Origin, mineralogy and properties
Bentonite Formation - Millions of years ago, during the Cretaceous period, the western United States experienced extensive volcanic activity due to the tectonic convergence of the North American and Pacific plates. During long periods of eruptions, immeasurable amounts of ash were disgorged into the prevailing easterly winds as the Pacific plate was forced under the North American plate deep into the earth’s crust. Over millions of years, the ash was repeatedly deposited in the mineral rich Mowry sea and interbedded with eroded silts and sediments. Slowly, the glass component of the ash was chemically altered in these low energy marine environments and consolidated into distinct layers of clay, often associated with Zeolite beds, marl, sandstone as well as shale and mudstone.
As plate drift continued, the North American plate was lifted and folded into mountains, typified by the Big Horn Mountains of Wyoming. The Mowry sea drained and ash deposition subsided as the clay / silt formations were heaved upwards. The Black Hills and the Big Horn mountains were two areas thrust up during this period. These areas were eroded and weathered over time, exposing numerous clay beds that are commercially mined today.
Definition - The term "Bentonite" is generally applied to the colloidal clays originally associated with the Cretaceous Benton Shale outcrops near Fort Benton, Wyoming. In the late 1880’s, the "clay of a thousand uses" was first called Taylorite, after William Taylor; one of the first commercial producers of the product in the Rock River area. Finding that name already taken, the clay was renamed for the Benton formation in which the outcrop was found, i.e. Bentonite.
Mineralogy – Natural sodium bentonite is characterized by having sodium as it’s predominant exchangeable ion. Sodium bentonite is not itself a mineral name, but more correctly, it is a smectite clay composed primarily of the mineral Montmorillonite. Montmorillonite is a three layer mineral formed of several layers of tetrahedron and octahedron sheets, electrostatically held together by isomorphic interlayer cations. As the electrostatic attraction is low, exposure to polar fluids will cause the formation of a monomolecular lattice of water between the silicate layers. The basis behind bentonite swelling is that several layers of water dipoles can form into weak "stacked" tetrahedral structures, causing the silicate layers to separate - this is termed intercrystalline swelling.
Purity of sodium bentonite will vary, as the depositional environment and subsequent weathering processes also differ by region and deposit. While not necessarily a sign of a clay’s quality for use in a GCL, the typical sodium bentonite used in a Geosynthetic Clay Liner product will contain 70 -90% Montmorillonite.
A chemical analysis would yield similar results to the following:
| SiO2 | 62.0% | TiO2 | 0.1% | |
| Al2O3 | 20.0% | Trace Elements | 2.8% | |
| Na2O | 2.5% | SO2 | 0.35% | |
| Fe2O3 | 3.5% | CaO | 0.6% | |
| FeO | 0.45% | Na2O | 0.35% | |
| MgO | 1.8% | K2O | 0.4% | |
| H2O (crystalline) | 5.5% |
Mining - Sodium bentonite is mined via open pit methods and can require as much as ten times the amount of overburden to be removed, compared to the thickness of the recoverable clay layers. Beds of bentonite can be as thin as several inches to as thick as several feet depending upon the volume of ash being deposited at the time, as well as the sea’s currents that may have concentrated deposits. Deposits can be quite expansive, stretching for miles in many directions.As with any naturally occurring ore, bentonite deposits vary in their qualities. Statistical process control as well as thorough deposit surveying and analysis allows today’s bentonite producers to minimize variability - ensuring consistent bentonite qualities in every product they deliver.
Properties - While the commercially packaged form of sodium bentonite is either a fine powder or one of numerous granulations, the dispersed particle size of sodium bentonite is actually less than 2 microns. In comparison, the thickness of a nonwoven fiber is approximetly 27 microns.
Sodium bentonite is commonly distinguished by it’s ability to swell 1000 % of it’s natural volume when exposed to water. The test method used for quantifying the swelling property for use in GCLs is ASTM D 5890 - Standard Test Method for Swell Index of Clay Mineral Component of Geosynthetic Clay Liners. This index test is useful for establishing the relative quality of a clay for use in a GCL.
For most environmental applications, sodium bentonite is also evaluated for use based upon it’s ability to create a seal. This test is ASTM D 5891 - Standard Test Method for Fluid Loss of Clay Component of Geosynthetic Clay Liners. Many consider this index test to be a quick qualitative test, suggesting the bentonite’s ability to work effectively in a GCL.
The bentonite in a GCL must act as a hydraulic barrier. It is the high swelling properties that provide sodium bentonite’s unique sealing qualities. As the clay hydrates and swells, the path for water to flow though becomes complex as the clay platelets intersperse. The most important test to evaluate the sealing qualities of bentonite in a GCL is a permeability or flux test. This test is ASTM D 5887 - Standard Test Method for Measurement of Index Flux Through Saturated Geosynthetic Clay Liner Specimens Using a Flexible Wall Permeameter.
