The Source of Life: Clay

Clay is old, tired rock. And it’s the source of life on Earth.

 

When granite erodes and weathers over millions of years, the minerals in the rock break down until they are very fine, flat, microscopic particles of clay. The essential quality that these particles have is that they trap water between them, and then slide over one another. This is what’s responsible for clay’s plasticity: clay is malleable, but it holds the shape you put it in until you alter it. Only when it is heated to high temperatures does it solidify into something durable.

 

After all of that weathering and eroding, the clay might be deposited at the foot of a former cliff or transported by river or glacier. And sometimes the clay finds itself in a geological circumstance where it is compressed over many more millions of years. Eventually, the clay compresses back into rock again, called shale. If you find shale, you can actually grind it into a useable clay. A related rock, called slate, is just shale that has been compressed even longer. The flat sheets that comprise these rocks reveal, to some extent, the molecular structure of clay.

 

This sort of cycle is nothing rare on our beautiful planet. Pottery is a part of a very long and satisfying cycle in which minerals deep inside the earth emerge in the form of superhot magma or lava, cool into rock, are ground down into clay, and then heated up again to form rock. It won’t happen in our lifetime, but even fired pottery will eventually be broken down into its constituent minerals and recycled by the planet.

 

The pure molecular formula for a basic clay (kaolin) is this: Al₂O₃+2SiO₂+2H₂O. It’s a mineral composed of alumina, silica, and water. This makes sense, because granite, which is what erodes to form clay, is composed largely of a class of minerals called feldspars. Feldspars, in turn, are defined by their composition of alumina and silica. (Plus other minerals: potassium, sodium, calcium). The feldspar in granite breaks down into ever finer and finer particles until it makes clay. Not surprisingly, other rocks that include similar minerals can be eroded into clay, too, like basalt. What these rocks have in common is that they are all volcanic or igneous rocks, which are formed when magma from a volcano cools. When you fire pottery in a kiln, you’re basically recreating that process of melting these minerals and then cooling them into a kind of stone once again. The kiln is the volcano, and our pottery is handmade stone. Hence some clay being termed “stoneware.”

 

Clay is rarely found in anything close to a really pure composition. The molecular formula itself varies, some clays having more or less silica or alumina. The shape and size of the particles vary significantly. And clay contains many impurites that alter the character of the clay body you work with. Granite is made up mostly of feldspars, but also quartz (sand is quartz) and mica (those shiny flakes of rock). It’s not surprising, then, that it’s common to find clay that has sand or mica in it.

 

On a microscopic level, clay is made up of platelets; you can actually see them and their flat, roughly hexagonal shape, under an electron microscope. These platelets vary in size depending on the clay—the smaller the size, the more plastic the clay, generally. The platelets are attracted to each other, and the weaker that attraction, the more plastic the clay. Kaolin isn’t very plastic, in part because the particle sizes are bigger, and in part because those platelets are strongly attracted to one another; bentonite is very plastic, because the platelets are small and they are bonded together weakly. The water you add to dry clay slips in between the platelets and allows them to slide over one another. They’re basically having a slip’n’slide party.

 

Obviously, the chemistry gets way more complex than what I’ve gone over here. One of these days, I’ll get around to writing an entire article devoted to pedantic disclaimers about oversimplifications, but for now, let me just acknowledge that the facts in this article are all true to the best of my knowledge but some are incomplete.

 

At the outset, I said that clay is the source of life on earth. Here’s what I mean. In the 1950s, scientists [Miller & Urey] conducted experiments where they tried to replicate the ecosystem of early Earth. They threw a bunch of stuff together in a sealed glass flask—basic stuff that would’ve been on the planet 4 billion years ago, like hydrogen, water, methane, and ammonia. Then they subjected the flask to extremes of heat and cold, and bursts of electricity that simulated lightning. And they found that these most basic molecules had come together in the flask to form amino acids. They had recreated primordial Earth, and spontaneously generated more complex stuff.

 

In 2009, two other scientists [Ricardo & Szostak] published an article about their own experiment. It was similar to the old one, but their sealed flask included one extra ingredient: clay. Like their predecessors, they found that simple amino acids spontaneously formed out of the more basic building blocks. But they also found that when given an environment of clay, those amino acids were attracted to each other, and they came together to form much longer chains of more complex amino acids. Those chains were a basic RNA—which is a precursor to DNA, the foundation of all life on Earth.

 

The authors say this: “The discovery reinforced the suggestion that life may have started in clay-rich muds at the bottom of pools of water formed by hot springs.”

 

All this, from some old, worn out rocks.