A MAS NMR study of a monoclinic/triclinic phase transition in an amphibole with excess OH; Na (sub 3) Mg (sub 5) Si (sub 8) O (sub 21) (OH) (sub 3)

This synthetic amphibole, with one excess OH pfu, undergoes a complex structural phase transition from a high-T monoclinic polymorph (C2/m) to a low-T triclinic polymorph (C<$E 1 back 40 up 40 -> or C1) at approximately 100-160 degrees C. A commensurate superstructure with a tripled b repeat forms at <<<$I> 100 degrees C. This phase transition has been studied by (super 29) Si MAS NMR with cross-polarization, (super 23) Na and (super 1) H MAS NMR. The results confirm that the monoclinic high-T polymorph has space group C2/m and are consistent with a tripled superstructure for the triclinic polymorph; they suggest that the phase transition may be caused by a positional disordering of the excess proton, which interacts with A-site Na to cause displacement of the latter from the special position at 2/m onto general positions.