MINERALOGICAL CHARACTERIZATION OF RIEBECKITE FROM ALINCI, REPUBLIC OF MACEDONIA
In this paper are presented the preliminary mineralogical characterization of riebeckite from Alinci, Republic of Macedonia. Several crystals of riebeckite were collected for research. The following three methods were used for the examination: scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectrometer (EDS), X-ray powder difraction (XRD) and Fourier Transform Infrared Spectroscopy using Diffuse Reflectance method (FTIR/DRS). With these methods was established that the investigated mineral is riebeckite. Riebeckite from Alinci appears in needle crystals with dark green-black colour, vitreous luster and greenish brown streak. The concentration of elements in riebeckite crystals is: Na (2.97–4.94%), Mg (4.89–6.96%), Fe (10.83–27.21%), Si (21.90– 26.42%), Ca (0.89–4.38%). The Fe/Mg ratio is from 1.8 to 3.8. Riebeckite (with Fe/Mg >> 1) is common mineral in alkaline granites and syenites. In alkaline granites and syenites, a continuous series exists between arfvedsonite and the riebeckite. Оuter surface of riebeckite is covered with the crust various thickness and width. In this crust concentration of elements is: P (0.60–15.22), Mg (0.58–5.14%), Fe(4.98–13.13%), Si (3.50–25.39%), Ca (0.59–1.63%), Al (0.73– 17.33%), Ti (0.12–0.40%). There is proportional dependence between phosphorus and silicon in riebeckite crystals and crust. In the crust content of phosphorus increases, while the content of silicon decreases. Fine-grained phosphates are formed by superficial alteration processes. The nucleation and crystallization of Al and Fe phosphate phases are directly from aqueous solutions. Al and Fe phosphate phases readily precipitate from supersaturated solutions. This crust is likely a result of surface chemical reaction through altered surface layers and riebeckites dissolution at Earth surface temperatures. The most intense registered maxima in the studied powder pattern [d values (in Å), I, (hkl)] are: 8.44 (100), 3.12 (51), 2.80 (17), 3.14 (10), 3.27 (10), 2.71 (5), 4.20 (4), 1.65 (4), 3.25 (3).
Key words: riebeckite; X-ray powder diffraction; scanning electron microscopy; Fourier transform infrared spectroscopy