A MAS NMR study of a monoclinic/triclinic phase transition in an amphibole with excess OH: Na3Mg5Si8O21(OH)3
Liu, Shuangxi; Welch, Mark D.; Klinowski, Jacek; Maresch, Walter V.
European Journal of Mineralogy Volume 8 Number 2 (1996), p. 223 - 230
published: Apr 26, 1996
manuscript accepted: Dec 18, 1995
manuscript received: Mar 3, 1995
ArtNo. ESP147050802001, Price: 29.00 €
Abstract The synthetic amphibole Na3Mg5Si8O21(OH)3, which contains one excess OH per formula, undergoes a complex structural phase transition from a high-temperature monoclinic polymorph (C2/m) to a low-temperature triclinic polymorph (Cl or Cl) at -100-160 °C A commensurate superstructure with a tripled b repeat forms below 100 °C. This phase transition has been studied by 29Si MAS NMR with cross-polarization, 23Na and 1H MAS NMR. The results (a) confirm that the monoclinic high-temperature polymorph has space group C2/m; (b) are consistent with a tripled superstructure for the triclinic polymorph; and (c) 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.