The anisotropy of itacolumite flexibility

Abstract

Abstract Itacolumites are very special rocks due to their high flexibility. The investigated Brazilian itacolumites and associated non-flexible quartzites are of comparable composition but differ in their rock fabrics. The shape and size of quartz is mainly controlled by the mica fabric. Quartz textures and grain boundary migration features are indications for deformation at temperatures of about 500°C. The flexibility is mainly related to a penetrative network of open grain boundaries which enable a limited body rotation of individual quartz grains. Continuous layers of white mica display deformation features indicative of shear along the layer-parallel cleavage planes. As demonstrated by simple bending experiments, the flexibility is a highly anisotropic phenomenon which can be related to a directional dependence of grain shape fabrics and corresponding grain boundary pore spacing. According to quantitative estimates, the amount and anisotropy of bending can be explained by the rotation of separated quartz grains between layers of mica which act as flexural slip planes and are also responsible for the observed elastic rebound. Solution along grain boundaries, volumetric strain by thermal contraction of quartz and bulk extension are processes discussed for the origin of the extreme values of secondary grain boundary porosity.