Mechanism of Gd3+ uptake in gypsum (CaSO4·2H2O): Implications for EPR dating, REE recovery and REE behavior. (1st August 2019)

• Record Type: Journal Article
• Authors: Lin, Jinru; Nilges, Mark J.; Wiens, Eli; Chen, Ning; Wang, Shaofeng; Pan, Yuanming
• Is Part Of: Geochimica et cosmochimica acta. Volume 258(2019)
• Page Start: 63
• Page End: 78
• Publication Date: 2019-08-01
• Subjects: Rare earth elements -- Gypsum -- EPR/ESR dating -- REE recovery -- REE behavior; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2019.05.019
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Rare earth elements (REE) in gypsum have long attracted interest for geochemical applications from the reconstruction of paleoclimate, paleoenvironment and paleohydrology to the monitoring of volcanic and seismic activities. Resurgent interests in REE in gypsum stem from the potential recovery of these high-tech metals from gypsum-rich tailings. Development of new geochemical applications and invention of effective recovery technologies all require knowledge about the mechanism of REE uptake in gypsum. In this contribution, we have investigated Gd-doped gypsum from the gel diffusion technique at ambient conditions in the pH range from 6.5 to 9.5. Inductively coupled plasma mass spectrometry analyses of Gd-doped gypsum yielded 752 ppm Gd at pH = 6.5 down to 2.8 ppm Gd at pH = 9.5. Synchrotron Gd L3 -edge X-ray absorption spectra show that Gd 3+ has a local environment similar to that of Ca 2+ in gypsum. Single-crystal electron paramagnetic resonance (EPR, also known as electron spin resonance or ESR) spectra reveal a dominant Gd 3+ center with weak satellites. The dominant Gd 3+ center arises from a substitutional Gd 3+ ion at the Ca site retaining its C 2 local symmetry, which is also confirmed by pulsed electron nuclear double resonance (ENDOR) data for proton hyperfine structures up to 5.2 Å away from Gd 3+ . The satellites are attributable to a Gd 3+ -Gd 3+ pair (dimer) along the crystallographic a-axis with a dipolar coupling constant D d /ge βe = −13 G, which corresponds to a distance of 12.6 Å between the two Gd 3+ ions and provides the first experimental confirmation for the substitution 2Gd 3+ + □ = 3Ca 2+ in gypsum. Hole trapping at cation vacancies associated with the incorporation of Gd 3+ (and other trivalent REE) in gypsum offers a new class of radiation-induced paramagnetic defects in this mineral for EPR/ESR dating. These results also have implications for developing new technologies for effective recovery of REE from gypsum-rich tailings and for understanding the behavior of REE in evaporites and aqueous solutions.