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Minerals from Macedonia XVIII. Vibrational spectra of some sorosilicates

The vibrational spectroscopy is used in order to identify and characterize three sorosilicate minerals originating from the localities in the Republic of Macedonia: epidote, Ca_2Al_2(Fe^3+, Al)(SiO_4)(Si_2O_7)O(OH), hemimorphite, Zn_4(Si_2O_7)(OH)_2.H_2O and ilvaite, CaFe_2^2+Fe^3+-(Si_2O_7)O(OH). The chemical composition of these minerals is not necessarily fixed, especially for epidote-clinozoisite mineral series: Ca_2Al_2(Fe^3+, Al)(SiO_4)(Si_2O_7)O(OH)-Ca_2Al_2Al(SiO_4)(Si_2O_7)O(OH), where the content of Fe^3+ and Al could substantially vary. In this context, it was shown that the intensity and especially the frequency of the IR band in the nu(OH) region could serve as a tool for reliable determination of the Fe^3+ and/or Al^3+ in epidote. The similarity between the sorosilicate infrared spectra in the 1300-370 cm^-1 region is based on their common structural characteristics. The bands in this region are tentatively assigned being mostly prescribed to the vibrations of the Si_2O_7 units (and SiO_4 tetrahedra in epidote). On the other hand, the similarities between their Raman spectra are less expressed, especially in the case of ilvaite spectrum. The authenticity of the studied minerals is checked by the powder X-ray diffraction. The results of the study are compared with the corresponding literature data for the analogous mineral species originating all over the world.

sorosilicate minerals; epidote; hemimorphite; ilvaite; vibrational spectroscopy

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