Bentonites are clay rocks consisting predominantly of smectite. They form mainly from alteration of pyroclastic and/or volcaniclastic rocks. Extensive deposits, linked to large eruptions, have formed repeatedly in the past. Bentonite layers are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration, favoured by fluids that leach alkali elements and by high Mg content. Smectite composition is partly controlled by parent rock chemistry. Recent studies have shown that bentonite deposits may display cryptic variations in layer charge - i.e. the variations are not visible at the macroscopic scale - and these correlate with physical properties.