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Ben­tonite is an absorbent alu­mini­um phyl­losil­i­cate, impure clay con­sist­ing most­ly of mont­mo­ril­lonite. The absorbent clay was giv­en the name ben­tonite by Wilbur C. Knight in 1898, after the Cre­ta­ceous Ben­ton Shale near Rock Riv­er, Wyoming in Amer­i­ca. There are dif­fer­ent types of ben­tonite, each named after the respec­tive dom­i­nant ele­ment, such as potas­si­um (K), sodi­um (Na), cal­ci­um (Ca), and alu­mini­um (Al). Experts debate a num­ber of nomen­cla­to­r­i­al prob­lems with the clas­si­fi­ca­tion of ben­tonite clays. Ben­tonite usu­al­ly forms from weath­er­ing of vol­canic ash, most often in the pres­ence of water. How­ev­er, the term ben­tonite, as well as a sim­i­lar clay called ton­stein, has been used to describe clay beds of uncer­tain ori­gin. For indus­tri­al pur­pos­es, two main class­es of ben­tonite exist: sodi­um and cal­ci­um ben­tonite. In stratig­ra­phy and tephrochronol­o­gy, com­plete­ly devit­ri­fied (weath­ered vol­canic glass) ash-fall beds are com­mon­ly referred to as K‑bentonites when the dom­i­nant clay species is illite. Oth­er com­mon clay species, and some­times dom­i­nant, are mont­mo­ril­lonite and kaoli­n­ite. Kaoli­n­ite-dom­i­nat­ed clays are com­mon­ly referred to as ton­steins and are typ­i­cal­ly asso­ci­at­ed with coal.