The real structure of tobermorite 11 angstrom: normal and anomalous forms,OD character and polytypic modifications

Specimens of tobermorite 11 Angstrom from Wessels mine, South Africa, an 'a nomalous' tobermorite, and from Bascenov, Urals, Russia, a 'normal' tobermo rite, have been studied through X-ray diffraction. Their structural disorde r, evidenced by the diffuseness of the reflections with k odd, has been dea lt with through an OD approach, which allowed us to derive a reliable model of the structure, to define the two main polytypes (MDO structures), which were subsequently refined, revealing the distinctive structural details of the two specimens. The MDO1 polytype of tobermorite 11 Angstrom from Wessels mine [orthorhombi c, space group F2dd, a = 11.265(2), b = 7.386(1), c = 44.970(9) Angstrom] w as refined to R = 0.128, whereas the refinement carried on the MDO2 polytyp e [monoclinic, space group B11m, a = 6.735(2), b = 7.385(1), c = 22.487(4) Angstrom, gamma 123.25(1)degrees] converged to R = 0.051. Two refinements w ere carried out for the MDO2 polytype of the specimen from the Urals [space group B11m, a = 6.732(2), b = 7.368(1), c = 22.680(4) Angstrom, gamma = 12 3.18(1)degrees] with data collected with conventional source (R = 0.089) an d with synchrotron radiation (R = 0.110). Common structural features are the infinite layers of calcium polyhedra, pa rallel to (001), with tetrahedral chains of wollastonite-type running along b and attached on both sides of these layers. The composite layers are sta cked along c and connected through formation of double tetrahedral chains, with 'ideal' composition [Si6O15(OH)(2)] and [Si6O16(OH)] in the specimens from Wessels mine and Urals respectively. The most distinctive feature: of the two specimens is the content of the structural cavities: whereas only w ater molecules were found in the specimen from Weasels mine, 'zeolitic' cal cium cations and water molecules were found in that from Urals. These struc tural aspects are compared and their relevance in explaining the different thermal behaviour of the two types of tobermorite 11 Angstrom is indicated.