Fluorine-, yttrium- and lanthanide-rich cerianite-(Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosion craters, Gregory Rift, northern Tanzania

Abstract

Cerianite-(Ce), ideally CeO2, occurs as rounded grains up to 5 μm across in a block of highly altered calcite carbonatite lava from the Kerimasi volcano, and as euhedral crystals up to 200 μm across in carbonatite-derived eluvial deposits in the Kisete and Loluni explosion craters in the Gregory Rift, northern Tanzania. X-ray powder diffraction data (a = 5.434(5) Å) and Raman spectroscopy (minor vibration modes at 184 and 571 cm—1 in addition to a strong signal at 449 cm—1) suggest the presence of essential amounts of large cations and oxygen vacancies in the Kisete material. Microprobe analyses reveal that the mineral contains both light and heavy trivalent rare earth elements (REE) (7.9-15.5 wt.% LREE2O3 and 4.9-9.7 wt.% HREE2O3), and that it is enriched in yttrium (7.1 — 14.5 wt.% Y2O3) and fluorine (2.2—3.5 wt.%). Single-crystal structure refinement of the mineral confirms a fluorite-type structure with a cation—anion distance of 2.3471(6) Å. The cerianite-(Ce) is considered to be a late-stage secondary mineral in the carbonatitic rocks.

Rare Earth Element Phases in Bauxite Residue

“…Based on the analysis of synthetic and natural cerianite specimens, this band can be attributed to the symmetric breathing mode of Ce-O-Ce bond [67,68]. The band at 396 cm −1 present on the reference spectrum R050379, but missing in current experimental spectrum, is rarely noted in different cerianite Raman spectra [67,68]. The band at 396 cm −1 could also be hidden due to the broadening of 457 cm −1 band.…”

“…Broadening as well as shifting of cerianite main band is noted to occur along with decreasing particle size [69]. Notably, the acquired spectrum lacks the presence of Raman bands that could be associated with the occurrence of carbonate (around 1100 cm −1 ) or hydroxide ions (3400-3600 cm −1 ) [67,70]. The matrix where cerianite is situated, is mainly controlled by hematite, following the Raman bands at 225, 293 and 409 cm −1 [61,71].…”

“…This textural fractionation has not been reported in the case of Parnassos-Ghiona bauxite deposit. The LREE phases were identified by µ-Raman spectroscopy being cerianite (CeO 2 ), following the main band at 457 cm −1 , when comparing with the reference spectrum R050379 from RRUFF database and those spectra given in the literature sources (Figure 1b) [61,67]. Based on the analysis of synthetic and natural cerianite specimens, this band can be attributed to the symmetric breathing mode of Ce-O-Ce bond [67,68].…”

“…The matrix where cerianite is situated, is mainly controlled by hematite, following the Raman bands at 225, 293 and 409 cm −1 [61,71]. Zaitsev et al have demonstrated that fluorine can also be present in cerianite [67]. Fluorine is also present in cerianite of Parnassos-Ghiona bauxite (Figure 1a).…”

“…The LREE phases were identified by µ-Raman spectroscopy being cerianite (CeO 2 ), following the main band at 457 cm −1 , when comparing with the reference spectrum R050379 from RRUFF database and those spectra given in the literature sources (Figure 1b) [61,67]. Based on the analysis of synthetic and natural cerianite specimens, this band can be attributed to the symmetric breathing mode of Ce-O-Ce bond [67,68]. The band at 396 cm −1 present on the reference spectrum R050379, but missing in current experimental spectrum, is rarely noted in different cerianite Raman spectra [67,68].…”