Characterization, axial anisotropy and formation conditions of celestine minerals from the Jabal Eghei (Nuqay) volcanic province (southeastern edge of the Sirt Basin, southern Libya)

Abstract

Five celestine crystals, sampled from the surface at the Miocene to Pliocene Jabal Eghei (Nuqay) volcanic province (southern Libya), are characterized by applying the combination of the SEM-WDS, XRPD and IR methods; and analyzed for their color variations and minerogenesis. Three samples have greenish-blue to blue (480.4−482.5 nm), whereas the other two samples are of blue-green color (cyan; 489.1−494.1 nm). The color purity is in the 1.36–7.16 range. Their chemical composition is mutually similar, fitting into the celestine near-end members, in which exclusively 1.6–4.1 at. % of Sr2 + contents was substituted by Pb2+ (0.7–0.9 at. %), Ba2+ (0.5–0.7 at. %) and Ca2+ (0.2–0.8 at. %); including the vacancies (1.0−1.9 at. %; only in three samples). The resulting unit-cell parameters have the following ranges: a0 = 8.3578(9)−8.3705(6) Å; b0 = 5.3510(5)−5.3568(4) Å; c0 = 6.8683(7)−6.8767(2) Å and V0 = 307.17(5)−308.34(4) Å3. The XRPD and IR results are mainly in accordance with the SEM-WDS analyses, having higher mutual correlativity. However, a few discrepancies among the results are observed, further yielding several possible interpretations. The overall results show that discrepancies were primarily induced by a slight unit-cell axial anisotropy, occurring as a consequence of their thermal expansion. Essentially, the investigated Sr-bearing celestines were formed as a secondary volcanic system, emplaced over the gypsum and/or anhydrite minerals, at ~ 368−430K (~ 95–157 oC) temperature range, in the ambient pressure conditions. The study further shows that the investigated celestine minerals are likely formed at a slightly elevated temperature and pressure subsurface environment, reaching over 250 bars.

Water–Rock Interactions across Volcanic Aquifers of the Lece Andesite Complex (Southern Serbia): Geochemistry and Environmental Impact

“…The Lece-Radan calc-alkaline ore-bearing volcanic complex or Lece district [8,25,26] is located within a complex tectonic assembly of southern Serbia (Figure 1a). A number of volcanic bodies crosscut the area of Central Serbia, in which plutonic rocks belong to considerably large magmatic provinces of the Cenozoic age [27][28][29].…”

“…The investigated area is the Lece andesite complex (hereinafter LAC), which is emplaced within the western realm of the gneissic basement unit or the Serbo-Macedonian Unit. The LAC occurs along the Tethyan active margin, and the Jurassic-Cretaceous members belong to the composite Vardar Zone (southern Serbia) [6][7][8]. Besides exposing the unaltered volcanic-type system (andesite) and its pyroclastic equivalents, the massif includes segments of hydrothermally altered (mostly propylitized rocks).…”