High-pressure and high-temperature transformation of Pb(ii)-natrolite to Pb(ii)-lawsonite

Abstract

We report on high-pressure and high-temperature chemical transformations of Pb²⁺-exchanged natrolite (Pb-NAT, Pb₈Al₁₆Si₂₄O₈₀·16H₂O) using a combination of in situ synchrotron X-ray powder diffraction and ex situ HAADF-STEM real space imaging. Three high-pressure polymorphs of natrolites (Pb-NAT-I, II, III) are observed via step-wise pressure-induced hydrations (PIH) up to 4.5 GPa, during which the number of H₂O molecules located inside the natrolite channel increases from 16 to 40 H₂O per unit-cell. At 4.5 GPa after heating the high-pressure Pb-NAT-III phase at 200 °C a reconstructive phase transits into a lawsonite phase (Pb-LAW, Pb₄Al₈Si₈O₂₈(OH)₈·4H₂O) with an orthorhombic space group Pbnm and a = 5.8216(9), b = 9.114(1) and c = 13.320(1) Å is observed. The structure of the recovered Pb-LAW phase was characterized using Rietveld refinement of the in situ synchrotron X-ray powder diffraction data and HAADF-STEM real space imaging. In the recovered Pb-LAW phase the Pb²⁺ content is close to 42 wt% and as bond valence approximations reveal the Pb²⁺ cations are more tightly coordinated to the framework oxygen atoms than originally in the natrolite phase.