A type of smectite, montmorillonite, can absorb 20-30 times its volume in water, and is of particular interest to the plastics industry. It is a 2:1 layered structure; a single layer of aluminum octahedra sandwiched between two layers of silicon tetrahedra. Each layered sheet is slightly less than one nanometer, or 10?, with surface dimensions extending to about a micron (1,000 nanometers). With aspect ratios approaching 1,000, surface area of the clay is in the range of 750 square meters per gram. Due to the isomorphous substitution by magnesium and iron in the octahedral aluminum layer, a certain charge deficiency occurs at the sheet surface. This charge is typically satisfied by sodium and calcium cations. In its natural state, when hydrated at 8-10 wt%, the sheets are stacked on each other (due to surface attraction), and the spacing between each sheet, called the gallery, is normally 3-5A. Additional water molecules can readily enter the structure and coordinate with sodium cations, and expand the distance between the sheets.