The minerals of the tobermorite group are hydrated calcium silicates characterized by wollastonite-like chains connected to sheets of seven-fold coordinated calcium cations. Additional calcium cations, as well as water molecules, may occupy the structural cavities between adjacent layers. The known members of the tobermorite group are: tobermorite 14 A (or plombierite), tobermorite 11 A, tobermorite 9 A (or riversideite), and clinotobermorite. The (002) spacings of 14, 11 and 9 A are related to the amount of H2O molecules between the layers. The partial dehydration of tobermorite 14A at 90°C results in the formation of tobermorite 11A. Moreover, it is known that some samples of tobermorite 11Å transform into tobermorite 9Å upon dehydration at 300°C. They were called ‘normal’ tobermorites (Mitsuda and Taylor, 1982?), to distinguish them from the ‘anomalous’ tobermorites that remain unchanged upon heating. Recently, a similar ‘normal’ behavior was observed also in clinotobermorite from Wessels mine, South Africa (Merlino et al. 1999), which decreases its c parameter from 22 to about 18 A upon heating to 300°C. The reaction was topotactic, so it was possible to obtain a single crystal of the dehydration product and to refine its crystal structure. This new phase was provisionally named ‘tobermorite 9Å’, even if some details in its diffraction pattern pointed to minor structural differences with tobermorite 9Å s.s. Actually, both the phases may be described as formed by the stacking along c of the same kind of layer, with C12/m1 layer symmetry. The OD theory indicates two possible OD groupoid families corresponding to this layer symmetry. For both the OD families, infinite polytypic modifications exist; the OD theory indicates that there are two MDO polytypes for each family. The phase we obtained from clinotobermorite belongs to the former OD family, whereas the product of dehydration of tobermorite 11Å should belong to the latter. The structural refinement of the MDO2 polytype for the phase obtained upon heating of clinotobermorite confirm the goodness of this model (Fig 1). The observed powder pattern of tobermorite 9Å s.s closely corresponds to the calculated one for the second OD groupoid (Fig. 2). [1] McConnell J.D.C.: ‘The hydrated calcium silicates riversideite, tobermorite, and plombierite’. Mineral. Mag.(1954), 30: 293-305. [2] Merlino, S., Bonaccorsi, E., Armbruster, T.: ‘The real structures of clinotobermorites and tobermorite 9Å: OD character, polytypes, and structural relationships’. Eur. J. Mineral. (2000), 12: 411-429
The Dehydration Products of Normal Tobermorite and Clinotobermorite
BONACCORSI, ELENA;MERLINO, STEFANO;
2001-01-01
Abstract
The minerals of the tobermorite group are hydrated calcium silicates characterized by wollastonite-like chains connected to sheets of seven-fold coordinated calcium cations. Additional calcium cations, as well as water molecules, may occupy the structural cavities between adjacent layers. The known members of the tobermorite group are: tobermorite 14 A (or plombierite), tobermorite 11 A, tobermorite 9 A (or riversideite), and clinotobermorite. The (002) spacings of 14, 11 and 9 A are related to the amount of H2O molecules between the layers. The partial dehydration of tobermorite 14A at 90°C results in the formation of tobermorite 11A. Moreover, it is known that some samples of tobermorite 11Å transform into tobermorite 9Å upon dehydration at 300°C. They were called ‘normal’ tobermorites (Mitsuda and Taylor, 1982?), to distinguish them from the ‘anomalous’ tobermorites that remain unchanged upon heating. Recently, a similar ‘normal’ behavior was observed also in clinotobermorite from Wessels mine, South Africa (Merlino et al. 1999), which decreases its c parameter from 22 to about 18 A upon heating to 300°C. The reaction was topotactic, so it was possible to obtain a single crystal of the dehydration product and to refine its crystal structure. This new phase was provisionally named ‘tobermorite 9Å’, even if some details in its diffraction pattern pointed to minor structural differences with tobermorite 9Å s.s. Actually, both the phases may be described as formed by the stacking along c of the same kind of layer, with C12/m1 layer symmetry. The OD theory indicates two possible OD groupoid families corresponding to this layer symmetry. For both the OD families, infinite polytypic modifications exist; the OD theory indicates that there are two MDO polytypes for each family. The phase we obtained from clinotobermorite belongs to the former OD family, whereas the product of dehydration of tobermorite 11Å should belong to the latter. The structural refinement of the MDO2 polytype for the phase obtained upon heating of clinotobermorite confirm the goodness of this model (Fig 1). The observed powder pattern of tobermorite 9Å s.s closely corresponds to the calculated one for the second OD groupoid (Fig. 2). [1] McConnell J.D.C.: ‘The hydrated calcium silicates riversideite, tobermorite, and plombierite’. Mineral. Mag.(1954), 30: 293-305. [2] Merlino, S., Bonaccorsi, E., Armbruster, T.: ‘The real structures of clinotobermorites and tobermorite 9Å: OD character, polytypes, and structural relationships’. Eur. J. Mineral. (2000), 12: 411-429I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
