Interpretation of the reflectivity behavior of ore minerals

A correlation exists between the percent reflectivity (R) of pyrite-type compounds and the effective number of free electrons (Neff) in the visible region, obtained from specular reflectance spectral measurements. Both R and Neff decrease in each of the series FeS2—CoS2—NIS2—CuS2 and CuTe2—CuSe2—CuS2. A molecular orbital model is described which relates the concept of Neff to the delocalisation of excited transition metal electrons into antibonding molecular orbitals of pyrite. The values of Neff are shown to depend on the number and proximity of empty energy levels above the Fermi level. The model is extended to explain reflectivity variations in other isomorphous and isocbemical series of ore minerals.