[chris (OP)]

What actually is clay? Why is it so sticky? What accounts for its water-retaining properties?

[paul.fr]

well, nobody else had a go.

Clay is defined by the size of it's particles, they are less less than 0.002 mm in diameter. I know clay absorbs and releases water/moisture very slowly so this could account for it's stickiness.

 

[Karen W.]

Clay
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For other uses, see Clay (disambiguation).
 
The Gay Head cliffs in Martha's Vineyard are made almost entirely of clay.Clay is a term used to describe a group of hydrous aluminium phyllosilicate (phyllosilicates being a subgroup of silicate minerals) minerals (see clay minerals), that are typically less than 2 μm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and tendency toward high plasticity.

http://en.wikipedia.org/wiki/Main_Page

[eric l]

"Clay" of course is a very vague term, and not all clays are equally water retaining.  In fact, the water retention capacity of most clays decreases sharply with increasing salt levels.  Lots of clays will retain sweet water, but not sea water.
The layered shape of the crystals is in important factor in the water retention, and so is probably the presence of aluminum (amphoteric material).

[JimBob]

Clay is a sheet silicate. This mean that there are large molecules of clay in a crystal structure that is flat as a pancake. These layers are held together by weak to fairly strong Van Der Waal bonds and metal atoms such as Potassium, Aluminum, magnesium and many other elements (ion substitution is common) that forms clumps of molecules that lay flat together. The weakness of the bonds holding the layers of montmorillonite (see below) allows water to replace the free metallic ions much more readily than Illite (see below). Montmorillonite is a swelling clay as water easily replace the metallic ions in the between layer spaces. That is why when you step on wet clay you slip - it is the clay molecules that slide across each other. Illite is much less likely to swell, i.e., have the inter-plate ions be replaced by water.

There are many types of clay. Montmorillonite and Illite are possibly the most recognized names for clays that are generically similar to the "official" chemical formula for these mineral (Illite is officially (K,H3O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H2O)] and Montmorillonite is officially (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH2O.) All clays are weathering products of other silicate minerals, mainly feldspars, biotite or muscovite, basalts, volcanic ash and other volcanic material.

Look up Phyllosilicates on Wikipedida and see all of the different types of clays and related sheet silicates.