W. van Bronswijk

Transport of uranium, thorium, and lead in metamict zircon under low-temperature hydrothermal conditions

Abstract Understanding the stability of radiation-damaged zircon under low-temperature hydrothermal conditions is crucial to the application of zircon for U–Pb geochronology and as a host phase for the disposal of plutonium waste. We report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 °C with a 2 M AlCl 3 and a 1 M HCl–CaCl 2 solution as hydrothermal fluids for 1340 h. Cathodoluminescence (CL) and backscattered electron (BSE) images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon. These zones have experienced severe loss of Si, U, Th, and Pb, and gain of Al or Ca, and a water species as revealed by electron microprobe, sensitive high-resolution ion microprobe (SHRIMP) analyses, and infrared spectroscopy. Micro-Raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered, whereas recrystallization of the embedding amorphous phase was not observed. No detectable structural or chemical changes were detected inside the unaltered areas. Intensive fracturing, which was most intense in the HCl–CaCl 2 experiment, occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions. We explain the formation of deep penetrating alteration zones by a percolation-type diffusion model, which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible. The idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon. The model was used to estimate for the first time diffusion coefficients for U, Th, and Pb diffusion in amorphous zircon at 175 °C by assuming that volume diffusion inside the amorphous domains is the loss rate-limiting process. These estimates show that volume diffusion in metamict zircon can cause significant loss of U, Th, and especially loss of radiogenic Pb over geological time scales, even at temperatures as low as 175 °C. Our results show that recent Pb loss discordias can be generated (1) by predominate Pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant, and (2) by leaching of Pb (and U and Th) from metamict zircon through an external fluid.

A study of the adsorption of acyclic polyols on hydrated alumina

Abstract The adsorption of ten selected C3, C4, C5 and C6 acyclic polyols on hydrated alumina, under Bayer liquor-like conditions, has been studied. It is found that adsorption is particularly sensitive to the polyhydroxy configuration and that it parallels the trends observed for polyol complexation with metal cations and oxyanions. In particular, adsorption increases with increasing numbers of vicinal hydroxy groups; threo-threo sequences promote adsorption whilst erythro–erythro sequences have little effect. A strong correlation is found between adsorption and the degree of gibbsite crystallisation inhibition reported for these compounds. Their effects on gibbsite morphologies in relation to mechanisms of crystal growth inhibition are discussed.

Biooxidation as pre-treatment for a telluride-rich refractory gold concentrate

Abstract Biooxidation is an attractive alternative pre-treatment to roasting for refractory gold concentrates because it eliminates toxic gaseous emissions. Biooxidation utilises the ability of acidophilic chemolithotrophic bacteria to oxidise pyrite and arsenopyrite and release the gold or gold telluride trapped within their grains. The gold can be extracted by cyanidation and recovered using CIl/CIP technologies. Gold telluride may require further oxidation to render it amenable to cyanidation. In this study chemical and bio-oxidative treatments of a refractory, telluride-rich gold concentrate have been compared. It was shown that mesophilic iron- and sulphur-oxidising bacteria enhanced the dissolution of pyrite and arsenopyrite significantly, compared with chemical (ferric ion) oxidation. The gold telluride (calaverite) trapped within the pyrite grains was exposed during the pretreatment and underwent rapid oxidation by ferric ions in solution. Bacteria did not oxidise the telluride directly but contributed to the process by oxidising ferrous ions to ferric ions, the so-called “indirect mechanism” of bacterial oxidation.

Apatite Mineralization in Teeth of the Chiton Acanthopleura echinata

Abstract. Raman spectroscopy has been used to demonstrate, for the first time, that calcium mineralization in the core of the major lateral teeth of the chiton Acanthopleura echinata takes place as an ordered process, with crystalline carbonated apatite being the first mineral deposited. Deposition begins at the top of the tooth core, under the so-called tab region, progresses down the interior surface of the tab and lepidocrocite layer, and then extends outwards to the anterior surface. Mineralization is not initiated until the lepidocrocite layer has isolated the core of the tooth from the magnetite cap. The last region to be infiltrated is the anterior basal region of the tooth cusp, immediately above the junction zone. The junction zone is also a region of high ion density, as determined by energy dispersive spectroscopy (EDS) analysis, but we show here for the first time that it is free of mineral deposits, acting instead as a transfer and storage region.

Source and heating rate effects upon maturity parameters based on ratios of 24-ethylcholestane diastereomers

Abstract Samples from two sedimentary sequences, one a typical shale and the other containing coals and dispersed coal particles, have been used to investigate the progress of epimerisation of 20R 5α(H),14α(H),17α(H) 24-ethylcholestane to the corresponding 20S diastereomer with increasing maturity of the sediments. The apparent activation energies and entropies for this reaction are different in these two sequences, and the differences have been attributed to differences in the rock matrixes in the two sequences. Furthermore, these kinetic parameters have been used to model the behaviour of the epimerisation process in coals and in shales under a range of sediment heating rates typical of those encountered in sedimentary basins. These calculations show that low heating rates cause the epimerisation to be more advanced in a coal than in a shale while the opposite applies under high heating rates. Some samples of closely occurring coals and shales were found to contain steranes with appreciably different relative abundances of 20S and 20R diastereomers. These observations are interpreted in terms of the different effects of different heating rates upon the epimerisation process in the two types of matrix. In turn, these results indicate that sediments which contain type III organic matter in the form of dispersed coals as the dominant source of hydrocarbons would produce petroleum with anomalously low sterane epimer ratios if they had experienced a high heating rate as a result of high geothermal gradients and high burial rates.

FLOCCULATION OF HAEMATITE IN SYNTHETIC BAYER LIQUORS

Abstract A simple turbidimetric method has been established for measuring the concentration of polyacrylate flocculants in synthetic Bayer liquors, as low as 0.5 μg g −1 . It has been used to determine the isotherm of a polyacrylate flocculant from synthetic Bayer liquor onto the surface of a well characterised haematite. The polyacrylate flocculant was found to adsorb strongly on haematite in synthetic Bayer liquor with a plateau coverage of 164 μg m −2 , contrary to previous belief that the presence of dissolved Ca 2+ is essential for bonding to occur under highly caustic conditions. It was calculated that at the plateau coverage the polyacrylate flocculant occupied less than 15% of the haematite surface, which may be indicative of a low number of active adsorption sites. Settling tests combined with the isotherm data showed that the mechanism of aggregation is bridging flocculation, with optimum performance obtained at a dosage corresponding to 20% of the plateau coverage.

Investigations into the initial composition of latent fingermark lipids by gas chromatography-mass spectrometry

A more comprehensive understanding of the variability of latent fingermark composition is essential to improving current fingermark detection capabilities in an informed manner. Gas chromatography-mass spectrometry was used to examine the composition of the lipid fraction of latent fingermarks collected from a population of over 100 donors. Variations in the appearances of chromatograms from different donors were apparent in the relative peak sizes of compounds including free fatty acids, squalene, cholesterol and wax esters. Principal component analysis was used as an exploratory tool to explore patterns in this variation, but no correlation to donor traits could be discerned. This study also highlights the practical and inherent difficulties in collecting reproducible samples.

Quantitative determination of silicon in silica dust by FT-Raman spectroscopy

Silicon was quantitatively determined in respirable silica dusts and silica fume by FT-Raman spectroscopy using barium sulfate as an internal standard. Barium sulfate was selected as the standard as its Raman spectrum does not overlap with that of silicon and its scattering cross-section is significantly lower. This allowed silicon admixed with barium sulfate in ratios as low as 0.00003 to be detected. The laser beam was defocussed to cover the sample area. 128 scans at 4 cm(-1) resolution with 600 mW of power at the sample achieved a detection limit of 0.1 mug of silicon in a 5 mg sample. Silicon to BaSO(4) ratios in the range 0.00003-0.06 showed a near linear response with any decade change being linear. The RSD for this analytical method was +/-2%. Silica fume and respirable silica dust showed silicon concentrations to be in the range 2.6-51.5 ppm and 0.05-0.7 mg m(-3) respectively.