Bentonite clay is found on every inhabited continent, but the largest deposits sit in the United States, China, India, Turkey, and Greece. These countries dominate global production, which totals roughly 20 million metric tons per year. The clay forms wherever ancient volcanic ash settled into water and slowly transformed over millions of years, so its deposits trace the footprints of long-extinct volcanoes and ancient seas.
How Volcanic Ash Becomes Bentonite
Bentonite starts as volcanic ash, typically from explosive eruptions that produce fine, silica-rich particles. When that ash lands in seawater, lake beds, or waterlogged soil, it undergoes a slow chemical transformation. Water seeps through the ash, breaking down volcanic glass and rearranging its minerals into the soft, swelling clay we call bentonite. The process takes millions of years and depends on the chemistry of the surrounding water, particularly which dissolved minerals are present.
The type of water determines the type of bentonite. Ash that weathers in the presence of sodium-rich water produces sodium bentonite, which swells dramatically when wet. Ash exposed to calcium-rich groundwater produces calcium bentonite, which swells less but is far more common worldwide. Sodium bentonite deposits are relatively rare. Brazil, for example, has no known sodium bentonite at all despite having calcium bentonite reserves.
The United States: Wyoming and Montana
The world’s most commercially important sodium bentonite comes from the Black Hills region of Wyoming and neighboring Montana. This deposit formed during the Cretaceous period, roughly 95 to 100 million years ago, when a shallow inland sea called the Mowry Sea stretched across much of western North America. Volcanic eruptions to the west blanketed this sea with rhyolitic ash, which settled on the seafloor and slowly altered into the sodium-rich clay mined today.
The specific formation most heavily mined is known as the Clay Spur bentonite, part of the Mowry Shale. Wyoming’s arid landscape actually makes prospecting easier: vegetation is sparse in bentonite-rich areas, leaving the geology exposed at the surface. Geologists use aerial photography to spot distinctive marker beds that signal where bentonite seams lie. All commercial bentonite in Wyoming is extracted through open-pit surface mining, with operations reaching depths no greater than about 50 feet.
The U.S. produced an estimated 4.7 million metric tons of bentonite in 2023, making it the largest or second-largest producer in the world depending on the year. Beyond Wyoming and Montana, smaller deposits exist in Texas, Mississippi, and other Gulf Coast states, though these tend to be calcium bentonite rather than the sodium variety.
China and India
China is the second-largest producer, with output around 2.1 million metric tons in 2023. Chinese deposits are spread across several provinces, with significant reserves in the northeast and central regions. India follows closely at an estimated 3.7 million metric tons, with major deposits in the states of Rajasthan, Gujarat, and Jharkhand. In a 2018 comparison, China led the world with 5,600 metric kilotons of mine production, with the U.S. close behind at 4,670 kilotons.
Turkey’s Split Geography
Turkey produces roughly 2.4 million metric tons annually, making it one of the top five bentonite-producing nations. What’s interesting about Turkey is how neatly its geology divides the two main types. Calcium bentonite deposits cluster in the western and northern provinces of Balıkesir, Edirne, Ordu, Giresun, and Konya. Sodium bentonite, the more commercially valuable type, is found further inland in Ankara, Çankırı, Çorum, and Tokat provinces. Calcium deposits significantly outnumber sodium ones across the country.
Greece and Europe
Greece is a major bentonite producer, with output centered on the volcanic islands of the Aegean Sea, particularly Milos. The island’s geology, shaped by relatively recent volcanic activity, created thick, accessible bentonite deposits close to the surface. Greece produced an estimated 1,360 metric kilotons in 2018 and remains one of the top global suppliers.
Elsewhere in Europe, the Czech Republic, Denmark, Spain, and Germany all have active bentonite operations, though at smaller scales. Denmark produced roughly 900,000 metric tons in 2023, while Spain and the Czech Republic each contributed a few hundred thousand tons. Romania, Hungary, and Poland also have known deposits. European bentonite tends to be calcium-type, and producers sometimes treat it with soda ash to give it swelling properties closer to natural sodium bentonite.
Other Deposits Around the World
Bentonite deposits exist across South America, the Middle East, and parts of Africa and Asia. Brazil has active mining operations producing around 220,000 metric tons per year, though only calcium bentonite has been identified there. Iran contributes roughly 850,000 metric tons annually. Mexico, Argentina, Uzbekistan, Japan, and Cyprus all have documented reserves as well.
The pattern is consistent: wherever ancient volcanic activity coincided with standing water, bentonite had the conditions to form. The deposits that matter commercially are the ones close enough to the surface for open-pit mining, thick enough to justify extraction, and pure enough to serve industrial needs without excessive processing.
How Bentonite Gets From the Ground to Market
Regardless of where it’s mined, the extraction and processing steps are similar worldwide. After the overburden (the rock and soil sitting above the clay seam) is removed, the bentonite is scooped out using heavy equipment like draglines, front-end loaders, or backhoes. The raw clay typically contains 30 to 35 percent moisture, so it’s often plowed and left to air-dry in stockpiles before processing begins.
Once the moisture drops to around 16 to 18 percent, the clay is crushed into pieces smaller than an inch, then fed through industrial dryers that bring the moisture content down to 7 or 8 percent. The dried material is ground into powder using roller or hammer mills. At this stage, processors may blend different grades together for consistency, or add soda ash to calcium bentonite to boost its swelling capacity. Some facilities run the ground clay through air classifiers to sort particles by size for specialized products.
The entire supply chain, from mine to finished product, depends on the deposit being shallow and accessible. Deep bentonite seams exist in many places around the world but aren’t economically viable to mine. The deposits that drive global production share a common trait: they’re close to the surface, in arid or semi-arid terrain that keeps mining costs low and the clay relatively dry before it ever reaches a processing plant.