The Geology of Bricks

bricks and mortar
Bricks and mortar are two very different types of artificial stone. Photo (c) Andrew Alden, licensed to (fair use policy)

The common brick is one of our greatest inventions, an artificial stone. Brickmaking transforms low-strength mud into strong materials that can endure for centuries when properly cared for.

Clay Bricks

The main ingredient of bricks is clay, a group of surface minerals that arise from the weathering of igneous rocks. By itself, clay is not useless—making bricks of plain clay and drying them in the sun makes a sturdy building "stone." Having some sand in the mix helps keep these bricks from cracking.

Sundried clay is little different from soft shale.

Many of the most ancient buildings in the early Middle East were made of sundried bricks. These generally lasted about a generation before the bricks deteriorated from neglect, earthquakes or the weather. With old buildings melted into piles of clay, the ancient cities were periodically leveled and new cities built on top. Over the centuries these city mounds, called tells, grew to considerable size.

Making sundried bricks with a little straw or dung helps bind the clay and yields the equally ancient product called adobe.

Fired Bricks

The ancient Persians and Assyrians made stronger bricks by roasting them in kilns. The process takes several days, raising the temperature above 1000 °C for a day or so, then cooling gradually. (This is much hotter than the mild roasting or calcination used to make top dressing for baseball fields.) The Romans advanced the technology, as they did with concrete and metallurgy, and spread fired brick to every part of their empire.

Brickmaking has been basically the same ever since. Until the 19th century, every locality with a clay deposit built its own brickworks because transport was so expensive. With the rise of chemistry and the Industrial Revolution, bricks joined steel, glass and concrete as sophisticated building materials.

Today brick is made in many formulations and colors for a variety of demanding structural and cosmetic applications.

Chemistry of Brick Firing

Over the period of firing, brickclay becomes a metamorphic rock. Clay minerals break down, release chemically bound water, and change into a mixture of two minerals, quartz and mullite. The quartz crystallizes very little in that time, remaining in a glassy state.

The key mineral is mullite (3AlO3 · 2SiO2), a blended compound of silica and alumina that is quite rare in nature. It is named for its occurrence on the Isle of Mull in Scotland. Not only is mullite hard and tough, but it also grows in long, thin crystals that function like the straw in adobe, binding the mix in an interlocking grip.

Iron is a lesser ingredient that oxidizes into hematite, accounting for the red color of most bricks. Other elements including sodium, calcium and potassium help the silica melt more easily—that is, they act as a flux. All of these are natural parts of many clay minerals.

Is There Natural Brick?

Earth is full of surprises—consider the natural nuclear reactors that once existed in Africa—but could it naturally produce true brick? There are two kinds of contact metamorphism to consider.

First, what if very hot magma or erupted lava engulfed a body of dried clay in a way that allows the moisture to escape? I would give three reasons that rule this out:

  • 1. Lavas are rarely as hot as 1100 °C.
  • 2. Lavas would cool quickly once they engulf surface rocks.
  • 3. Natural clays and buried shales are wet, which would draw even more heat from lava.

The only igneous rock with enough energy to even have a chance to fire proper brick would be the superhot lava known as komatiite, thought to have reached 1600 °C. But the Earth's interior has not reached that temperature since the Early Proterozoic Era more than 2 billion years ago. And at that time there was no oxygen in the air, making the chemistry even more unlikely.

On the Isle of Mull, mullite appears in mudstones that have been baked in lava flows.

(It also has been found in pseudotachylites, where friction on faults heats dry rock to melting.) These are probably a far cry from real brick, but I should go there myself to make sure.

Second, what if an actual fire could bake the right kind of sandy shale? In fact, that does happen in coal country. Forest fires can start coal beds burning, and once started these coal-seam fires may go on for centuries. Sure enough, shale overlying coal fires can turn into a red clinkery rock that's close enough to true brick.

Unfortunately this occurrence has become common as human-caused fires start in coal mines and culm piles. A significant fraction of global greenhouse-gas emissions arises from coal fires. But today we outdo nature in this obscure geochemical stunt.