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Abstract: The substructure of natural clinotobermorite Ca-5[Si3O8(OH)](2) . 4 H2O - Ca-5[Si6O17] . 5 H2O from the Wessels mine, Kalahari manganese field, South Africa, was determined from single-crystal X-ray data. The diffraction pattern (monoclinic: a = 11.19 Angstrom, b = 7.29 Angstrom, c = 22.46 Angstrom, beta = 96.97 degrees) is characterised by rather diffuse reflections of the type hkl with h, k = 2n + 1. The structure (space group I2/m, Z = 1, a/2 = 5.593(6) Angstrom, b/2 = 3.645(4) Angstrom, c = 22.46(3) Angstrom, beta = 96.97(2)degrees) was solved from the sharp Bragg reflections (hkl, with h, k = 2n) and refined on the basis of 1877 unique reflections to R(F) = 6.7%.
The structure of clinotobermorite is closely related to the one of so called 11 Angstrom tobermorite Ca5Si6O16(OH)(2) . 2H(2)O (pseudo-orthorhombic, a = 11.17 Angstrom, b = 7.38 Angstrom, c = 22.6 Angstrom). In both minerals layers parallel (001), formed by seven-fold co-ordinated Ca, are flanked on either side by wollastonite-like [Si3O9] single chains (ca. 7.3 Angstrom periodicity along b) creating a sandwich-like assemblage. The short b/2 axis (3.645 Angstrom) and the short a/2 axis (5.593 Angstrom) of the subcell are caused by an overlay of two half-occupied single chains of tetrahedra with one shifted by b/2 against the other. In clinotobermorite the sandwich-like layers are stacked along c yielding an interlayer distance of 4.5 Angstrom with a free aperture of 3.2 Angstrom where additional Ca ions and H2O molecules connect the sandwich units. Clinotobermorite with 4-5 H2O represents a much denser structure than 11 Angstrom tobermorite with only 2 H2O or 14 Angstrom tobermorite with ca. 6 H2O (pseudo-orthorhombic, a = 11.17 Angstrom, b = 7.38 Angstrom, c = 27.94 Angstrom). The substructure of clinotobermorite can be interpreted as either composed of 'Dreiereinfachketten' leading to the formula Ca-5[Si3O8(OH)](2) . 4 H2O, or of 'Dreierdoppelketten' leading to the formula Ca-5[Si6O17] . 5 H2O, or of a mixture of both variants.
The surface structure of clinotobermorite was studied by atomic force microscopy (AFM) where low magnifications reveal a complex layering of twinning parallel (001). High resolutions scans parallel (100) establish enhanced contrast fringes with a periodicity of ca. 10 Angstrom interpreted as the repeat distance of the Ca polyhedral layers parallel to c.