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Crystallographic insights into monovalent thallium incorporation: Exploring hydropyrochlore structure for environmental remediation

  • Alice Taddei , Luca Bindi ORCID logo EMAIL logo , Giovanni O. Lepore , Henrik Skogby and Paola Bonazzi
Published/Copyright: November 4, 2024
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American Mineralogist
From the journal American Mineralogist Volume 109 Issue 11

Abstract

Hydropyrochlore, ideally (H2O,□)2Nb2(O,OH)6(H2O), is a cubic mineral (space group Fd3m, a = 10.56–10.59 Å, Z = 8) belonging to the pyrochlore supergroup (general formula: A2–mB2X6–wY1–n). The K-rich variety of this species is unique to the Lueshe syenitic-carbonatitic deposit (D.R. Congo), where it occurs as the alteration product of primary (Ca,Na)2Nb2O6F pyrochlores. The structure of this mineral is made of a B2X6 (B = Nb, Ti; X = O, OH) framework that generates tunnels along the [110] direction, where the interstitial sites are partially occupied by water molecules and minor amounts of different cations. These features form the basis for the ion-exchange properties of hydropyrochlore, making it a promising candidate as a mineral sink for heavy metals (e.g., Tl+) dispersed in aqueous matrices, with interesting environmental implications.

Tl+ incorporation was induced through imbibition experiments in a diluted Clerici solution using single crystals of hydropyrochlore from Lueshe (D.R. Congo); the modifications induced by Tl+ incorporation were then evaluated using single-crystal X-ray diffraction (SCXRD), electron microprobe analyses (EMPA), and Fourier transform infrared (FTIR) spectroscopy.

After Tl+ imbibition, a dramatic increase of the A-site electron density (n.e– from ~4 to ~60) confirms the entry of a substantial amount of Tl+ at this site (up to about 70%), leading to a lengthening of the A-X distances and the consequent expansion of the unit cell. A decrease of the site scattering at the Y′ site (from ~9 to ~4 e–) also occurs, suggesting a loss of aqueous species. Although the predominance of neutral H2O molecules at the interstitial sites of hydropyrochlore from Lueshe is widely accepted by the mineralogical community, our crystal-chemical and FTIR data provide evidence that the dominant species might be the hydronium ion, with significant implications on the nomenclature of the pyrochlore supergroup.

Understanding the crystallographic aspects of hydropyrochlore as a potential waste form for monovalent thallium immobilization not only addresses a pressing environmental concern but also contributes to the broader field of waste management.

Keywords: Hydropyrochlore; hydrokenopyrochlore; thallium; ion exchange; single-crystal X-ray diffraction; electron microprobe analysis; Fourier transform infrared spectroscopy

† Deceased March 18, 2024.

Acknowledgments and Funding

The authors acknowledge CRIST, Centro di Studi per la Cristallografia Strutturale, Università degli Studi di Firenze (Italy), and Naturhistoriska Riksmuseet, Stockholm (Sweden) for X-ray diffraction (single-crystal and powder, respectively) measurements. Electron microprobe analyses were carried out at LaMA, Laboratorio di MicroAnalisi, Università degli Studi di Firenze (Italy), and FTIR spectra were collected at the Naturhistoriska Riksmuseet, Stockholm (Sweden). C. Ferraris (Muséum National d’Histoire Naturelle, Paris, France) and M. Batoni (private collector) are warmly thanked for providing the samples. The paper benefited from the useful suggestions and comments of the Associate Editor E.S. Grew, D. Atencio, and two anonymous reviewers. This research received financial support by Università degli Studi di Firenze through the project BONAZZI_RICATEN23- Pyrochlore supergroup minerals: potential sinks for toxic elements in aqueous matrices, prot. 58517.

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Received: 2023-12-20
Accepted: 2024-02-21
Published Online: 2024-11-04
Published in Print: 2024-11-26

© 2024 by Mineralogical Society of America

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https://doi.org/10.2138/am-2023-9289
Keywords for this article
Hydropyrochlore; hydrokenopyrochlore; thallium; ion exchange; single-crystal X-ray diffraction; electron microprobe analysis; Fourier transform infrared spectroscopy

Articles in the same Issue

  1. Tetrahedral aluminum in tourmaline from a spinel-pargasite-metamorphosed mafic-ultramafic rock
  2. Lorenz number and transport properties of Fe: Implications to the thermal conductivity at Earth’s core-mantle boundary
  3. Structure and equation of state of Ti-bearing davemaoite: New insights into the chemical heterogeneity in the lower mantle
  4. Solfataric alteration at the South Sulfur Bank, Kilauea, Hawaii, as a mechanism for the formation of sulfates, phyllosilicates, and silica on Mars
  5. Plastic deformation and trace element mobility in sphalerite
  6. Crystallographic insights into monovalent thallium incorporation: Exploring hydropyrochlore structure for environmental remediation
  7. Distribution of REE between amphibole and pyroxenes in the lithospheric mantle: An assessment from the lattice strain model
  8. Elastic anomalies across the P21nm→Pnnm structural phase transition in δ-(Al,Fe)OOH
  9. Hyper-enrichment of heavy rare earth elements in highly evolved granites through multiple hydrothermal mobilizations
  10. In-situ zircon and cassiterite LA-ICP-MS geochronology and implications for granite-hosted Sn deposit models and exploration: Insights from the Cameroon Line
  11. Compressibility, thermal expansion, and Raman spectroscopy of synthetic whitlockite Ca9Mg(PO3OH)(PO4)6 at high pressures and high temperatures
  12. Experimental determination of tin partitioning between titanite, ilmenite, and granitic melts using improved capsule designs
  13. Low-temperature crystallization of kumdykolite, a polymorph of albite, during mineral carbonation within fluid inclusions in hornblendite from the Dabie orogen, central China
  14. Louisfuchsite, Ca2(Mg4Ti2)(Al4Si2)O20, a new rhönite-type mineral from the NWA 4964 CK meteorite: A refractory phase from the solar nebula
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