Specimens of tobermorite 11Å from Wessels mine, South Africa, an ‘anomalous' tobermorite, and from Bašenov, Urals, Russia, a ‘normal’ tobermorite, have been studied through X-ray diffraction. Their structural disorder, evidenced by the diffuseness of the reflections with k odd, has been dealt 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 11Å from Wessels mine [orthorhombic, space group F2dd, a = 11.265(2), b = 7.386(1), c = 44.970(9) Å] was refined to R = 0.128, whereas the refinement carried on the MDO2 polytype [monoclinic, space group B11m, a = 6.735(2), b = 7.385(1), c = 22.487(4) Å, γ = 123.25(1)°], converged to R = 0.051. Two refinements were 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) Å, γ = 123.18(1)°] with data collected with conventional source (R = 0.089), and with synchrotron radiation (R = 0.110).

Common structural features are the infinite layers of calcium polyhedra, parallel to (001), with tetrahedral chains of wollastonite-type running along b and attached on both sides of these layers. The composite layers are stacked 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 water molecules were found in the specimen from Wessels mine, ‘zeolitic’ calcium cations and water molecules were found in that from Urals. These structural aspects are compared and their relevance in explaining the different thermal behaviour of the two types of tobermorite 11Å is indicated.

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