Characterization, axial anisotropy and formation conditions of celestine from the Jabal Eghei (Nuqay) late Neogene – Pleistocene volcanic province, southern Libya: Constraints on the mineralogical geothermometer

Abstract

Five celestine crystals are sampled from the (paleo)surface intervening between the late Miocene to Pleistocene basaltic sequences of the Jabal Eghei(Nuqay) volcanic province (southern Libya). The celestine specimens are characterized by applying the combination of the SEM-WDS, ICP/OES, XRPD, and IR methods. The celestine minerals are further analyzed for their color variations and minerogenetic framework. Three samples have greenish-blue-to-blue (480.4-482.5 nm), whereas the other two samples have blue-green color (cyan; 489.1-494.1 nm). The color purity ranges from 1.36-7.16. Their similarity of chemical content is fitting into the celestine near-end members, in which exclusively 1.6-4.1 at. % of Sr2+ content was substituted by Pb2+ (0.7-0.9 at. %), Ba2+ (0.5-0.7 at. %) and Ca2+ (0.2-0.8 at. %). The composition includes vacancies ranging from 1.0 to 1.9 at. % (observed only in three samples). The content of other chemical elements is minor. 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 results, having a higher level of correlativity. However, the analysis exposed a few discrepancies yielding several possible interpretations. The illustrated discrepancies were primarily caused by a slight unit-cell axial anisotropy i.e., thermal expansion. In this manner, the results yield a new geothermometric tool that is based on the unit-cell axial anisotropy. The investigated Sr-bearing celestines were formed during a Miocene intraplate volcanism, basaltic magmas, and associated brines lifted by the structural conduits (normal faults crosscutting the Sirt basin). The Sr-bearing fluids were then poured into and over the faulted and fractured lagoon-type gypsum, anhydrite Eocene sediments. The celestine minerals were produced within a ~ 368-430K (~ 95-157 oC) temperature range. The celestine is formed at slightly elevated temperature and pressure conditions, close to the shallow subsurface environment (over 250 bars).