Macroscopically-zoned grandite from the garnetite skarn of Meka Presedla (Kopaonik Mountain, Serbia)

Abstract

The macroscopically-zoned grandite from the garnetite skarn of Meka Presedla (Kopaonik Mountain, Serbia) was studied with optical microscopy, electron microprobe analysis (EMPA), Fourier transform infra-red (FT–IR), and Raman methods. The EMPA results indicate that the main core–rim compositional variations (Ca2.93–2.97Mn0.05–0.06Mg0.00–0.01Al1.14–1.26Fe3+0.72–0.83Ti0.00–0.02Si2.97–3.02O12) slightly differ along the zones, showing evidence for a quasi-cyclic alternation of the oscillatory zoning nature. Among this, considerable variation is observed only by the Al–Fe3+ substitutions in the octahedral site. The EMPA also indicate that the grandite zones compositionally vary, mostly within ±1 and ±2 mol% of the homogeneity level range, that is, Grs64±1Adr36±1Sps2 (A), Grs62±1Adr38±1Sps2 (B), Grs59±2Adr40±2Sps2 (C), Grs58±2Adr41±2Sps2 (D), and Grs58±1Adr41±1Sps2 (E). Therefore, the investigated garnet can be considered as relatively highly homogeneous. The majority of compositions lie within the narrow miscibility region of 0.58±2<XGrs<0.64±1, without gaps, and with only three outliers near the zone boundaries of approximately 0.38<XGrs<0.52. FT–IR and Raman bands are almost constant within the zones and adequate to the chemical compositions. All of the zones should be considered as anhydrous. From the results, formation temperatures between ~600 and 720oC, and pressures of ~2–3 kbars, are derived. Among five possible causes for the slightly optical anisotropy of grandite, three were reconsidered, and consequently rejected.