VARIATIONS OF RESISTIVITY AND CARRIER TYPE FOR SYNTHETIC PYRITE DOPED WITH AS, CO, AND NI: IMPLICATIONS FOR OXIDATION

The rate of pyrite oxidation may be related to its resistivity, which is affected by crystal imperfections such as nonstoichiometry and impurities. In this study large synthetic pyrite crystals both undoped and doped with As, Co, and Ni, which are common impurities in natural pyrite, were characterized for resistivity, Hall mobility, and carrier concentration using a four probe van der Pauw and Hall measurement system.

The charge carriers for undoped pyrite and pyrite doped with Ni and Co are electrons (n-type) while carriers for As doped pyrite tend to be holes (p-type). Pyrite doped with 0.22 wt% Co has resistivity 4.3x10^-3 ohm*cm with carrier concentration 1.1x10¹⁹ cm^-3 and Hall mobility of 137 cm²/v*sec. Pyrite doped with 0.13 wt% Ni has resistivity of 3.3 ohm*cm with carrier concentration of 1.7x10¹⁶ cm^-3 and Hall mobility of 136 cm²/v*sec. Pyrite doped with 0.03 wt% As has resistivity of 11.9 ohm*cm with carrier concentration of 9.5x10¹⁸cm^-3 and Hall mobility of 2.4 cm²/v*sec. Undoped pyrite has resistivity of 51.3 ohm*cm with carrier concentration of 1.1x10¹⁶ cm^-3 and Hall mobility of 44 cm²/v*sec.

Chemical reactions at one site on a semiconducting crystal lattice may affect the electronic structure and thus reactivity at remote sites due to electron transfer through the substrate. The rate of such reactions should be inversely related to resistivity of the semiconductor. The doped pyrites in our study have lower resistivity than undoped pyrite. The large difference in resistivity between Co bearing pyrite and the other types may affect the oxidization potential of Co bearing material. The tendency for arsenian pyrite to be a p-type semiconductor may also influence its oxidation potential. When areas of As rich pyrite are found within grains rich in either Ni or Co or both, the potential for enhanced oxidation exists due to voltage gradients arising from p-n conductivity junctions and variable resistivity.