Kerri Jansen (narrator): Meet goethite.
It's an iron mineral that's abundant in the
soil beneath our feet.
And it's a major component of rust.
Because goethite is so common and because
it binds to heavy metals, scientists want
to better understand how the mineral and its
relative hematite could be used to help clean
up contaminated soil.
Some scientists, like Sandra Taylor, think
goethite might even inspire a new generation
of rust-resistant materials.
But before goethite can be put to work cleaning
up soil or helping to make new materials,
we need to understand exactly how it interacts
with the atoms in its environment.
And that's not yet well understood.
Sandra D. Taylor: We're interested in understanding
how these iron minerals form to get a better
understanding of how they form in the environment
and also how rust forms.
Kerri: To get a handle on how goethite interacts
with its surroundings, Sandra is using a technique
called atom probe tomography.
This technique aims a laser at a thin needle
of whatever material a scientist is studying.
The laser is powerful enough to cause atoms
to evaporate from the tip of the needle.
The instrument then detects individual atoms
as they fly off a sample.
Sandra and the rest of the team at Pacific
Northwest National Lab hoped they could soak
goethite particles in a solution containing
a minor isotope of iron as a tracer and then
use the atomic probe to see how goethite interacted
with those isotopes.
But goethite doesn’t lend itself nicely
to making the needles that an atomic probe
needs.
So the researchers had to encase their goethite
particles in a carrier material, then carefully
mill away most of the excess to expose a small
chunk of goethite at the end of a tiny, needle-shaped
tip.
And it worked.
They were able to analyze the goethite-tipped
microneedle in the atom probe instrument.
Sandra D. Taylor (voice over): We can see
how atoms collect on the detector, we can
map their coordinates in x and y and track
their location along the tip.
And we can see in real time where atoms are coming off
and where their coordinates are.
Kerri: The team’s measurements produced
a 3-D map of the atoms within the mineral
structure.
Green dots represent the carrier material.
Pink is a common iron isotope normally found
in goethite.
And blue is the tracer isotope that the researchers
soaked their sample in.
The gray cube is just shown for scale.
It's 5 nm wide.
They found that the tracer atoms from solution
didn’t just coat the surface of the goethite.
They also penetrated more than 3 nm into the
mineral structure, something that scientists
had suspected but never directly observed.
The atoms concentrated in several “hot spots”
within the goethite, which the researchers
think happens because the tracer iron atoms
penetrate into flaws within the mineral.
That could be an important clue to creating
rust-resistant structures.
Sandra D. Taylor: We can then design better
materials that can withstand these chemical
reactions occurring at metal surfaces and
causing them to deteriorate.
Kerri: Penn State’s Christopher Gorski says
he thinks the research could have large implications
for geochemistry, where a clearer understanding
of chemical reactions in minerals could help
better interpret past conditions on Earth.
