Strain and Texture Investigations by Means of Neutron Time-of-Flight Diffraction: Application to Polyphase Gneisses

Two polyphase gneiss samples (Western Alps, Switzerland) consisting of quartz, albite, muscovite and microcline have been investigated with respect to anisotropic physical properties - residual and applied strain - and crystallographic preferred orientations. The samples were taken in close vicinity to a fault structure. For texture evaluation the 'Rietveld texture analysis' has been applied, because reliable texture analysis of polyphase rocks from pole figures is problematic due to multiple peak overlaps. The quartz textures of both samples are strong and rather similar, whereas albite and microcline show a nearly random distribution. In contrast, the mica texture is pronounced and complex. The basal plane normals of muscovite are preferably oriented parallel to [-direction of the structural reference frame and switch to [-direction with residual orientations in [ and a transition into the [-[-plane within the fault. This might be the result of quartz recrystallization, if mica is used as reference according the applied stress field. A residual strain scan has been carried out along the axis of a cylindrical sample (d = 30 mm, l = 60 mm). Residual strain values of quartz have been detected with a minimum of-1.6×10-3, and maximum strain values of 1.2×10-3. Uniaxial compression experiments in discrete steps up to 53 MPa were performed, for each step the applied and residual strains were determined. The strain increases up to 28 MPa; surprisingly, lower strain values are detected at higher applied stresses.