Lutite

Lutite is a sedimentary rock composed of clay-sized particles (less than 1/256 mm or 4 μm). It is a general term and encompasses various more specific rock types based on the predominant clay mineral composition. Common clay minerals found in lutites include kaolinite, illite, and smectite.

Lutites are typically formed in low-energy environments, such as lake bottoms, deep marine settings, and floodplains, where fine-grained sediments can settle out of suspension. The depositional environment significantly influences the mineralogy and texture of the resulting lutite.

The term "lutite" is less frequently used in modern geological literature, with more specific terms like shale, mudstone, claystone, and siltstone being preferred based on particle size and fissility (the tendency to split along parallel planes). Shale refers to lutites that exhibit fissility, while mudstone describes non-fissile lutites composed of a mixture of silt and clay. Claystone is primarily composed of clay minerals, and siltstone is composed mostly of silt-sized particles, though these grain sizes may overlap significantly in usage.

Lutites can be important source rocks for petroleum and natural gas, as well as serving as aquitards (layers that restrict groundwater flow) in groundwater systems. Their composition and properties are also important considerations in civil engineering projects.