A c-axis spacing

Figure 3.8. Schematic of the smectite structure showing one gibbsite sheet between two silicate sheets. The basic unit is repeated many times in the horizontal directions to produce layers. The basic unit with the 9.6 A c-axis spacing expands to 14 A when water enters between layers. The exchangeable cations located between the layers produce the counter charge for the isomorphous substitution. It occurs in the layers marked with an asterisk (from Taylor and Ashcroft, 1972, with permission).

Unlike Na -smectite, Ca -smectite expands to a c-axis spacing of 19 to 20 A at 100 percent relative humidity, and expands no further. Since the osmotic force is repulsive at all separation distances, this limited expansion implies the existence of... 

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