Clay mineral
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Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Oxford Clay (Jurassic) exposed near Weymouth, England
Hexagonal sheets of the clay mineral kaolinite (SEM image, 1,340× magnification)
Structure of clay mineral groups
Smectite
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A smectite is a mineral mixture of various swelling sheet silicates, which have a three-layer 2:1 structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often
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
Typical volcanic eruption plume whose ashes' weathering after contact with seawater is the main source of smectite. Leaching of most of amorphous silica leads to partial dissolution of obsidian, the main constituent of volcanic glass.