Thermal stability of barrerite and Na-exchanged barrerite: An in situ single crystal X-ray diffraction study under dry conditions

Cametti, Georgia; Armbruster, Thomas Michael; Nagashima, Mariko (2016). Thermal stability of barrerite and Na-exchanged barrerite: An in situ single crystal X-ray diffraction study under dry conditions. Microporous and mesoporous materials, 236, pp. 71-78. Elsevier 10.1016/j.micromeso.2016.08.023

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The thermal stability of barrerite (Na8.2K3.0Ca2.2Mg0.2Al17Si55O144·50H2O) from Kuiu Island, Alaska, was investigated from 25 °C to 500 °C by in situ single-crystal X-ray diffraction under dry conditions. Corresponding experiments were performed on Na-exchanged barrerite Na17Al17Si55O144·45H2O from the same locality. The natural sample showed a gradual transformation of the A phase with dehydration to the B phase associated with rupture of T-O-T links and formation of new connections. Though, the high temperature modification (phase D) was not observed in the studied temperature range. In contrast to natural barrerite (space group Amma), Na-exchanged barrerite was found to be monoclinic F2/m at room temperature and transformed to A2/m (same cell setting) with beginning dehydration at 50 °C. Its dehydration trend, monitored by unit-cell volume, corresponded to structural modifications observed for natural barrerite. The D phase of this sample was produced by ex situ experiments at 525 °C under ambient conditions. A new model for transformation from the disordered B phase to the ordered highly condensed D phase (space group A21ma) is presented. In addition, the new in situ equilibrated single-crystal experiments are compared with dynamic X-ray powder diffraction studies.

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