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Pyrite, the most abundant sulfide on Earth and a common component of gold deposits, can be a significant host for refractory gold. This is the first documentation of pore-attached, composite Au-telluride nanoparticles in “arsenic-free” pyrite. Trace elements mapping in pyrite from an intrusion-hosted Au deposit with orogenic overprint (Dongping, China) shows trails of tellurides overlapping Co-Ni-zonation. Intragranular microfracturing, anomalous anisotropy, and high porosity are all features consistent with devolatilization attributable to the orogenic event. The pyrite-hosted nanoparticles are likely the “frozen,” solid expression of Te-rich, Au-Ag-Pb-bearing vapors discharged at this stage. Nanoparticle formation, as presented here, provides the “smallest-scale” tool to fingerprint Au-trapping during crustal metamorphism

Lead-doped zircon crystals, which were synthesized under three different conditions (Watson et al. 1997): dry at 1430 °C at atmospheric pressure without P 2 O 5 ; wet at 900 °C at 1.5 GPa in the presence of P 2 O 5 ; and wet at 800 °C at 1.0 GPa without P 2 O 5 , have been investigated to understand the mechanisms of Pb incorporation into zircon at the sub-micrometer scale, using various electron microscopy techniques including high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Four different mechanisms in which Pb may be incorporated into zircon have been identified. In the P-free synthesis, Pb-oxide hydrate particles, 50–200 nm in size, are embedded in zircon. Each Pb-particle is associated with a single vesicle, ~50 nm. Lead in the zircon structure is possibly incorporated under the detection limit value of energy-dispersive X-ray analysis (EDX) by means of: (1) Zr 4+ = Pb 2+ + 2H + at less than ~0.1 wt%. In the system with P, Pb-phases occur in three different forms: Pb-rich domains concentrated along cleavage planes or grain boundaries without any evident crystal form; numerous Pb-phosphate particles, as large as 100 nm, embedded heterogeneously in the zircon crystal; and homogeneous distribution of Pb in the zircon structure at less than ~1 wt% as determined by EDX. These results suggest that charge balance is maintained by the xenotime-type coupled substitution: (2) Zr 4+ + 2Si 4+ = Pb 2+ + 2P 5+ with a possible minor contribution from mechanism 1. The apparent solubility limit of Pb, 2+ in natural zircon is consistent with the low-level Pb allowed by substitution mechanism 2, with only a minor contribution from substitution mechanism 1, the latter of which causes distortion in the local structure.

We have examined a paleosol in Pronto, Canada, which formed at ~2.45 Ga and under very low O 2 conditions, to reconstruct ancient weathering processes. Chlorite, which replaced biotite in parent granite, consists of almost complete chlorite layers as revealed by high-resolution transmission microcopy. Chlorite grains were fragmented gradually with increased intensity of weathering. The fragmented chlorite grains were partly or largely replaced by sericite formed from K-metasomatism, which strongly suggests that almost complete chloritization of biotite preceded sericitization. In the upper part of the paleosol, chlorite-biotite mixed layers formed but the formation of these layers cannot be explained by mere chloritization and sericitization, which are usually considered to occur concurrently during post-weathering events. Our observations and analyses suggest that the chlorite-biotite mixed layers were formed as follows: chlorite, with no or little biotite, was formed from biotite in parent granite prior to weathering. Chlorite was then weathered by dissolution and fragmentation at ~2.45 Ga, and chlorite-vermiculite (or smectite) mixed layers were formed by layer-by-layer transformation in the weathering profile (note that vermiculite is not actually observed in the Pronto paleosol). Following burial of the weathering profile, the vermiculite layers in the chlorite-vermiculite mixed layers were converted to biotite layers, by means of K-metasomatism, to form the chlorite-biotite mixed layers. The chlorite-biotite mixed layers in the Pronto paleosol were therefore chlorite-vermiculite mixed layers at the time of weathering.

In soil near tailings from an antimony (Sb) mine, we found micro-grains coated with an antimony-rich layer. These grains were characterized in detail using multiple advanced analytical techniques such as micro-X-ray absorption near edge structure (μ-XANES), micro-extended X-ray absorption fine structure (μ-EXAFS), micro-X-ray diffraction (μ-XRD), transmission electron microscope (TEM), and electron probe microanalysis (EPMA). The EPMA showed that one soil grain (grain A) locally accumulated a large amount of Sb in the secondary phases (40–61 wt% Sb 2 O 5 ) with significant Fe (20–28 wt% Fe 2 O 3 ). The spatial distribution of Sb in the grain was similar to that of iron. Both Fe μ-XANES and μ-XRD of the Sb hot spots in grain A consistently showed that the secondary products were dominantly composed of ferric antimonate, tripuhyite (FeSbO 4 ). Fits to the Sb K -edge μ-EXAFS of this phase showed second-neighbor coordination numbers ~30% smaller than in bulk tripuhyite, indicating that the tripuhyite included in grain A is nanoparticulate and/or has a high structural disorder. The TEM analysis suggests that the particle size of tripuhyite in grain A was around 10 nm, which is consistent with the size range indicated by μ-XRD and μ-EXAFS. This is the first report showing tripuhyite with nanocrystallinity in natural soil to date.

The uranyl peroxide, studtite (UO 4 ·4H 2 O, C 2/ c , Z = 4), is expected to form as a consequence of alpha radiolysis of water in contact with spent nuclear fuel (SNF) in a geologic repository. Investigation of its stability is, therefore, of critical importance because secondary U(VI) phases may incorporate trace amounts of radionuclides and thus retard their mobility away from a repository site. To examine the effect of ionizing radiation on uranyl peroxides, electron-beam irradiation experiments have been conducted on two synthetic uranyl peroxides: studtite and metastudtite (UO 4 ·2H 2 O, Immm , Z = 2). All experiments were done using a transmission electron microscope (TEM) with an acceleration voltage of 200 kV at room temperature. The fluence required to completely amorphize studtite was 0.51–1.54 × 10 17 e/cm 2 , which is equivalent to an absorbed dose of 0.73–1.43 × 10 7 Gy. Metastudtite becomes amorphous at a higher absorbed dose (1.31 × 10 7 Gy) than studtite, most likely because it contains fewer water molecules in its structure. These uranyl peroxides partially amorphize at doses that are one-tenth of the dose required for complete amorphization. With continued irradiation, uraninite nanocrystals form that are a few nanometers in diameter, at 4–20 × 10 10 Gy. In a geologic repository, for spent nuclear fuel, the estimated absorbed doses due to ionizing radiation may be as high as 10 8 –10 11 Gy after 10 6 years. This is well in excess of doses in the laboratory experiments that caused the uranyl peroxides to become amorphous and decompose.

A new mode of arsenic incorporation into arsenian pyrite has been discovered. Electron microprobe analyses and elemental maps of arsenian pyrite from Pueblo Viejo, Dominican Republic, show that its chemical composition varies from (Fe0.998As0.003)1.001S2 to (Fe0.963As0.050Cu0.003Ag0.001) 1.017S2 and that arsenic is inversely correlated with Fe. High-resolution transmission electron microscopy (HRTEM) images show that some arsenic in this pyrite is present as nanoscale inclusions of amorphous As- Fe-S in a matrix of arsenian pyrite. The amorphous inclusions display negative facets with a cubic or rectangular morphology, typical of negative inclusions of pyrite and arsenopyrite or marcarsite, respectively, and they are oriented parallel to the lattice fringes (100) of the arsenian pyrite matrix. Elemental maps collected by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show that the inclusions have a higher content of arsenic than the surrounding pyrite, and TEM-EDX analyses indicate approximate atomic proportions of 62 at% S, 28 at% Fe, and 10 at% As for the inclusions, which are near the minimum melting temperature in the As-Fe-S ternary. These observations suggest that the inclusions were trapped as liquids during growth of the surrounding arsenian pyrite. Although not a new mineral, this constitutes a third form for arsenian pyrite, which has previously been shown to contain arsenic as either As1- or As3+.

Although As-rich, hydrothermal overgrowths on pyrite have been recognized as the primary host phase for Au in Carlin-type deposits in Nevada, the chemical and structural state of the Au has remained unresolved. Spectroscopic and electron imaging techniques have suggested that Au is either structurally bound (e.g., Au 1+ ) or occurs as particles of native Au (Au 0 ), but the latter has never been observed directly. We have determined that Au is present in signiÞ cant quantities as discrete nanoparticles of native Au (~5–10 nm) in As-rich overgrowths on pyrite from the Screamer deposit in the Carlin trend, Nevada, using analytical and high-resolution TEM and high-angle annular dark-Þ eld (HAADF) imaging in STEM mode. Electron microprobe and secondary ion mass spectrometry (SIMS) analyses of the As-rich rims containing the Au-particles reveal that these rims (1–20 ∝m) contain up to 0.8 wt% Au, among the highest Au-contents ever reported for arsenian pyrite. These observations suggest two possible mechanisms for nanoparticle formation: that Au exceeded its solubility limit in arsenian pyrite causing it to be deposited as nanoparticles of native metal; or that exsolution of native metal from metastable arsenian pyrite was caused by a later event in the history of the deposit.

Bedrock samples were examined from a region in south-central New Hampshire in order to un- derstand the factors responsible for elevated As-concentrations (up to 180 ppb) in the groundwater. Although the As is predominantly from natural sources, the precise mineralogy of the As-bearing phases was unknown. As-bearing samples were examined in detail using advanced electron micros- copy techniques, including high-angle annular dark field scanning electron microscopy (HAADF- STEM), STEM elemental mapping, and high resolution TEM. Numerous As-bearing minerals were observed, predominantly arsenopyrite (FeAsS) with some westerveldite (FeAs). The arsenopyrite was partially altered to nanocrystalline (~20 nm) magnetite and westerveldite, most likely during hydrothermal circulation of fluids following the emplacement of a nearby granitoid pluton. A reac- tion rim surrounds the pristine cores of arsenopyrite, with compositionally variable amorphous zones (containing Fe, As, K, and O), a Cu-sulfide, and a uranium-bearing phase, suggesting at least three distinct fluid compositions during alteration. Because the K-Fe-As-oxide is amorphous and the nanocrystalline FeAs has a high surface area per unit mass, the dissolution rate of As-bearing phases during recent low-temperature weathering is increased. The rapid dissolution of the reduced As phase (FeAs), which is unstable under oxidizing conditions, may be an important factor in the el- evated As-concentrations in the groundwaters of this region.

High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that uraninite from the Oklo-Okelobondo natural fission reactors that occur in near surface environments, have low δ 1 8 O values and nanotextures that are consistent with interaction with ground water. These low δ 1 8 O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and post-depositional alteration of U-bearing phases.

Biotite dissolution in the laboratory and in nature was examined and compared to elucidate certain aspects of the weathering processes. Batch dissolution experiments of fresh biotite [(K 0 . 9 1 Na 0 . 0 1 )(Mg 0 . 4 0 Fe 2 . 0 7 Mn 0 . 0 5 Al 0 . 1 4 Ti 0 . 1 9 )(Si 2 . 8 2 Al 1 . 1 8 )O 1 0 (OH) 2 in granite were carried out at 150 °C for 1 to 56 days. Examination by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) revealed that, in the early stage, dissolution proceeds from the edges of crystals inward and secondary minerals such as Fe oxide are precipitated mostly at the edges, with only a few secondary minerals found on the basal surfaces. A dissolution experiment using a mixture of biotite and muscovite, done at 150 °C for 7 days, indicated that hematite crystals formed mostly at the edges of biotite but not on muscovite. This observation suggests that released Fe is precipitated before it diffuses into the bulk solution. Because a dissolution rate at the edge is larger by two orders of magnitude than at the basal surface (Turpault and Trotignon 1994), precipitation before diffusion of dissolved elements to bulk solution well explains the preferential secondary mineralization at the edges in the laboratory experiments. SEM-EDS of fresh to slightly weathered biotites revealed that early stage weathering proceeds in the same way as in the laboratory with the edges being preferentially weathered and secondary minerals being precipitated mostly at the edges. Because of the similarity between the occurrence of secondary minerals in the laboratory experiments and in nature, the laboratory results elucidate the early stage weathering conditions, namely that (1) supersaturation with respect to secondary minerals in a solution occurs around biotite, i.e., the solution is poorly connected to a main flow pathway of water, and (2) once supersaturation is achieved, secondary minerals are precipitated mainly at the edge before some released elements diffuse into the solution. The immobility of water immediately adjacent to primary minerals partly explains the large difference in dissolution rate between the laboratory and natural samples. A phlogopite dissolution experiment was done at 150 °C for 5.5 days to examine the effect of biotite composition on vermiculitization. High-resolution transmission electron microscopy revealed that vermiculite layers are formed by layer-by-layer transformation within phlogopite after 5.5 day dissolution but not within biotite after 56 day dissolution or within slightly weathered biotite. A comparison of our data with results of other studies indicates that the formation of vermiculite layers between biotite layers occurs even in the early stage when biotite is not Fe-rich [more than 0.8 Mg per O 1 0 (OH) 2 ]. When biotite is Fe-rich [less than 0.4 Mg per O 1 0 (OH) 2 ]-like the present one-released Fe is precipitated as, for example, Fe hydroxide, and vermiculite is rarely formed because of limited availability of Mg. Thus, a higher Mg content in biotite facilitates the formation of vermiculite, at least in the early stage. Because vermiculite dissolves at a much slower rate than biotite, Mg-rich biotite dissolves at a slower rate than Fe-rich biotite. In the late stage, biotite dissolution still continues from the edge and within biotite, which results in a fine comb-like texture for weathered biotite; vermiculite occurs as a domain within chemically and structurally altered biotite as well, as at the edge even during weathering of Mg-poor biotite.

The susceptibility of uranophane, a uranyl sheet silicate, ideally Ca(UO2)2(SiO3OH)2(H2O)5, to ionizing irradiation has been evaluated by systematic irradiations with 200 keV electrons over the temperature range 94 to 573 K. High-resolution transmission electron microscopy revealed that amorphous domains formed locally, concurrently with a gradual disordering of the entire structure. Amorphization doses at room temperature were 1.1 × 1010 Gy for uranophane, 1.3 × 1010 Gy for Sr-substituted uranophane, and 1.9 × 1010 Gy for Eu-substituted uranophane; thus, there was an increase in amorphization dose with increasing average atomic mass. At 573 K, the amorphization dose of uranophane was 2.0 × 1011 Gy. The temperature dependence of the amorphization dose of uranophane has two stages; ď≤413 K and >413 K. Based on a defect accumulation model, the effective activation energies for amorphization at each stage are 0.0440 eV and 0.869 eV, respectively. This suggests that the presence of H2O (and OH−) reduce the energy deposition required to cause–amorphization. Above 413 K, the amorphization dose increased due to the absence of H2O and OH and the absence of radiolytic decomposition of H2O and OH−.