SYNTHESIS AND CHARACTERIZATION OF LARGE CO, NI, AND AS DOPED PYRITE CRYSTALS PRODUCED BY CHEMICAL VAPOR TRANSPORT

Pyrite crystals up to 7mm diameter have been synthesized with chemical vapor transport (CVT) in evacuated quartz tubes. Both doped and undoped crystals have been produced for use in oxidation experiments aimed at understanding the effect of minor element concentration on the rate of pyrite oxidation. Pyrite is a semi-conductor whose electrical properties are a function of impurity concentration, non-stoichiometry, and crystal perfection. Evidence suggests that the oxidation mechanism is related to electrical properties. Further analysis will include measuring the conductivity, carrier concentration, and Hall mobility to relate the electrical properties to the dopant.

The synthesis is accomplished with a two step process. First, troilite (FeS) is produced by grinding S under acetone with equal molar amounts of Fe powder. This charge is sealed in an evacuated quartz tube and heated to 550ºC. The result, FeS, has been confirmed with X-ray diffraction. FeS is then ground under acetone with equal molar amounts of S. Doping involves adding Co, Ni, and/or As to the S/FeS mixture. The charge is sealed in an evacuated quartz tube with FeBr3 as a transport agent. This is placed in a tube furnace for 14 days with a temperature gradient of 700ºC to 650ºC over 15cm. The crystals form in the cool end of the tube and are typically 1 to 2 mm diameter. Their habit is most often a combination of pyritohedral and octahedral.

The resulting crystals were characterized with laser ablation inductively coupled plasma mass spectroscopy (LA-ICPMS). For analysis, the crystals are washed in acetone and then HCl. LA-ICPMS is done in an Ar atmosphere using a 213 nm Nd-YAG laser coupled with a quadrupole mass spectrometer with dual detectors. The results, using spot, line, and raster geometries, indicate homogenous pure pyrite crystals and pyrite crystals with Ni, Co, and As. Crystals doped with combinations such as Ni and Co, Ni and As, or Ni, Co, and As have also been produced though homogeneity is not consistent.