Bentonite forms from alteration of which material?

Bentonite forms from alteration of which material?

• A. Limestone
• B. Dolostone
• C. Sandstone
• D. Volcanic ash

Answer: (D)

Bentonite forms when volcanic ash is altered by water during diagenesis. The ash supplies glassy and feldspathic material that, with groundwater, weather into smectite clays, especially montmorillonite, which are the swelling clays that define bentonite. The other rock types don’t provide the same source material: carbonate rocks like limestone or dolostone weather to calcite/dolomite minerals and various clays but not the swelling smectite needed for bentonite, and sandstone typically weathering yields clays like kaolinite or illite rather than smectite. So volcanic ash is the source that leads to bentonite formation.

Bentonite forms when volcanic ash is altered by water during diagenesis. The ash supplies glassy and feldspathic material that, with groundwater, weather into smectite clays, especially montmorillonite, which are the swelling clays that define bentonite. The other rock types don’t provide the same source material: carbonate rocks like limestone or dolostone weather to calcite/dolomite minerals and various clays but not the swelling smectite needed for bentonite, and sandstone typically weathering yields clays like kaolinite or illite rather than smectite. So volcanic ash is the source that leads to bentonite formation.