Au Grains Research Articles

Supergene gold transformation: Biogenic secondary and nano-particulate gold from arid Australia

Biofilms on Au grains from sub-tropical and tropical sites promote the dispersion of gold (Au) by continuously recycling coarse Au and releasing nano-particulate Au. In this study we investigated the influences of (bio)geochemical processes on the transformation of Au grains in arid environments. At these sites water and nutrient availability are limited and episodic, hence abiogenic evaporative mechanisms were thought to control the formation of secondary and especially nano-particulate Au. Gold grains were collected from eight arid sites in three Australian Au provinces, i.e., Lawlers (Western Australia), Tanami (Northern Territory), and Flinders Ranges (South Australia). Sites were chosen based on contrasting deposit styles, i.e., primary underground and epithermal deposits as well as secondary eluvial-, colluvial- and alluvial placers at increasing distances from primary mineralization. Gold grains were studied using optical microscopy, FEG-SEM, FIB–SEM–EDXA, EPMA and PIXE. Gold grains from all surface environments displayed supergene transformation features, i.e., morphotypes indicative of Au and Ag dissolution, Au aggregation and Au neoformation. The latter included spheroidal and bacteriomorphic Au, which increased in abundance with distance to source. Viable biofilms containing abundant Au nano-particles and spheroidal Au μ-crystals were detected on all grains from the Flinders Ranges. Gold grains from the Lawlers and the Tanami provinces are covered by polymorphic layers containing abundant nano-particulate, spheroidal and bacteriomorphic Au. The polymorphic layers consist of vermiform clays and organic matter, suggestive of remnant biofilms. These results demonstrate that biofilms capable of transforming Au grains develop episodically on Au grains in arid environments. In conclusion, this study shows that microbial processes play a critical role for the transformation of Au grains and contribute to the dispersion of Au and the formation of geochemical anomalies in arid environments.

Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand

Biogeochemical processes drive the transformation of gold (Au) in surface environments. In this study we assess the link between surface morphologies of Au grains and supergene transformation processes, with a focus on the formation of nano-particulate Au in temperate settings. Gold grains were collected from six localities across the South Island of New Zealand. Deposit styles vary from eluvial-, alluvial-, and beach placer deposits in areas of moderate to very high levels of precipitation. Gold grains were assessed using optical microscopy (OM), field emission scanning electron microscopy (FEG-SEM), focused ion beam-scanning electron microscopy (FIB-SEM) coupled with X-ray dispersive analysis (EDXA) and electron microprobe analyses (EPMA). Morphologies indicative of Au- and Ag dissolution, e.g., grain boundary dissolution, as well as abundant Au neoformation- and aggregation morphologies were observed on all grains. The latter include a variety of secondary Au morphotypes, in particular nano-particulate- and μ-crystalline forms as well as bacteriomorphic Au, sheet-Au and porous, branched Au networks. Pervasive dissolution features on grains from an outcropping quartz-vein system as well as extensive nano-particle formation on weathered quartz-vein- and placer grains from the west coast of New Zealand's South Island, which is subject to very heavy orographic precipitation, suggest that these climatic conditions enhance the transformation of Au grains. At these sites, Au nano-particles are most abundant in the polymorphic layer (i.e., a coating of biofilms, secondary Au, siliceous and carbonaceous materials on the surface of transforming Au grains), and in soil materials associated with the grains. Nano-particulate Au is also highly abundant in carbonaceous, likely exopolymeric, coatings on Au grains from Orepuki Beach, suggesting that Au dissolution in seawater and microbial biomineralization are important contributors to Au alteration in beach placer deposits. In conclusion, surface morphologies of Au grains from New Zealand are the result of supergene transformations occurring in current environments. The formation of nano-particulate Au, which was previously thought to be evaporation-driven, is in these high-rainfall environments likely due to other mechanisms, such as biomineralization.

Tuning surface plasmons in interconnected hemispherical Au shells

We present a new approach for making interconnected hemispherical shells by stripping Au from templates of anodized aluminum, where the metal thickness can be adjusted without affecting the outer radius of curvature, film roughness and the sharpness of the hemisphere contact areas. This provides increased understanding of the surface plasmon resonances (SPRs) observed for Film-On-Nanospheres (FONs) by decoupling these parameters, which are coupled in the case of FONs. Investigating the influence of the shell thicknesses on the spectral positions of SPRs for FONs involves a dielectric core with a fixed radius encased by a metal film with adjustable thickness. By performing linear reflection spectroscopy, we demonstrate a wide tunability of the SPR by tailoring the inner hemisphere diameter, while keeping the outer diameter fixed. Deposition of extra Au on top of thick, previously stripped hemispherical shells isolates optical response contributions from Au grain- and island-mediated roughness, and unsharpening contact areas in form of decreasing LSPR quality factor. Two-photon luminescence scanning optical microscopy of shells with different thicknesses, applying several different laser wavelengths, is exploited to map local electromagnetic hot spots and correlate the high field enhancements with the linear reflection spectroscopy measurements.

Scanning electron microscopy and energy-dispersive X-ray investigations of gold grains in quartz veins from the Seiga gold mine, south Egypt

Seiga Au mineralization is located in the Wadi Seiga which intersected the Wadi Allaqi in Southeast Egypt. It has had a long history of ancient Au working since 4000B.C. and discontinzuous mining until 1950s. This study presents a detailed microchemical characterization of Au grains in quartz veins and veinlets, aiming to obtain further understanding of Au occurrences, chemistry of Au bearing phases, their internal structure, inclusions and genesis, and type of source mineralization for evaluation and exploration purposes. The scanning electron microscope fitted with energy dispersive X-ray spectrometer techniques has been applied to achieve this paper.The results revealed that the Au mineralization is considered as a single major episode of mineralization formed by hydrothermal ore bearing fluid rich in sulfides, epithermal character and metamorphic origin. It formed through four stages, including four mineral phases varied in their dimensions, distribution and composition. The first two phases were homogeneous and included Au+Ag and Au+Ag±Cs±Rb phases that derived from two separated homogeneous ore bearing fluids. They existed as discrete individual grains in quartz veins and veinlets, caged in secondary iron oxides pseudomorphed after early formed Au bearing pyrites and as zoned Au grains including the cores of Au+Ag phase and the rims of younger Au+Ag±Cs±Rb phase. The third phase was represented by pyrites and chalcopyrites, whereas the fourth phase included carbonate (ankerite and calcite) and muscovite minerals. Both the last two phases were barren and free of Au signals. Seiga Au mineralization has been subjected to dissolution, remobilization and re-precipitation processes that control the dimension and distribution of both Au phases. The obtained results give a complete image of quantitative and qualitative characterization of Au alloy phases and their associated minerals for genetic history of Au mineralization.

The effects of pretreatment of polycrystalline gold with OH• radicals on the electrochemical nucleation and growth of platinum

The nucleation and growth of platinum on polycrystalline gold was studied by chronoamperometry, cyclic voltammetry, and atomic force microscopy before and after treatment of the gold surface with hydroxyl (OH•) radicals. Two different procedures of mechanical polishing of the gold surface (“coarse polish” and “fine polish”) were applied before the treatment with OH• radicals. The nucleation and growth of Pt was much better reproducible on electrodes which underwent a “coarse polish”. The treatment of the Au surface with OH• radicals decreased the number of active sites; however, the nucleation growth mode remained the same (3-D instantaneous). The spontaneous Pt deposition (no externally applied potential) on Au was unaffected by the treatment with OH• radicals. In situ atomic force microscopy experiments showed that the Pt starts to grow only on some of the Au grains, most probably on those which have active sites on their surface. This leads to a roughening of the electrode surface upon Pt deposition. Treatment with OH• radicals did only quantitatively diminish the amount of deposited Pt, but qualitatively the imaging of the Pt growth remained the same. Obviously, the OH• radicals lead to a knockout (decreasing number) of active sites for Pt nucleation, while the nature of the remaining active sites stays unaffected.

Electronic properties of Au-graphene contacts

The effects of Au grains on graphene conduction and doping are investigated. To obtain a clean Au-graphene contact, Au grains are deposited over graphene before any chemical processing. The bulk and the effective contact resistance versus gate voltage demonstrate that Au grains cause p-doping in graphene. The Fermi level shift is in disagreement with published first-principles calculations, with larger than predicted separation between the graphene and the topmost Au layer. The differential resistance versus bias voltage display anomalies at zero bias and at higher voltages, the latter likely caused by inelastic tunneling across the Au-graphene interface.

Low-dose-rate Interstitial Brachytherapy Preserves Good Quality of Life in Buccal Mucosa Cancer Patients

To determine the results and long-term changes in radiation toxicity of stage I-II buccal mucosa cancer patients treated by low-dose-rate (LDR) brachytherapy with (198)Au grains. A total of 133 stage I-II buccal mucosa carcinomas patients received (198)Au grain implantation brachytherapy between January 1982 and July 2005: 75 of them were treated by (198)Au grain implantation alone and 58 were treated by (198)Au implantation in combination with external irradiation. The average (198)Au-grain dose was 70 Gy in 7 days. Gross tumor areas ranged from 2.4 cm(2) to 9 cm(2), and the clinical target areas ranged from 6 cm(2) to 15 cm(2). The follow-up periods ranged from 3 months to 20 years (mean: 5 years 11 months and median: 5 years 1 months). Failure at the site of the primary lesion occurred in 17 patients. Post-treatment mucosal ulceration developed in 15 patients, and all were cured within 25 months by conservative treatment. Osteoradionecrosis was diagnosed in 8 patients, but only one patient required surgical treatment. No severe complications or aggravation of complications developed more than 10 years after treatment. The results of low-dose-rate (LDR)-brachytherapy (BT) alone and LDR-BT in combination with external irradiation at a total dose of 25 Gy were acceptable from the standpoint of cure rate and QOL.

Nanoparticle factories: Biofilms hold the key to gold dispersion and nugget formation

Biofilms living on gold (Au) grains play a key role in the biogeochemical cycle of Au by promoting the dispersion of Au via the formation of Au nanoparticles as well as the formation of secondary biomorphic Au. Gold grains from Queensland, Australia, are covered by a polymorphic, organic-inorganic layer that is up to 40 μm thick. It consists of a bacterial biofilm containing Au nanoparticles associated with extracellular polymeric substances as well as bacterioform Au. Focused ion beam (FIB) sectioning through the biofilm revealed that aggregates of nanoparticulate Au line open spaces beneath the active biofilm layer. These aggregates (bacterioform Au type 1) resulted from the reprecipitation of dissolved Au, and their internal growth structures provide direct evidence for coarsening of the Au grains. At the contact between the polymorphic layer and the primary Au, bacterioform Au type 2 is present. It consists of solid rounded forms into which crystal boundaries of underlying primary Au extend, and is the result of dealloying and Ag dissolution from the primary Au. This study demonstrates that (1) microbially driven dissolution, precipitation, and aggregation lead to the formation of bacterioform Au and contribute to the growth of Au grains under supergene conditions, and (2) the microbially driven mobilization of coarse Au into nanoparticles plays a key role in mediating the mobility of Au in surface environments, because the release of nanoparticulate Au upon biofilm disintegration greatly enhances environmental mobility compared to Au complexes only.

Fabrication of freestanding gold nanotubes

Ag nanowhiskers were grown on a C-coated Si substrate at 800 °C and subsequently coated with Au by physical vapor deposition. The Au forms a single crystalline, epitaxial layer on the nanowhisker and a polycrystalline Au film on the substrate surface. Annealing at 150 °C for 70 h activates diffusion of Ag into the Au grain boundaries of the polycrystalline film, emptying the Ag nanowhisker core. The remaining epitaxial Au film forms the single crystalline wall of Au(Ag) nanotubes.

Information on deformation mechanisms in nanocrystalline Pd–10% Au inferred from texture analysis

There is still a lack of understanding of deformation mechanisms in nanocrystalline (nc) materials. Studies on microstructures formed in nc Pd–10% Au (grain size about 15 nm) after high pressure torsion revealed signatures of various deformation processes as cooperative grain boundary sliding (GBS), shear banding, dislocation slip and twinning. In order to estimate contributions of these processes to total strain, a comparison was made between torsion textures formed in nc and coarse grained (cg) samples after identical shear strain. The textures were measured with synchrotron radiation. Intensities of characteristic components of the shear texture are two times stronger in the cg sample than in the nc one, indicating that dislocation slip is less significant in the nc sample. It is proposed that numerous planes of cooperative GBS revealed by TEM contribute to plasticity of nc alloy.

Electrons scattered inside small dust grains of various materials

The dust grain charge in an electron beam is given by a difference in numbers of electrons that fall onto the grain and those leaving it. Electrons with energies exceeding 1 keV can penetrate through submicron-sized dust grains. If the grain is small enough, a yield of these electrons reaches unity but they leave a part of their energy inside the grain and this energy excites secondary electrons. The paper presents a hybrid Monte Carlo code that simulates paths of the primary electrons inside a spherical grain and provides the yield of scattered electrons and their energy spectrum as a function of the grain size and material. This code is based on the Richterovaet al. [Phys. Rev. B 74, 235430 (2006)] model but it includes several corrections important for light materials like carbon or ice. The model was verified using experimental results obtained on large planar samples. For spherical samples, we have found that the yield of scattered electrons reaches unity for 50 nm Au grains illuminated by 5 keV electrons, whereas the same effect can be observed on approx =1000 nm carbon grains.

Reductive dissolution of Fe3O4facilitated by the Au domain of an Fe3O4/Au hybrid nanocrystal: formation of a nanorattle structure composed of a hollow porous silicananoshell and entrapped Aunanocrystal

The Fe(3)O(4) grain of a Fe(3)O(4)/Au hybrid nanocrystal encapsulated in a silica nanosphere was rapidly and exclusively dissolved through a reductive process facilitated by the attached Au grain, resulting in the formation of a nanorattle structure which has utility as a nanoreactor to template the growth of nanocrystals inside the cavity.

Guided phase separation of polymer blend thin films on ion beam-induced pre-patterned substrates

A technique for fabricating pre-pattern induced phase-separated polymer blend films is presented. The gold layer coated silicon substrates were bombarded with focused ion beams (FIB) to sputter away Au grains in the irradiated regions and expose silicon oxide surface underneath, thereby creating pre-patterns of relatively hydrophobic (Au)/hydrophilic (silicon oxide) regions. In the pre-patterned regions, the spin-coated films consisting of immiscible PS (polystyrene) and PtBA (poly-tert-butyl acrylate) blend exhibited phase separation induced by the underlying pre-patterns and formed two distinct ordered morphologies with periodicities much smaller than that of the pre-patterns. The influence of varying periodicities of the pre-patterned structures, blending ratios and spin-coating parameters on the resulting morphologies are presented.

Mechanisms of gold biomineralization in the bacterium Cupriavidus metallidurans

While the role of microorganisms as main drivers of metal mobility and mineral formation under Earth surface conditions is now widely accepted, the formation of secondary gold (Au) is commonly attributed to abiotic processes. Here we report that the biomineralization of Au nanoparticles in the metallophillic bacterium Cupriavidus metallidurans CH34 is the result of Au-regulated gene expression leading to the energy-dependent reductive precipitation of toxic Au(III)-complexes. C. metallidurans, which forms biofilms on Au grains, rapidly accumulates Au(III)-complexes from solution. Bulk and microbeam synchrotron X-ray analyses revealed that cellular Au accumulation is coupled to the formation of Au(I)-S complexes. This process promotes Au toxicity and C. metallidurans reacts by inducing oxidative stress and metal resistances gene clusters (including a Au-specific operon) to promote cellular defense. As a result, Au detoxification is mediated by a combination of efflux, reduction, and possibly methylation of Au-complexes, leading to the formation of Au(I)-C-compounds and nanoparticulate Au(0). Similar particles were observed in bacterial biofilms on Au grains, suggesting that bacteria actively contribute to the formation of Au grains in surface environments. The recognition of specific genetic responses to Au opens the way for the development of bioexploration and bioprocessing tools.

In situdiffraction strain analysis of elastically deformed polycrystalline thin films, and micromechanical interpretation

In situtensile tests have been carried out under synchrotron radiation on supported gold (Au) thin films exhibiting a pronounced crystallographic texture. The 2θ shift of X-ray diffraction lines has been recorded for different specimen orientations and several loading levels in the elastic domain. The data obtained demonstrate the large strain heterogeneities generated within the specimen because of the intergranular interactions associated with the large elastic anisotropy of Au grains. To interpret these results, the use of a multi-scale micromechanical approach is unavoidable. The theoretical background of such methods is described, and the points where exact results can be obtained and where approximations have to be introduced are highlighted. It is shown that the Vook–Witt model, for which a general formulation is provided, is the exact solution for polycrystals exhibiting a laminate microstructure, which is a significant departure from the standard thin-film microstructures. Among several standard models used in the field, the self-consistent model is the only one that reproduces the experimental data correctly. This is achieved by accounting for the actual crystallographic texture of the specimen, and assuming pancake-shaped two-point statistics for the morphological texture. A discussion of the limitations of this approach, originally developed for bulk materials, is given for the specific case of thin films.

3D strain imaging in sub-micrometer crystals using cross-reciprocal space measurements: Numerical feasibility and experimental methodology

Direct inversion of coherent X-ray diffraction patterns is a powerful method to image strains in individual crystals with a high spatial resolution, less than 10 nm. The possibility to invert the diffraction pattern is in principle ensured by the oversampling of the measurement. In addition, the resolution of the reconstructed object requires the measurement of the intensity distribution as far as possible in reciprocal space. Thus, fulfilling the oversampling and resolution constraints, in 3D, implies very long acquisition times, hardly compatible with the stability of the X-ray synchrotron source or sample. To overcome this problem, we present a method based on partial crossed intensity measurements: it consists in two irregularly spaced intensity measurements taken in two orthogonal directions. This paper demonstrates that partial crossed intensity measurements are in principle sufficient to reconstruct 2D objects using a phase retrieval algorithm. We also describe how such measurements can be achieved in practice for 3D imaging. 3D intensity measurements taken in two orthogonal directions for a 111 Bragg peak of a sub-micrometer Au grain are shown.

Poorly crystalline Pd-Hg-Au intermetallic compounds from Corrego Bom Sucesso, southern Serra do Espinhaco, Brazil

Potarite, ideally PdHg, is reported in the literature to have compositions varying from PdHg or Pd(Hg,Au) to Pd3Hg2. Such a Pd3Hg2 phase is unknown in the synthetic Pd–Hg binary system. For the first time, Pd–Hg grains recovered from the historical Bom Sucesso alluvium, regarded as the type locality of Pd, are shown to consist of arborescent and lamellar intergrowths of two intermetallic compounds, compositionally close to empirical Pd(Hg,Au), i.e. auriferous potarite, and (Pd,Au)3Hg2. The Pd–Hg–Au grains have a rim of palladiferous Pt. The otherwise sharp Pd–Hg–Au intergrowths become diffuse at the contact with the palladiferous Pt rim. Both the Pd–Hg–Au compounds and the palladiferous Pt rim did not diffract using the electron-backscattered diffraction (EBSD) and powder X-ray microdiffraction techniques, indicating that they are poorly crystalline. Their poor crystallinity and the diffuse zone between the Pd–Hg–Au core and the Pt-rich overgrowth are suggestive of electrochemical metal precipitation from dilute solutions within the alluvium.

Immobilization of β-cyclodextrin on gold surfaces by chemical derivatization of an 11-amino-1-undecanthiol self-assembled monolayer

A new methodology for the covalent functionalization of a SAM of 11-amino-1-undecanethiol, previously adsorbed on polycrystalline Au, was successfully applied in order to immobilize a β-CD layer on surface. A two steps synthetic strategy is proposed based on the activation of the SAM with di-(N-succinimidyl) carbonate for the further inclusion of the β-CD. The modification of the SAM was followed by PM-FTIRRAS, AFM imaging, CV, and EIS which confirmed the introduction of β-CD layer. The AFM images allowed the identification of homogeneously distributed β-CD aggregates over the Au grain microstructure. The electrode was characterized in the presence of electroactive species in solution, with the ability to form inclusion complexes with the β-CD cavity. Contrary to the reported for other thiolated CD derivative films, the results of this study showed the formation of well-packed and compact structures which strongly reduce non-specific adsorption phenomena. The redox response of the probes at the β-CD electrode was shown to appear at higher potentials with respect to the response at bare Au. Good correlation was found between the increase of the hydroquinone oxidation peak and, both, the scan rate used in CV experiments (typical behavior of surface-confined species) and the hydroquinone concentration. In the case of dopamine, the processes seem to shifted out of the potential window of SAM stability. The results suggest that this problem could be overcome by improving the design of the device.

Electrolytic Gold Deposition on Dodecanethiol-Modified Gold Films

The electrochemical nucleation and growth of Au from a Au sulfite electrolyte onto dodecanethiol-modified Au surfaces is investigated by a combination of microscopy and chronoamperometry methods. The self-assembled dodecanethiol monolayers are continuous but exhibit defects in correspondence of the Au grain boundaries and on top of Au terraces. Nucleation of Au films occurs initially at these defect sites, but only a small fraction of these nuclei survive an initial competition process. The remaining nuclei expand through three-dimensional progressive nucleation followed by diffusion-limited growth. The resulting Au films exhibit microstructures which are widely different from those observed in the electrochemical growth of Au on Au and that depend on the applied potential: while at low overpotentials the film grows as an assembly of hemispherical clusters, at intermediate overvoltages the films are smooth and at high overvoltages become dendritic. Metal growth onto self-assembled monolayer-modified substrates can thus provide an alternative method for controlling film morphology for a wide range of applications.

A CRYPTICALLY ZONED AMALGAM (Au1.5-1.9Ag1.1-1.4) 2.8-3.0Hg1.0-1.2 FROM A PLACER DEPOSIT IN THE TULAMEEN - SIMILKAMEEN RIVER SYSTEM, BRITISH COLUMBIA, CANADA: NATURAL OR MAN-MADE?

We describe an unusual pattern of zoning in an amalgam of Au and Ag; it occurs as peripheral zones in a 0.3 × 0.15 mm grain in a placer deposit associated with the Tulameen – Similkameen river system, British Columbia. This amalgam formed at the expense of primary Au–Ag alloy, which is present as relics in the core: Au 54–55 Ag 45–46 (Hg-free; not zoned). Abundances of Au gradually increase outward and complement decreasing levels of Ag in the amalgam. Values of Au/Ag increase from 1.1–1.2 in the core to 1.8 at the margin of the grain. The observed range of compositions, (Au 1.50–1.87 Ag 1.06–1.40 ) ∑2.81–3.00 Hg 1.00–1.19 , is limited. Two possibilities are discussed to explain the formation of the zoned amalgam. (1) This phase could be natural, formed by reaction between primary Au–Ag alloy and a late melt or oxidizing fluid, which may have remobilized and transported Hg as a result of destabilization and oxidation of cinnabar. If so, the observed zonation would reflect an increasing activity of Au species (and decreasing activity of Ag) during crystallization. (2) This phase may have formed under supergene conditions, as a result of anthropogenic introduction of Hg into placer gravels during historic mining operations in this region; we favor this hypothesis. Zoning in the amalgam may have been controlled principally by electrochemical factors related to the process of “self-electrorefining”. Gold is more “noble” than silver, so that ions of Au probably migrated and were preferentially deposited closer to the margin of the grain. This mechanism is similar to that responsible for the “secondary” rim rich in Au (poor in Ag) in placer grains of Au–Ag alloy, as reported from other localities worldwide.