montmorillonite
/ mont-mor-ILL-oh-nite /
Montmorillonite is the swelling clay, the drama queen of the clay world. It is the main mineral in bentonite, the clay that puffs up when it gets wet, seals ponds, thickens drilling muds, and clumps in cat litter. Add a little of it to a stiff porcelain body and the whole mass suddenly becomes silky and plastic; that outsized effect on plasticity is why potters keep it on the shelf as a tiny but powerful additive.
Structurally montmorillonite is a 2:1 (TOT) sheet silicate, the same tetrahedral-octahedral-tetrahedral sandwich as mica, but with a crucial difference in the interlayer. Its layers carry only a modest negative charge, balanced not by tightly locked potassium but by loosely held, exchangeable cations (Na+, Ca2+) that come surrounded by water. Water molecules can march right in between the TOT layers, prying the spacing open from about 0.96 nm when dry to well over 1.5 nm when wet, so the clay swells, sometimes to many times its dry volume. The particles are also extraordinarily fine and thin, giving montmorillonite a huge surface area (hundreds of square metres per gram) and a high capacity to exchange ions, which is why it is such an effective plasticiser, binder, and adsorbent.
That swelling power is a double-edged tool. In small doses montmorillonite is a superb plasticiser and suspension-stabiliser (deflocculated bentonite keeps a casting slip fluid yet non-settling). But too much brings trouble: a montmorillonite-rich body shrinks enormously on drying and cracks easily, and swelling clay in a foundation or a raw material can heave and crack structures. The same interlayer water that makes it so plastic is exactly what makes it so shrink-prone, a reminder that in clays a virtue and a vice usually share one mechanism.
Drop a spoon of dry bentonite into water and watch it swell into a jelly many times its original size, as water forces the TOT layers apart. That same water uptake is why just one or two percent added to a short porcelain body transforms it into a smooth, throwable clay.
Water walks in between the layers: montmorillonite swells, which makes it intensely plastic and intensely shrink-prone.
More plasticity is not always better. Because montmorillonite swells and shrinks so much, a body over-loaded with it dries and fires with severe cracking; it is a seasoning, used in small percentages, not a main clay.