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1 Universitá di Pisa, Dipartimento di Scienze della Terra, Via S. Maria 53, I-56126 Pisa, Italy
2 Universität Bern, Laboratorium für chemische und mineralogische Kristallographie, Freiestrasse 3, CH-3012 Bern, Switzerland
* E-mail: merlino{at}dst.unipi.it
Clinotobermorite, Ca5Si6O17·5H2O, is a rare mineral, structurally related to tobermorite 11 Å. It is characterized by the presence of structural disorder (evidenced by the diffuseness of the reflections with k odd) which, until now, prevented the determination of its real structure. In this paper, a model for the real structure of clinotobermorite is proposed on the basis of OD theory, through examination of the X-ray diffraction pattern of a sample coming from the Wessels mine (South Africa). The proposed model, which assumes the presence of silicate double chains of wollastonite-type, is confirmed by structural refinements carried out for the two polytypes with maximum degree of order (MDO). The MDO1 polytype of clinotobermorite (monoclinic, space group Cc, a = 11.276(2), b = 7.3427(8), c = 22.642(4) Å, β = 97.28(1)°) was refined up to R = 0.15, whereas the two refinements performed on the MDO2 polytype (triclinic, space group C1, a = 11.274(2), b = 7.3439(7), c = 11.468(2) Å, 
= 99.18(1), β = 97.19(1), 
= 90.09(1)°) converged to R = 0.12 and R = 0.10, respectively. In clinotobermorite infinite calcium polyhedral layers parallel to (001) are connected through double silicate chains [Si6O17]10- running along b; additional calcium cations and H2O molecules are placed in the channels of the resulting framework.
By dehydration at 225°C, clinotobermorite transforms topotactically into a new phase, which also displays an OD character. The results of the structural refinement carried out for its triclinic MDO2 polytype (space group Cc1, a = 11.156(5), b = 7.303(3), c = 9.566(5) Å, = 101.08(4), β = 92.83(5), = 89.98(4)°) indicate that this phase, with crystal chemical formula Ca5Si6O16(OH)2, exhibits single chains of wollastonite-type, resulting from decondensation of the double chains.
On the basis of the new detailed structural information a possible explanation for the enigmatic thermal behaviour of tobermorite 11 Å has been proposed.
Key-words: clinotobermorite, crystal structure, OD theory, polytypism, tobermorite 11 Å, tobermorite 9 Å.
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