Tobermorites: Their real structure and order-disorder (OD) character

The real structures of clinotobermorite, tobermorite 9 Angstrom, and toberm orite 11 Angstrom were determined through the application of OD approach, w hich allowed us to explain their peculiar disorder and polytypic features a nd to derive the main polytypes for each of them. The structural arrangemen ts will be described and discussed for one polytype of each compound: clino tobermorite, triclinic polytype C1, a = 11.274, b = 7.344, c = 11.468 Angst rom, alpha = 99.18 degrees, beta = 97.19 degrees, gamma = 90.03 degrees; to bermorite 9 Angstrom, triclinic polytype C (1) over bar a = 11.156, b = 7.3 03, c = 9.566 Angstrom, alpha = 101.08 degrees, beta = 92.83 degrees, gamma = 89.98 degrees; tobermorite 11 Angstrom, monoclinic polytype B11m, a = 6. 735, b = 7.385, c = 22.487 Angstrom, gamma = 123.25 degrees. Common structu ral features are infinite layers, parallel to (001), formed by sevenfold-co ordinated calcium polyhedra. Tetrahedral double chains, built up through co ndensation of "Dreiereinfachketten" of wollastonite-type and running along b, link together adjacent calcium layers in clinotobermorite and tobermorit e 11 Angstrom, whereas single tetrahedral chains connect adjacent calcium l ayers in tobermorite 9 Angstrom. The relatively wide channels of clinotober morite and tobermorite 11 Angstrom host "zeolitic" calcium cations and wate r molecules. The present structural results now allow for a sound discussio n of the crystal chemical relationships between the various members of the tobermorite group and an explanation of the peculiar thermal behavior of to bermorite 11 Angstrom.