Raman microspectrometric and EPMA characterization of Sphalerite from Zawar Group of Mines, Rajasthan, India

Raman microspectrometry and EPMA have been used to compositionally characterize different types of sphalerite based on its FeS mole fraction. This is in reference to some previous studies where the mole fraction of FeS in sphalerite was quantified by measuring the relative intensities of peaks in Raman spectra. Based on petrographic study of samples, sphalerite was distinguished on the basis of its colour (transparent, honey yellow translucent and dark brown translucent). This was followed up by Raman microspectrometric analysis. The acquisitions were made using the Horiba LabRAM HR Evolution with a multichannel Peltier cooled (-70℃) CCD detector, using a 532 nm frequency-doubled Nd:YAG laser, 30s exposure and a laser power of 25% in the spectral range of 200-700 cm^-1. Three distinct peaks were obtained, of which the ones at ~299 and ~329 cm^-1 are assigned to Fe-S vibration modes, while the one at ~350 cm^-1 is assigned to the Zn-S vibrations. From Fig. 1, it can be observed that there is a strong correlation between the peak intensities and the Fe content, with some peaks dominating the other within individual spectrum depending on their respective FeS and ZnS contents. After applying a baseline correction, the spectra were truncated within the range of 250-360 cm^-1. The Pseudo-Voigt function (a linear combination of Gaussian and Lorentzian functions) was employed for deconvolution and peak-fitting. By measuring the heights of the peaks at 300 cm^-1 (h₁) and 350 cm^-1 (h₃), the h₁/h₃ ratio was computed. The FeS mole fraction was then calculated by substituting the values of the ratio in a linear equation. To validate the extracted sphalerite chemistry based on the iron content from Raman studies, elemental analysis was carried out using a Cameca SX-Five EPMA. The Fe concentrations, thus obtained, are in good agreement with the EPMA data (Fig. 2).

Keywords: Raman Microspectrometry, EPMA, Sphalerite, FeS mole fraction