapatites (HAPs), containing carbonate, HPO42-, sodium and chloride impurities. No other phases were found to be present in any of the precipitates. The presence of citrate resulted in a decrease in crystal size and a higher degree of apatite lattice imperfection in the precipitated HAP. Furthermore, IR spectroscopy showed a higher amount of carbonate present in that HAP, compared with the ones formed in the control and acetate experiments. An additional absorption band, in the infrared spectrum of the HAP formed in the presence of citrate, was observed at 1570 cm(-1); this is interpreted as carboxyl groups bound to HAP.

Synthesis of novel composite materials via the deposition of precious metals onto protonated titanate (TiO2) nanotubes

Abstract Methods for the deposition of precious metals (Au, Pt, Pd and ruthenium hydrated oxide) onto the surface of nanotubular titanates are considered. Viable techniques include preliminary ion exchange of precious metal cations onto the nanotubes followed by chemical, electrochemical or photochemical reduction to the metal. The morphology and size of the metal nanoparticles ranged from spheroidal particles of a few nanometres to larger, rod like particles. The deposits, which were densely loaded onto the surface and were uniformly distributed, had a high surface area and good chemical stability. The size of metal nanoparticles ranged from 1 to 50 nm.

The crystal structure of Mg end-member lizardite-1T forming polyhedral spheres from the Lizard, Cornwall

Abstract The crystal structure of lizardite-1T from Gew-graze, Cornwall, has been refined by single-crystal X-ray diffractionto R1 of 0.0263 and 0.0557, for two crystals having P31m space group and lattice parameters a = 5.2905(5) Å, c = 7.2815(8) Å, and a = 5.3111(10) Å, c = 7.2907(15) Å, respectively. The Gew-graze lizardite is very close to the end-member composition, so these refinements confirm the existence of well crystallized pure Mg-lizardite and show this to be the most weakly H-bonded lizardite ever refined. With regard to its lattice parameters, Si−O bond distances and geometrical parameters controlled by interlayer interactions (i.e. H-bonding), the Gew-graze Mg end-member lizardite yields slight but observable geometrical differences compared with previous refined structures for specifically, aluminous, lizardites.

A model for the composite nanostructure of bone suggested by high-resolution transmission electron microscopy

Abstract We have imaged the spatially-preserved microtexture of biogenic apatite, retained together with its collagen template, in non-demineralized human bone using high-resolution transmission electron microscopy. Using ion-beam thinning, a specimen preparation method generally employed for inorganic minerals rather than for biological materials, we have imaged a composite nanostructure of bone not previously reported, and we propose a model for this nano-architecture that involves a boxconstruction of apatite plates and apatite sheets. This observation provides a new understanding of bone strength at the nanometre scale and suggests how post mortem enhancement of this texture by recrystallization probably accounts for the durability of ancient bone. Modern sheep bone (a close analogue for recently dead human bone) imaged in the same way also shows evidence of this composite architecture.

Cu and Zn ordering in aurichalcite

Abstract The advantages of X-ray absorption spectroscopy have been utilized to assess the Cu and Zn ordering in aurichalcite, (Cu5−xZnx)(OH)6(CO3)2. We have examined one hydrozincite sample and three aurichalcite samples in which the Cu:Zn ratios are in the range 1:3 to 2:3. Copper 2p XAS confirms that there is no monovalent copper in aurichalcite and that in each sample the copper might be distributed across more than one metal site. EXAFS, at the Cu and Zn K-edges, shows that the copper atoms preferentially enter the Jahn- Teller elongated, octahedral (M2) and trigonal bipyramidal (M4) sites, with the zinc atoms entering the more regular octahedral (M1) and tetrahedral (M3) site. Substantial solid solution towards the zinc rich region is facilitated by the substitution of copper by zinc on the M2 and M4 sites. This information, not easily obtained by X-ray diffraction, substantially enhances the understanding of the structure of aurichalcite.

Polyhedral serpentine: a spherical analogue of polygonal serpentine?

Abstract Vugs in late hydrothermal veins in the serpentinite at Gew-graze, Lizard, Cornwall, UK, contain serpentine spheres ≤0.7 mm in diameter composed of a crystallographically controlled radial array of well crystallized lizardite-1T crystals. Examinations with optical and scanning electron microscopy reveal that the spheres actually have polyhedral morphology. The polyhedral facets at the sphere surface are the (0001) terminations of individual single crystals of lizardite. Each lizardite crystal is a hexagonal prism and tapers inwards to the core. The angle from prism axis to prism axis is always ~24º, and this angle is consistent even though individual prisms have not maintained contact during growth. The space between prisms is filled by smaller crystals of lizardite in more random orientations, forming a solid sphere. Collectively, the tapering prisms form a growth array that produces a surface tessellation consisting of mainly 6-fold neighbours, but with some 5-fold arrangements to accommodate a closed spherical structure. A ‘buckyball’, modified by adding face-centring points to each hexagon and pentagon, provides a useful model to describe the space filling adopted by the polyhedral lizardite spheres. Cross sections (close to an equatorial plane) through these polyhedral spheres resemble cross sections of polygonal serpentine, with 15 sectors at 24º to each other, though very much larger in diameter.

Low-Temperature Synthesis of Titanate Nanotubes in Aqueous KOH

Although hydrothermal alkaline treatment of TiO2 in a concentrated, aqueous solution of KOH usually results in the formation of solid fibrous titanates, analysis of the temperature dependence of Ti(iv) concentration in KOH solution and comparison of these data with that for NaOH solution suggests that, at low temperatures, the treatment of TiO2 with KOH may result in formation of titanate nanotubes. This result was confirmed by 12 days treatment of TiO2 in 10 mol dm?3 KOH at 56?C, resulting in the formation of nanotubular titanates with similar morphology to those produced in a shorter time at higher temperatures using NaOH.The mechanism of nanotube formation and the necessary conditions of nanotube phase formation are considered.

The development of X-ray photo-emission electron microscopy (XPEEM) for valence-state imaging of mineral intergrowths

Abstract We demonstrate the potential of X-ray photo-emission electron microscopy(XPEEM) to reveal valence-state images of the spatial distribution and relative concentration of metals in specific oxidation states. Additionally, XPEEM allows X-ray absorption spectra to be extracted from chosen pixel areas of the images. Using an in-house-built XPEEM instrument we show an application of the method in providing valence-state images of complex mineral intergrowths. The image resolution achieved with this instrument of simple design was ~5 μm and reasonable quality X-ray absorption spectra were extracted from areas of ~5×5 μm. These initial results suggest that by using commercial XPEEM instruments on 3rd generation, high-brightness synchrotron sources a spatial resolution of 100 nm or better could be achieved, with the ability to extract high-quality X-ray absorption spectra from areas of 0.1 μm2. Given that standard thin sections or polished blocks can be studied by XPEEM, and that each XPEEM image records ~1000 μm2, XPEEM can be used in conjunction with other analytical methods such as EMPA and TEM-EELS/PEELS.

The significance of the aluminium content of a lizardite at the nanoscale: the role of clinochlore as an aluminium sink

Abstract Lizardite-1T crystals from Gew-graze at the Lizard, Cornwall, of an apparent composition (Mg2.94Fe0.03Al0.03)(Si1.97Al0.03)O5(OH)4, have been observed to contain 2-5% of thin interstratifications of clinochlore when investigated by analytical transmission electron microscopy (TEM). The clinochlore is usually only a few unit layers thick but has an extensive lateral dimension parallel to the lizardite layers. Analytical TEM confirms the existence of very low-Al lizardite with a planar structure interstratified with occasional thin units of clinochlore containing substantial Al. Compositionally, such clinochlore (invisible by X-ray diffraction) interstratified throughout the lizardite crystals, could account for most of the Al present. We suggest that an occasional influx of Al causes the nucleation and growth of clinochlore at the expense of lizardite. The possibility of clinochlore contributing to the measured Al content in lizardite samples highlights the need for future investigations of Al-bearing serpentines to include careful examination and interpretation using imaging and analysis by TEM. It has crept into serpentine discussions that lizardite cannot form without the coupled substitution of Al to relieve the misfit between the sheets of octahedra and tetrahedra. The lizardite at Gew-graze is almost Al-free, forms well-crystallized planar crystals and demonstrates that Al is not an essential element for lizardite crystallization.

Development of X-ray photoemission electron microscopy (X-PEEM) at the SRS.

The use of synchrotron radiation sources for X-ray spectroscopy is a well known and developed field. The majority of applications, however, have been limited to studies of materials containing only a single phase of the element of interest. Owing to limited availability of suitable instrumentation, the study of materials comprising intergrowths of different phases has presented difficulties in analysis. The majority of natural materials, including mineralogical samples, fall into this category. However, by applying the technique of photoemission electron microscopy (PEEM) to view the X-ray stimulated photoemission generated at an absorption edge, micro-area-selectable spectroscopy becomes possible. An instrument for X-ray PEEM (X-PEEM) is being developed at the Daresbury SRS and this paper shows how it can be used to obtain characteristic L-edge XANES spectra from finely intergrown iron oxide minerals.