A 29Si MAS NMR study of tetrahedral site distributions in the layered silicic acid H+-magadiite (H2Si14O29 · nH2O) and in Na+-magadiite (Na2Si14O29 · nH2O)

The structural compositions of the layered silicic acid H+-magadiite (H2Si14O29 · 5.4H2O) and the corresponding Na+ salt (Na2Si14O29 · 9–11H2O) have been investigated by 29Si MAS NMR. Both compounds exhibit resonances characteristic of Q3 and Q4 environments in which three and four oxygen atoms of the silicate tetrahedra are bridging adjacent silicons, respectively. The Q3 and Q4 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, NaHSi2O5 · 3H2O, a layered silicate with only Q3 environments. The Q3 : Q4 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)).