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The real structure of tobermorite 11Å

¹ Università di Pisa, Dipartimento di Scienze della Terra, Via S. Maria 53, I-56126 Pisa, Italy
² Universität Bern, Laboratorium für chemische und mineralogische Kristallographie, Freierstrasse 3, CH-3012 Bern, Switzerland

View larger version (44K): [in this window] [in a new window]   Fig. 1. Single layer in tobermorite 11Å, as seen along b.

View larger version (47K): [in this window] [in a new window]   Fig. 2. Sequences of -operators (heavy marks) and operators (light marks) in the orthorhombic MDO1 (a) and monoclinic MDO2 (b) structures of tobermorite 11Å, as seen along a* The unit cells and the sequences of the stacking vectors t1 and t2 are outlined.

View larger version (9K): [in this window] [in a new window]   Fig. 3. Relationships between the C centred cell, which presents additional lattice points at 1/4,1/4,1/2 (empty circles), and the B centred cell of the MDO2 polytype of tobermorite 11Å.

View larger version (9K): [in this window] [in a new window]   Fig. 4. Drawing of the diffraction pattern of tobermorite 11Å, as seen along the b axis. The reciprocal vectors A* and C* corresponding to the subcell are indicated. Large circles indicate family reflections. For sake of clarity, the characteristic reflections of only MDO1 and MDO2 polytypes are reported (small circles and crosses, respectively; differently oriented crosses correspond to different k values). The characteristic reflections of the twin MDO2’ polytype correspond to those of MDO2, once the symbols + and x are inverted.

View larger version (48K): [in this window] [in a new window]   Fig. 5. Crystal structure of the monoclinic polytype MDO2 of anomalous tobermorite 11Å, as seen along [010] (a) and down [210] (b). Silicate double chains (dark grey) are connected to layers of sevenfold coordinated calcium polyhedra (light grey). Grey circles indicate the ‘zeolitic’ water molecules W1, W2, and W3. The stacking vectors are drawn in (a), with the indication of their component along b.

View larger version (53K): [in this window] [in a new window]   Fig. 6. Crystal structure of the monoclinic polytype MDO2 of normal tobermorite 11Å, as seen along [010] (a) and down [210] (b). Symbols are as in Fig. 5. Dark grey dots indicate additional calcium cations Ca2.

View larger version (58K): [in this window] [in a new window]   Fig. 7. Crystal structure of the orthorhombic polytype MDO1 of tobermorite 11ä, as seen along [010]. Symbols are as in Fig. 5.

View larger version (96K): [in this window] [in a new window]   Fig. 8. Connection of silicate chains (dark grey) to the layer of calcium polyhedra (light grey), as seen along [001].

View larger version (37K): [in this window] [in a new window]   Fig. 9. Schematic drawing of the structural cavities, as seen along [010] in (a) anomalous tobermorite 11Å from Wessels mine; (b) normal tobermorite 11Å from Urals; (c) clinotobermorite from Wessels mine (Merlino et al., 2000). In (b) one of the two possible ordered arrangements of Ca2, W1, and W3 is reported.

View larger version (21K): [in this window] [in a new window]   Fig. 10. Hydrogen bond system in normal to bermorite from Urals, whenever Ca2 is present. The atoms are seen approximately along [001]; the letters ‘u’ and ‘d’ mean ‘up’ and ‘down’ with respect to the plane at z, = 1/2. Large empty circles represent H2O molecules belonging to the scaffolding (W6), whereas large grey circles represent ‘zeolitic’ H2O molecules (W1, W2, W3). Small empty circles indicate oxygen atoms. The Ca2 cation is indicated with a small grey circle.