PYRITE DISSOLUTION EXPERIMENTS: PRODUCTION OF CLEAN, UNIFORM PARTICLE SIZE FRACTIONS

Pyrite reactions are challenging to study experimentally, and comparison of dissolution rates is often difficult, in part because methodologies regarding pyrite mineral preparation are inconsistent within the literature. Pyrite powders are generally prepared by grinding a homogenous, massive pyrite sample using a mortar and pestle or a mixer mill. To achieve a specific size range of material for experiments, samples are either dry sieved, wet sieved, or both. Sieved samples are then cleaned in various ways to remove fine particles adhering to the mineral surface and oxidation products prior to use. Given the number of geochemical experiments using pyrite, and the current wide range of preparation techniques, there is clearly a need for standardization to allow for experimental and interlaboratory comparison of experimental results under diverse conditions.

Our objective was to assess preparation methodologies as presented in the literature, and to develop a reproducible technique that yielded clean material within a narrow size range of interest (45-75 micron diameter). After evaluating several techniques, we developed a wet sieving preparation method successful at concentrating the intended particle size range. This procedure, a modification of existing vacuum filtration techniques, involves drawing the powdered sample through sequential sieves using water and ethanol. No further cleaning is required to produce a uniform pyrite fraction, as determined by SEM analysis. This work was conducted as part of a comparative investigation of dissolution rates for hydrothermal and sedimentary pyrite. Results from dissolution experiments indicate that dry-sieved samples exhibited higher dissolution rates than those separated using our wet sieving technique. The elevated dissolution rates exhibited by the dry sieved material can be attributed to a high percentage of very fine particles remaining in the sample, even after vigorous sieving. The presented methodology uses readily available equipment within the laboratory and provides improved control over particle size separation.