Preferred orientation of phyllosilicates: Effects of composition and stress on resedimented mudstone microfabrics

Development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material (Boston Blue Clay). Boston Blue Clay (BBC), an illitic glacio-marine clay composed of 57% clay-sized particles (<2 μm) was resedimented and step-loaded to a vertical effective stress of 0.1 MPa, 1 MPa, 6 MPa, and 10 MPa. These four samples show a small increase in preferred orientation of mica and chlorite with increasing vertical effective stress. Furthermore, pure BBC powder was admixed with silica (silt-sized quartz) in five different ratios of BBC:silica to form sediments with 57%, 52%, 48%, 44%, and 36% < 2 μm particles and were loaded to a maximum vertical effective stress of 2.4 MPa. Mica preferred orientation decreases significantly with decreasing clay content at one stress state. We used and compared three different techniques to characterize and quantify the preferred orientation of phyllosilicates: transmission-mode X-ray texture goniometry, reflection-mode X-ray texture goniometry, and grain and pore networks imaged using secondary and backscattered electron micrographs gathered on argon-ion-milled surfaces. Preferred orientation of the clay minerals shows good agreement between transmission-mode X-ray texture goniometry and reflection-mode X-ray texture goniometry. Mercury porosimetry further illuminates vertical effective stress and compositional controls on microfabric.