Synthesis and properties of phosphate cancrinite (PO4-CAN) a synthetic counterpart of depmeierite

Abstract

Phosphate cancrinite (PO4-CAN) has been synthesized under mild hydrothermal conditions at 473 K using kaolin, sodium orthophosphate-dodecahydrate and strong alkaline conditions (8 M NaOH solution). Cancrinite crystals of rod-like habit and maximum lengths up to 2 μm were observed and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and Fourier transform infrared spectroscopy (FTIR). Magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) of the nucleus 31P, thermogravimetry (TG/DTG) and heating experiments under open conditions in a furnace were additionally performed. The empirical formula Na8[SiAlO4]6(PO4)0.58(CO3)0.13(H2O)4.6 was calculated from EDXS analyses and TG data under consideration of a minor carbonate content, detected by FTIR spectroscopy. PO4-CAN is stable up to 800 °C before thermal destruction rapidly occurs at 825 °C. The final product of TG analyses at 1000 °C is a mixture of a nepheline-like phase and the phosphate content of the initial cancrinite sample. XRD and FTIR results of the synthesis product are in good agreement with literature data of the rare mineral depmeierite.