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The Geology of Bricks
by Andrew Alden
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 sun-dried 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 levelled and new cities built on top.
Over the centuries these city mounds, called tells, grew
to considerable size.
Making sun-dried 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, brick clay 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 deposits.
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.
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 you
should go there yourself 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. Today we outdo nature in this obscure geochemical
stunt.
Andrew Alden
Professional geologist, writer, photographer, and
geological tour guide
Thirty-seven years of experience writing about geological
subjects
Six years as a research guide with U.S. Geological Survey (USGS)
Experience
Andrew Alden is a former writer for ThoughtCo who
contributed hundreds of articles for more than 17 years. Andrew works as a
geologist, writer, editor, and photographer. He has written on geological
subjects since 1981 and participates actively in his field. For example, Andrew
spent six years as a research guide with the U.S. Geological Survey,
leading excursions on both land land and at sea. And since 1992, he has hosted
the earthquakes conference for the online discussion platform, The Well,
which began as a dialogue between the writers and readers of the Whole Earth
Review.
In addition, Andrew is a longtime member of
the member of the Geological Society of America — an international
society that serves members in academia, government, and industry; and
the American
Geophysical Union — a community of earth and space
scientists that advances the power of science to ensure a sustainable future.
Andrew lives in Oakland, California; and though he writes
about the whole planet and beyond, Andrew finds his own city full of
interest too and blogs about its geology.
Education
Andrew Alden holds a bachelor's (B.A.) degree in
Earth Science from the University of New Hampshire, College of Engineering and
Physical Sciences, in Durham, N.H.
Awards and Publications
Andrew Alden on Earthquakes (The Well Group, Inc., 2011)
Assessment of River — Floodplain Aquifer Interactions (Environmental
and Engineering Geoscience, 1997)
Andrew Alden
on Hosting (The Well Group, Inc., 1995)
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