The occurrence of phosphorus and other impurities in Australian iron ores

The characterisation of high-P iron ores by fi eld emission gun electron probe microanalyser (FEGEPMA) was carried out to determine elemental associations in iron oxides sensus lato and, using this knowledge, to speculate on possible P incorporation mechanisms in goethite. Quantitative data was collected from two goethite-rich bulk ores and results showed that in both samples goethite was the main repository for phosphorus. The P-rich goethite also contained elevated levels of both aluminium and silicon. To understand the mechanism of P incorporation in goethite, high-P grains were located by coarse mapping of polished blocks with the FEG-EPMA. Analysis of element distribution maps collected on the P-rich grains showed that low-P regions within grains were typically associated with low-Al and high-Si, whereas high-P regions were associated with high-Al and low-Si. Based on follow-up quantitative analyses, a coupled substitution mechanism for phosphorus incorporation within goethite was proposed: 2Si ⁴⁺ = 1P ⁵⁺ + 1Al ³⁺. Additional quantitative analyses on further examples indicated the mechanism was only valid for Si contents in goethite of <1 wt per cent. Preliminary transmission electron microscope (TEM) studies confi rmed the presence of P, Al and Si in goethite.