Abstract
A calcined pyritic gold ore concentrate was examined using both ion–milled and ultramicrotomed samples to determine the distribution of gold and the porosity of the matrix supporting the gold. Examinations of the same specimen prepared by different techniques yielded complementary information about the overall characteristics of the material.Figure 1 is a secondary electron image (SEI) of a piece of roasted pyritic ore. The particles are highly porous with morphology suggesting that the iron pyrite (FeS2) particles virtually exploded when labile sulfur was released during oxidative calcination. The particles produced by this process are not large enough to permit ordinary solid specimen preparation techniques. Therefore, a compacted bar of material was made by compressing the powder with a binder to form a disk which was subsequently impregnated with epoxy in a PARR bomb under isostatic pressure of a few thousand psi. From this dense material two types of specimen were prepared, one by diamond knife ultramicrotomy and the other by standard thin foil techniques. For the latter, the sample was sliced with a diamond saw, core–drilled into 3mm disks, polished to about 100μm, dimple ground, and Argon ion milled to perforation. This yielded specimens which show the porosity and connectivity of the iron oxide frameworks formed during calcination. Figure 2 is a 10,000X annular dark field (ADF) image of an ion–milled particle taken with a VG HB501 STEM. The channel structures evident in this image are easily related to the SEI image in Figure 1. Figure 3 is a 200,000X ADF image which illustrates the connectivity of individual iron oxide crystallites that form the skeletal structure. This sample was electron transparent only near the central perforation and did not have a large usable area. Also, the outer surfaces ot the particles may have been milled away during specimen preparation and therefore it is not easy to determine where the remaining structures were located in the original particles.
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