My wife Lori asked me to explain to her pottery class in a fairly simple way what clay is, where it comes from, and how it got here. So here's an attempt at a non-technical explanation.
Clay is feldspar rusting. This is an analogy, but not that far from the actual process. We all know what happens if you buy a nice, shiny piece of cast iron from the hardware store and leave it outside in the sun and rain. It quickly rusts. If you leave it out long enough, it turns to almost all rust. So what is rust?
Rust is primarily the minerals limonite and goethite, created when iron combines with oxygen from the atmosphere and oxygen in water. We all know that iron things tend to rust faster when wet than when dry. Moisture hastens rusting.
Feldspar is not iron. Iron is one element, iron. Feldspar is a large family of minerals made from oxygen, silicon, aluminum, sodium, potassium and calcium. Feldspar does not form on the Earth's surface. It only forms miles beneath the Earth's surface, where solid rock is naturally in a semi-liquid, molasses-like state.
Feldspar is only released from its 'natural' home and to the Earth's surface either when it is forcibly ejected from a volcano as lava or when, after hundreds of millions of years, the 2-3 miles of solid rock above the feldspar is eroded away, leaving the feldspar nakedly exposed on the Earth's surface. This is usually in the form of granite, which is a rock made of feldspar and quartz and some mica.
To add another analogy, just like a piece of fine pottery on the edge of a shelf 'wants' to fall on the floor and smash, feldspar 'wants' to turn to clay when it is exposed to the Earth's surface. The agent for the pot on the shelf wanting to fall down and smash is gravity (in outer space, pottery does not break, it orbits). The agent for feldspar wanting to turn to clay is a bit more complex, but similar in design to iron rusting. In both, the agents are primarily air and water.
In the presence of air and water at the Earth's surface, the most natural and restive state for feldspar is to re-align its molecules into clay molecules. Clay is a mineral, just like quartz or feldspar. It has a very regular and ordered crystalline structure, like a diamond or a cube of salt. The three predominant clay minerals are kaolinite, illite and montmorillonite. With a scanning electron microscope you can get pictures of very nice, well formed, plate-like clay crystals growing right next to a crystal of feldspar.
Feldspar becomes clay by slowly bringing water into its crystal structure, like a sponge left in a puddle of water. This water becomes part of the very fabric of the feldspar; like how iron becomes part of your blood cells. The feldspar wants the water. It likes it. Which brings us back to rust.
What we call rust is the natural state of iron on the Earth's surface. Iron readily combines with oxygen to make rust. It wants to become rust. In fact, we have to do all kinds of crazy things to prevent iron from becoming rust. We coat it with oils, with paint (like Rust-Oleum) or galvanize it with zinc, all to keep the iron from contacting oxygen in the air and oxygen in water, sort of like teacher chaperones at a high school dance. Left to its own device, feldspar becomes clay because it wants to; that is its most stable and natural state on the Earth's surface. Like a thrown ball 'wants' to come back down, feldspar wants to become clay. Clay is rusted feldspar; and the actual chemical reactions are not that different.
In Maine, where I live, from 1880-1930 there was a flourishing industry where large feldspar deposits were quarried and mined for use as ceramic pottery glaze. This was feldspar that had not yet had time to weather into clay. It is still solid enough to make a house foundation. But if you crush into a fine enough powder, it works beautifully as a glaze ingredient. Most of the feldspar mined in Maine was shipped to pottery works in New Jersey as a basic glaze ingredient for everything from fine plateware to toilet bowls. It was an 'industrial mineral,' as the saying goes.
The only reason Maine does not have deposits of natural, 'primary' clay is because for the past million years Maine has been scoured by successive, mile high glaciers every 100,000 years or so, which like a steel plow on a snow-filled driveway, scraped away all the clay and softened rock right down to hard bedrock and dumped the residue in the Atlantic Ocean. In the U.S., you have to go south of the line of glaciers, ie. Kentucky or Tennessee, to find clay deposits still intact and near where they were first formed. What we in Maine call 'marine clay' is actually the finely ground-up residue from the glaciers' scraping and grinding that has partly altered into true clay minerals and is on its way to doing so, give another 10 million years. That said, it is still perfectly usable as a slip or a low-fire earthenware body. Be patient, Maine !!!