Smectite

The TOT elementary layers are not rigidly connected to each other but are separated by a free space: the interlayer hosting hydrated cations and water molecules.

Only more expandable vermiculite and some rarer alumino-silicate minerals (zeolites) with inner channel structure can exhibit a higher CEC than smectite.

Smectites are formed from the weathering of basalt, gabbro, and silica-rich volcanic glass (e.g., pumice, obsidian, rhyolite, dacite).

The highly porous (with a large and easily accessible specific surface) and very reactive volcanic ashes rapidly reacted with seawater.

In civil engineering works, it is routinely used as a thick bentonite slurry when excavating deep and narrow trenches in the ground to support the lateral walls and to avoid their collapse.

Smectites, more commonly called bentonite, are candidate as buffer and backfill materials to fill the space around high-level radioactive waste in deep geological repositories.

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
2:1 clay minerals crystallographic structure made of three superimposed sheets of tetrahedra-octahedra-tetrahedra (TOT layer unit), respectively
Detailed molecular structure of pure montmorillonite , the best known end-member of the smectite group. The interlayer space between two successive TOT layers is filled with hydrated cations (mainly Na +
and Ca 2+
ions) compensating the negative electrical charges of the TOT layers and with water molecules causing the interlayer expansion.
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 .