Smectite

Scanning electron microscope (SEM) photograph of smectite clay – magnification 23,500 – U.S. Geological Survey – Tuckup Canyon
Typical cracks pattern of a smectite-rich bentonite after its desiccation and shrinkage

A smectite (from Ancient Greek σμηκτός (smēktós) 'lubricated'; from σμηκτρίς (smēktrís) 'walker's earth, fuller's earth'; lit. 'rubbing earth; earth that has the property of cleaning') is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secondary minerals such as quartz and calcite.