Soil clay minerals dominate the colloidal fraction and specific surface area ( E SSA ) of soil. Clay minerals exhibit a wide range of microstructures and hydration responses that affect soil macroscopic hydraulic, chemical, and mechanical properties. We focus on kaolinite and smectite as end members of the clay mineral family due to their contrasting surface areas, differences in activity (shrink-swell behavior), high abundance in natural soils, and their general separation among climatic regions (e.g., kaolinite dominates in tropical soils that receive high values of mean annual precipitation, see Figure 1). Due to differences in their basic building blocks and properties, kaolinite comprises tightly bound clay platelets that form large tactoids (aggregated stacks of platelets) and is considered inactive, whereas smectite is considered a highly active clay mineral (Jefferson & Smalley, 1997;Skempton, 1953).In many land surface and Earth system models (the latter term includes simulations of aspects of the Earth system affecting the occurrence of natural hazards), information on 'clay' is often used in an undifferentiated manner as soil 'clay content' (defined as the mass fraction of soil particles smaller than 2 μm in diameter) to derive spatially distributed soil hydraulic and mechanical properties using pedotransfer functions (Gutmann & Small, 2007;Van Looy et al., 2017). The large differences in microstructure and hydration response between kaolinite and smectite, along with their remarkable spatial segregation in climatic regions, have a large effect on the regional clay mineral-dependent soil hydraulic and mechanical properties. We seek to