Structural links between zeolite-type and clathrate hydrate-type materials: strands of small clusters of water molecules interconnect oligomeric silicate Si 8 O 18 (OH) 2 6– anions to generate the 3D host structure of the heteronetwork clathrate DMPI 6 Si 8 O 18 (OH) 2 ·48.5h 2 O (DMPI = 1,1-dimethylpiperidinium)