Structural changes of allophane during purification procedures as determined by solid-state (super 27) Al and (super 29) Si NMR

Allophanes are poorly crystalline and quasi-stable aluminosilicate minerals, the structures of which are sensitive to chemical treatment. In the present study, solid-state (super 27) Al and (super 29) Si nuclear magnetic resonance (NMR) spectra of allophane samples were monitored as they went through several purification procedures. It was confirmed that no significant structural changes were caused by boiling with 6% H (sub 2) O (sub 2) to remove organic matter, by size fractionation (sonification), by sedimentation, by precipitation at pH 4.0, or by dithionite - citrate - bicarbonate treatment for the removal of Fe (hydr)oxides. Hot 5% Na (sub 2) CO (sub 3) treatment for the removal of reactive silica-alumina gels and adsorbed citrate from allophane samples, however, decreased signal intensity corresponding to imogolite-like Si (Q (sub 3) 3 (super VI) Al, -78 ppm in (super 29) Si NMR) and increased signal intensities corresponding to (super IV) Al (55 ppm in (super 27) Al NMR) and possibly X-ray amorphous aluminosilicates (centered at -85 ppm in (super 29) Si NMR). Cold (room temperature) 5% Na (sub 2) CO (sub 3) treatment for 16 h proved to be effective in avoiding these structural changes.