PACROFI VI - Electronic Program

Characterization of IR-transmittance of sulfides and other opaque minerals by FTIR-spectroscopy

Martin A. Ziemann* & Volker Lüders**

* University of Potsdam, Institute of Geosciences, Telegrafenberg C 7

** GeoForschungsZentrum Potsdam, PB 4.3 Lagerstättenbildung, Telegrafenberg A 50, D-14473 Potsdam, Germany

The IR-transmittance of minerals and any other solids depends on their individual electronic band structures and infrared-active vibration modes. Especially semi-conducting minerals, such as sulfides, show a very large variation and quality in IR-transmittance. The quality of a minerals' IR-transmittance strongly depends on its crystal structure and chemistry, which both define its band gap energy. Impurities of trace elements within the lattice can diminish the band gap energy and cause a decrease in IR-transmittance. As shown for pyrites, the band gap energy seems to be sensitive for quantities of As and/or other elements (Richards & Kerrich, 1993).

FTIR-spectroscopic investigations of doubly polished sections (90 microns) of various opaque minerals have been performed in order to characterize their IR-transperancy in the range lambda = 1.0 to 3.3 microns (= 1.000 to 3.300 nm). The IR-transmittance was measured by a Perkin Elmer 2000 FTIR-spectrometer with a microchamber facility, which allows a spatial resolution up to 20 microns.

Using a high-resolution Hamamatsu IR camera with a selected tube, infrared-microscopy observations in the near infrared (NIR) up to 2.6 mm are possible. FTIR-spectroscopic measurements in the range under consideration therefore allow a preselection of minerals for IR-microthermometry.

The results of FTIR- spectroscopic investigations of different opaque minerals show that, especially, stibnite and many sulfosalts display an excellent IR-transmittance which increases continuously from 1.0 to 3.3 microns. In contrast, samples of arsenopyrite which has a very low band gab energy, do not show any transparency in the NIR. Galena always shows steeply increasing maxima of transmittance, which starts at about 3.0 microns. This maximum lies well above the detection capacities of all IR cameras that are available at the moment. In contrast, the maxima of IR-transmittance of some pyrites start at 1.4 microns and therefore fall into the detection range of a high-resolution IR equipment. The spectra of pyrite and/or galena indicate mineral specific maxima of IR-transmittance of varying quality. Other opaque minerals such as wolframite show FTIR spectra that exhibit two maxima of IR-transmittance.

FTIR spectra of several opaque minerals indicate varying qualities of IR-transmittance at different wavelengths. Fluid inclusions and internal features can be observed in the wavelength range lambda = 0.8 - 1.2 (1.3) microns (IR resolution capacity of Research Devices Model-F IR microscope and usual video cameras with removed IR filter, respectively) in some opaque minerals such as wolframite, stibnite, Sb-rich sulfosalts and dark sphalerite. Since most of the FTIR spectra of opaque minerals indicate better IR-transmittance at higher wavelengths, the use of high-resolution IR systems must be favored.

References

Fig. 1 FTIR spectra of stibnite and bournite from hyfrothermal vein mineralization near Wolfsberg/SE Harz Mts. (Germany)
Fig. 2 FTIR spectr of arsenopyrite, Freiberg/Saxony (Germany)
Fig. 3 FTIR spectra of galena from various occurrences

Fig. 4 FTIR spectra of pyrites from various occurrences
Fig. 5 FTIR spectra of wolframites from Panasqueira (Portugal) and Rossgrabeneck/Black Forest (Germany)