31P and 27Al solid-state nuclear magnetic resonance study of taranakite

Abstract

High-resolution 27Al solid-state nuclear magnetic resonance (NMR) spectroscopy indicates that aluminum in taranakite is probably restricted to six-coordinate sites. High-resolution 31P solid-state NMR spectroscopy reveals that phosphate groups exist in two environments in taranakite. The rate of 1H-induced dipolar dephasing of the 31P signals in cross-polarization, magic-angle-spinning NMR spectra of taranakite suggests that one or more oxygens of one of two phosphates are directly protonated. The same experiments suggests that the oxygens of the second form of phosphate are not directly protonated but may be hydrogen-bond receptors. The ratio of protonated phosphate to non-protonated phosphate, as measured from 31P single-pulse excitation, magic-angle-spinning spectra, is approximately one to three.