STRUCTURAL CHANGES OF ALLOPHANE DURING PURIFICATION PROCEDURES AS DETERMINED BY SOLID-STATE ~(27)Al AND ~(29)Si NMR

摘要

Allophanes are poorly crystalline and quasi-stable aluminosilicate minerals, the structures of which are sensitive to chemical treatment. In the present study, solid-state ~(27)Al and ~(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_2O_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_2CO_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~33~ⅥAl, -78 ppm in ~(29)Si NMR) and increased signal intensities corresponding to ~ⅣAl (55 ppm in ~(27)Al NMR) and possibly X-ray amorphous aluminosilicates (centered at -85 ppm in ~(29)Si NMR). Cold (room temperature) 5% Na_2CO_3 treatment for 16 h proved to be effective in avoiding these structural changes.