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.