COMPOSITION AND CRYSTAL STRUCTURE OF SYNTHETIC PYRITE DOPED WITH AS, CO AND NI

In preparation for electrical characterization and oxidation rate studies, pyrite was synthesized using the chemical vapor transport method, with FeBr₃ added as a transport agent. Undoped crystals and crystals doped with Co, As, or Ni were characterized by electron microprobe, laser ablation ICP-MS, and synchrotron X-ray diffraction. EMP and LA-ICP-MS indicate nearly homogeneous crystals when Ni or Co is the dopant. Crystals doped with As contain more defects such as voids and cracks. In these crystals, a phase with stoichiometry similar to orpiment but enriched in Br commonly coats defect surfaces and crystal faces; this coating appears to be residual condensation from vapor in the synthesis tubes. Lattice parameters calculated from SXRD are 5.4174(2) Å for undoped pyrite and pyrite with 0.3 wt.% Co. Pyrite with ~250 ppm As has a slightly expanded lattice parameter of 5.4186(1) Å.

Major element stoichiometry (S/Fe) varied from 1.95 to 2.04 (mean of 9 - 29 EMP measurements per grain). The low values were from As-doped samples while high values were from Ni or Co-doped samples; S/Fe for the undoped sample was 1.99. Results are consistent with As substitution for S and Ni or Co substitution for Fe. Trace element concentrations from LA-ICP-MS are consistent with EMP results for highly doped samples, giving confidence in LA-ICP-MS measurements for samples where trace element concentrations are below EMP detection limits. Results indicate that doping efficiency (wt% in crystal / wt% in reagents) was much higher for Ni and Co (2 to 30%) than for As (< 1%). Doping efficiency generally increased with dopant concentration. The highest concentrations of trace elements incorporated were 0.4 wt% Co, 0.25 wt.% Ni, and 800 ppm As. These are much lower than concentrations typically found in natural pyrite. Further synthesis experiments are in progress to assess the effect of synthesis temperature and gradient on crystal homogeneity and doping efficiency.