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+0.72–0.83 Ti0.00–0.02Si2.97–3.02O12) 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 720°C, 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.

Structural dissymmetrization of optically anisotropic Grs64±1Adr36±1Sps2grandite from Meka Presedla (Kopaonik Mt., Serbia)

“…However, there were also structure studies performed with constraints in , , , Fddd , I 2/ c , C 2/ c , and space groups; and the mixture of the two phases. These constraints were 〈Si–O〉 = 1.644(5) Å, 〈 Y –O〉 = 1.960(5) Å, 〈 X (1)–O〉 = 2.336(5) Å, and 〈 X (2)–O〉 = 2.494(5) Å distances; and for Al = 64 and Ca = 98 sofs , adequate to the observed Grs 64±1 Adr 36±1 Sps 2 chemical composition (Tančić et al ., 2012), and calculated from data given by Novak and Gibbs (1971). All angles were treated without constraints.…”

“…A third important criterion is that there should be a present difference between Al sofs over the octahedral Y sites. Other six significant criterions are average 〈Si–O〉, 〈 Y –O〉, 〈 X –O〉, and 〈 D –O〉 distances; and average sofs for Al and Ca at the octahedral and dodecahedral sites, consistent with the grandite chemical composition (Tančić et al ., 2012). And last two chosen criterions are R B and R F reliability factors, whose values should be at least better than those obtained for the space group (Supplementary Material 6, third row).…”

“…On the basis of the { X } cation contents (Ca 2+ , Mn 2+ , and Mg 2+ ), as well as of the [ Y ] cation contents (Al 3+ , Fe 3+ , and Ti 4+ ) at the A–E zones (Tančić et al , 2012, Table III), it was calculated mean cation ratios at the dodecahedral (〈r{x}〉) and octahedral (〈r[y]〉) sites, using the data given by Shannon and Prewitt (1969). Such calculated values and distributions of 〈r{x}〉 = 1.1168–1.1169 Å and 〈r[y]〉 = 0.5750–0.5813 Å are presented in the right part of Supplementary Material 7.…”

“…The macroscopically zoned grandite from the garnetite skarn of Meka Presedla (Kopaonik Mt., Serbia) studied in this paper was previously characterized with optical microscopy, electron microprobe analysis (EMPA), FT-IR, and Raman methods (Tančić et al ., 2012). The optical microscopy study showed that grandite is entirely through the sample slightly anisotropic without the presence of isotropic areas.…”

“…In the present study, core (zone A) of the previously studied grandite (Tančić et al ., 2012) with the Grs 64±1 Adr 36±1 Sps 2 composition was reconsidered with regard to the two remaining possible causes for its optical anisotropy (i.e. causes 4 and 5).…”