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Smectite is a group of expandable clay minerals with a variety of structural and chemical differences that affect their surface charge and chemistry. Montmorillonite, commonly identified in ‘healing clays’, is one type of smectite. It has a structure of 2 tetrahedral sheets that sandwich a single octahedral sheet (Fig 1). The 2:1 structure forms layers that stack like cards with the space between cards called the ‘interlayer’. The collective of interlayer, octahedral, and tetrahedral sites of montmorillonite forms a mineral with an ideal chemical formula; R_0.33⁺ (Al_1.67 Mg_0.33) Si₄ O₁₀ (OH)₂ (where R represents interlayer cations; Moore and Reynolds, 1997). Many elements can substitute into these various structural sites. In addition water and organic compounds may be found in the interlayer sites and/or adsorbed on the edges and exterior surfaces of the crystal structure. Zeolites have properties similar to clays, but form tubes or cage-like structures that can also incorporate a variety of molecules and ions. Over time, if the bentonite is deeply buried and subjected to an increase in temperature, or if hot volcanic water percolates through the ash layer, the smectites will alter to form illites or other minerals that are stable under the higher temperature conditions. The point here is that a bentonite usually does not consist of a single pure smectite, and each natural deposit is mineralogically variable.