The presence of mica changes the mechanical behavior of sandy soils. In this study, micro- and macroscale tests are used to explore the unique packing characteristics that develop in mixtures made of round and platy particles, and the effects that mica exerts on small, medium, and large strain parameters. Mixtures are prepared with different mica contents and various mica-to-sand size ratios, L_mica /D_sand. Results provide a comprehensive characterization of mixtures made of spherical and platy particles. Mica plates change sand packing through pore filling, bridging, ordering, and mica–mica interaction; bridging prevails and leads to open fabrics when L_mica /D_sand≥1. As the mica content increases in mixtures with L_mica /D_sand≥1, the shear-wave velocity decreases and it becomes more sensitive to the state of stress; the constraint modulus decreases; and the peak and residual friction angles decrease. Remixing during disordered granular flow prevents mica alignment and diminishes the otherwise weakening effect of mica on large-strain strength.