Beidellite

Beidellite is a clay mineral in the smectite group, a 2:1 phyllosilicate composed of two tetrahedral sheets sandwiching a central octahedral sheet. Like other smectites, it exhibits swelling upon hydration and has a notable cation exchange capacity, properties that make it important in soils and engineered barriers.

A distinguishing feature of beidellite is the distribution of its structural charge. In beidellite, the negative charge is primarily located in the tetrahedral sheets due to substitution of Al3+ for Si4+ in the tetrahedra, whereas in montmorillonite and some other smectites the charge is more commonly concentrated in the octahedral sheet. This tetrahedral substitution pattern influences properties such as swelling behavior, cation selectivity, and thermal stability.

Occurrence and formation: Beidellite forms through weathering and alteration of aluminosilicate minerals in a range of rocks, often yielding clay-rich horizons in soils and sediments. It has been reported in numerous regions worldwide, including parts of Asia, Europe, and North America, where it commonly occurs with other smectites.

Identification and classification: Beidellite is identified through mineralogical techniques such as X-ray diffraction and infrared spectroscopy, which reveal typical smectite basal spacings and reflections. In natural samples, it frequently coexists with montmorillonite and other clay minerals, and its tetrahedral charge distribution helps distinguish it from related smectites.

Applications: The swelling and high cation exchange capacity of beidellite render it relevant to soil science, environmental engineering, drilling fluids, and barrier materials. Its properties influence water retention, nutrient exchange, and hydraulic conductivity in soils and engineered systems.