BentoniteBentonite beds are clay deposits that form from the alteration of vol…


Bentonite beds are clay deposits that form from the alteration of volcanic ash. Explosive volcanic eruptions can eject tons of ash and other pyroclastic material up into the atmosphere, eventually falling back to earth, covering the ground in a layer of ash. Volcanic ash is comprised of a combination of vitric (glassy) and lithic particles which are less than 2mm in diameter. In large eruptions this material can cover hundreds to thousands of square kilometres.

Over time these ash layers become buried beneath other layers of sediment and undergo diagenesis (alteration of rocks at temperatures and pressures less than what is required to form a metamorphic rock). Diagenesis causes devitrification and chemical alteration of the vitric ash to form bentonite beds comprised of smectite group clay minerals, primarily montmorillonite [(Na,Ca)0,3(Al,Mg)2Si4O10(OH)2•n(H2O)], a hydrous aluminum phyllosilicate.

For sedimentologists, bentonite beds can be a key feature in correlating sedimentary sequences across an area. Today the same bed at different localities may be seen at differing depths. But at the time of the volcanic eruption, the ash layer must have been on the surface at each location. This helps scientists reconstruct what the geology of an area was like at the time. If the age of the bentonite (or the date of the volcanic eruption) is known, the presence of a bentonite layer within a sedimentary sequence can be used to indirectly date or refine the minimum or maximum depositional ages of the sediments surrounding it.

Bentonites are also an important industrial mineral. The unique properties of the clay allow it to absorb large amounts of water, up to roughly 14 times its dehydrated volume. Due to this, bentonite clays are often used as an absorbent material in chemical spills or waste cleanup, in water purification systems, as an additive in cements and concrete, as pet litter, and many other uses.

– CD

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James St. John