A 29Si MAS NMR study of tetrahedral site distributions in the layered silicic acid H +-magadiite (H 2Si 14O 29 · nH 2O) and in Na +-magadiite (Na 2Si 14O 29 · nH 2O)

Abstract

The structural compositions of the layered silicic acid H +-magadiite (H 2Si 14O 29 · 5.4H 2O) and the corresponding Na + salt (Na 2Si 14O 29 · 9–11H 2O) have been investigated by 29Si MAS NMR. Both compounds exhibit resonances characteristic of Q 3 and Q 4 environments in which three and four oxygen atoms of the silicate tetrahedra are bridging adjacent silicons, respectively. The Q 3 and Q 4 chemical shifts assignments for Na +-magadiite (−102 and −113 ppm) and H +-magadiite (−105, −115 ppm) were supported by the chemical-shift position (−101 ppm) for kanemite, NaHSi 2O 5 · 3H 2O, a layered silicate with only Q 3 environments. The Q 3 : Q 4 site ratios are approximately 1 : 3 for both H +- and Na +-magadiite. A previously reported value of 1 : 1 based on estimates of the surface density of OH groups (cf., Ref. ( 6)) is not confirmed. Some interlayer cross-linking involving the condensation of silanol groups occurs in H +-magadiite upon dehydration at 230°C, a temperature considerably lower than that previously estimated from thermal analysis data for the onset of interlayer siloxane bond formation (cf., Ref. ( 3)).