Strain Scan and Texture of an Anhydrite-Dolomite Rock from the Piora-Syncline (Switzerland)

An anhydrite-dolomite rock was investigated in order to find correlations between intracrystalline strain and texture. The sample represents a composite rock, called Zuckerdolomit, occurring for instance in the central Alps. Zuckerdolomit may occasionally respond to mechanical impulses with a complete break down of its structure. Residual strain was scanned at the strain/stress diffractometer EPSILON-MDS and the crystallographic texture was obtained by neutron time-of-flight diffraction at the texture diffractometer SKAT at the pulsed neutron source IBR-2 in Dubna, Russia. Texture was additionally determined by the U-stage microscopy. The texture of the rock could be derived from the main constituents: anhydrite and dolomite. The anhydrite texture appears to be characterised by an orthorhombic symmetry. One plane of the symmetry trends parallel to the rock’s foliation plane, with basal planes subparallel to and a small circle distribution within the foliation plane. There is a subordinated small circle orientation pattern relating in 45° to the rock’s foliation plane. On the other hand, the subordinated regulation pattern of anhydrite is fixed to the orientation of the foliation plane. The determined residual strain data for anhydrite and dolomite reflect a generally opposed directed state, regarding both, compression and dilatation. Moreover, taking into account the elastic constants for both minerals, which differ approximately by the factor two, the special geomechanical behaviour of the studied Zuckerdolomit appears to be considerably determined by interactions between its residual strain and textural properties. This may be of particular importance for underground activities (mining, tunnelling etc.). The dangerous geomechanical behaviour of the rock seems to be determined by its residual straintexture relations more than so far believed (anhydrite hydration).