Diffuse Reflectance Spectra and Optical Properties of Some Sulphides and Related Minerals

B. J. Wood and R. G. J. Strens
Department of Geology, The University, Manchester M13 9PL
School of Physics, The University, Newcastle upon Tyne NE1 7RU

Summary: The results of measurements of diffuse reflectance over the wavelength range 200 < λ < 2500 nm are reported for sphalerite, cinnabar, alabandite, chalcopyrite, bornite, orpiment, stibnite, bismuthinite, enargite, and pyrargyrite, and for eight pyrite-type and four NiAs-type compounds. Some spectral assignments have been made.

Optical properties are related to the absorption spectrum (and through this to composition and structure) in a rational way. Absolute reflectances tend to increase with mean atomic number (z) through the operation of the ‘z-sum rule’, and at constant z they decrease as the band gap increases. Bireflectance is structurally controlled, being weak in derivatives of the cubic sphalerite and pyrite structures, moderate in derivatives of wurtzite and NiAs, and strong in anisodesmic structures such as that of stibnite. Extreme bireflectance occurs in anisodesmic structures with strong dichroic absorption bands in the visible (molybdenite, covelline).

The dispersion of reflectance (dR/d2) depends on the position of the centre of the main absorption envelope (λ¯) in relation to the visible spectrum. For λ>λ¯, dispersion is normal (Rblue > Rred, dR/dλ negative), the streak is light or coloured, and polished surfaces tend to be bluish. For λ<λ¯, dispersion is reversed (Rred > Rblue), the streak is dark, and polished surfaces are yellowish. Polished surfaces are white or grey if absorption varies little through the visible or strongly coloured if it varies rapidly (covelline, chalcopyrite).

Mineralogical Magazine; December 1979 v. 43; no. 328; p. 509-518; DOI: 10.1180/minmag.1979.043.328.11
© 1979, The Mineralogical Society
Mineralogical Society (www.minersoc.org)