Local clay worth more than gold?
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Although my interests generally favor living
things, a few geological treasures occupy my desk at work: half a geode, dull
on the outside but sparkling within; a thumb-sized chip of granite from the
“elephant rock,” a giant boulder left by a glacier in southern Illinois; and a
rounded, flat stone I’ve had so long I don’t remember where it came from—the
shore of Lake Superior, where my wife and I stopped on our honeymoon?
To this collection I recently added a small bag of
clay fragments, which I acquired on a field trip held in conjunction with the
50th annual meeting of the Clay Minerals Society on the University
of Illinois campus.
Is it worth more than gold?
That’s what another participant on the trip was
overheard to say as he received his sample bag.
The clay fragments aren’t much to look at. They
were moist when we collected them and very dark gray, but in the process of
drying they’ve lightened to the color of an elephant, and they crumble to a
fine powder when they’re ground together.
I would like to be able to tell you more about the
clay based on simple observation, but I can’t. That’s not a dismissal of my
powers of discernment, but a fact of life for people who study clay.
[Photos by author: (above) U of I geology major Evan Gragg collects samples from the Fithian illite for field trip participants; (below) Steve Altaner (white t-shirt), directs field trip participants as they take turns examining the Fithian illite.]
Geologists can distinguish among other minerals by
looking at them. Quartz looks like quartz, and mica looks like mica, even
without any kind of magnifying lens. But clay grains are too tiny to be
classified that way.
Geologists gained the ability to distinguish among
different types of clay only in the early decades of the twentieth century,
when they developed a process for analyzing it using x-ray diffraction. This process
enables them to observe distances between planes of atoms in crystal structures.
Since the set of these distances is unique in each mineral, taken together they
can be thought of as a fingerprint.
And that brings us to the value of the clay
fragments on my desk. They are composed of illite, which is one of three (or
four, depending on who is counting) recognized groups of clay minerals, and one
with a special local connection—as you may have gathered from the name. Illite
was first described by Ralph Grim, who was a faculty member in the UI
Department of Geology from 1948 to 1967, and who is widely recognized as the
“founding father” of the scientific discipline of clay mineralogy.
My illite holds extraordinary value for a clay
specialist because it was collected from the Fithian illite, a deposit on the
south bank of the Salt Fork River in Vermilion County that our field trip
visited. The Fithian illite is what geologists call a “type locality,” which is
to say it’s the very place Grim and his collaborators collected the samples
they used to analyze and describe it.
The intrinsic value of illite is much more modest.
As Steve Altaner, a UI professor of geology and organizer of the field trip
said to me when we spoke, it’s really the “average Joe” of clays—useful for
making bricks, perhaps, but not very sexy from a broad perspective. Illite, he
explained, lacks the moldable quality of kaolinite, a type of clay most people
come into contact with regularly—it’s the main component of porcelain. And it
lacks the super absorbing power of smectite, or “swelling clay.” Smectite’s
power to absorb and swell can be useful, as in cat litter, or destructive, as
when it deforms and breaks concrete structures that are anchored in it.
Altaner went on to point out that illite does
matter to geologists, in part because it is the second most abundant mineral in
sedimentary rock after quartz. In addition, it plays a key role in a process
used they use to gauge the timing of events that happened long ago (which for
them means millions or even billions of years), events such as the formation of
mountains or movements along geological faults. But hat may be a story for
another day.
