The effect of substitution of Cu by Zn, Fe and Ag on the optical properties of synthetic tetrahedrite, Cu12Sb4S13

Abstract

The spectral reflectances of polished specimens of synthetic tetrahedrites were measured in air and oil using normal-incident linearly polarised monochromatic light (λ : 440 nm to 800 nm). The optical constants were calculated using the Fresnel equation. Compositions of the grains studied were determined by electron microprobe. Qualitative observations of absorption were made in polished thin section using transmitted white and infra-red light. Substitution of Cu led to significant changes in the spectral reflectance curves and dispersion curves of the refractive indices of synthetic tetrahedrite. In general the optical properties of the tetrahedrites are those of a semi-conductor with a narrow band gap. The rapid increase in absorption at wavelengths shorter than 500-600 nm corresponds to excitation of electrons across the forbidden band gap to the conduction band. At wavelengths longer than 600 nm, the absorption is low for all the tetrahedrites and the reflectance values follow, in a general way, the refractive index curves. The small differences in the optical properties are due to the type of substituting element and to the varying role of the d electrons in the chemical bonding. The only tetrahedrite with a simple d10 closed shell configuration for all the metal atoms is the composition Cu₁₀¹⁺Zn₂²⁺Sb₄³⁺S₁₃²⁻ which is the only example found to be transmitting in the red and near infra-red.