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

Hydropyrochlore, ideally (H2O,◻)2Nb2(O,OH)6(H2O), is a cubic mineral (space group Fd3 ̅m, 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 through single-crystal X-ray diffraction (SCXRD), electron microprobe analyses (EMPA) and Fourier transform infrared (FT-IR) 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 FT-IR 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.