Angela Halfpenny

Microelectronic junctions in arsenian pyrite due to impurity and mixed sulfide heterogeneity

Abstract Impurities and crystal defects within the semiconducting bulk of a metal sulfide introduce energy levels within the forbidden bandgap. These levels in turn control semiconducting type and local electrical properties within single and multi-phased sulfide assemblages. Heterogeneity in sulfide semiconductivity linked to these impurities can lead to p-n micro-junction formation and potential distributions near the surface that may alter redox reactivity. Secondary gold ore genesis via a micro-galvanic effect related to heterogeneity has in the past been hypothetically linked to such micro-junctions. Understanding these regions and their interaction with weathering fluids in the regolith for example requires large-scale imaging of potential distributions associated with near-surface micro-junctions and correlation with the responsible elemental distributions. Here we investigate the existence of micro-electronic junctions in a mixed sulfide assemblage using scanning laser beam induced current (LBIC) and correlate them with pyrite-chalcopyrite interfaces mapped using combined energy-dispersive spectroscopy (EDS) and wavelength-dispersive spectroscopy (WDS) on an electron hyper-probe. Junctions in a natural assemblage are positively identified for the first time.

Crystallography of Natural and Synthetic Gold Alloy Microstructures

To improve our understanding of the mechanisms of gold deposition, a comparison was made of the microstructures of a natural gold sample with a synthetic gold foil of similar alloy composition (approximately Au 90%, Ag 10%). The aim was to identify any similarities between the samples that could help increase our knowledge of how the natural gold microstructures formed and were modified post-mineralisation. The samples were analysed using electron backscatter diffraction to map their microstructure, with the synthetic gold foil then heated to and mapped at 400°C, 500°C, 600°C and 700°C. Both the natural and synthetic sample exhibited a dominance of ∑3 twin boundaries, but these were much less abundant in the synthetic sample prior to heating. The natural sample is dominated by coarse grains exhibiting lattice distortion and low angle grain boundaries, which more closely resemble the synthetic gold foil microstructure after recrystallisation has taken place, than the initial microstructure, implying that the grains have had time to grow. Performing experiments such as these allows direct comparison of gold microstructures where the formation conditions are known and the controlling mechanisms can be determined. This will improve our understanding of the important mechanisms behind gold deposition.