As described by Schultz (1969) based on chemical and thermal analysis, beidellite-type montmorillonite is a type of montmorillonite characterized by a large net negative layer charge of -0.85 to -1.20 per O20(OH)4, with tetrahedral substitutions causing more than 50% of the total layer. Current (chemical) nomenclature for montmorillonite is that it is an Al-rich, dioctahedral smectite with an ideal structural formula of (Al3.15.Mg0.85)Si8O20(OH)4X0.85.nHO with layer charge from primarily octahedral substitutions of Mg, whereas beidellite has a net layer charge that occurs from tetrahedral substitutions of Al, with an ideal structural formula of Al4.0(Si7.15Al0.85)O20(OH)4X0.85.nH2O. The term ”beidellite-type montmroillonite” is obsolete and should not be used.