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Release: Sept. 14
Contact: Jacquelyn Savani (609/258-5729)
Geologist Discovers Deep Structure of
Agates Natural System Provides
Example of Self-Assembly
Princeton, N.J.--Peter J. Heaney, an assistant professor of
geological and geophysical sciences at Princeton University, has
discovered that the iris banding pattern of agates (rocks that are
semi-precious jewels) arises from the alternation of two crystal
configurations of silica. His discovery about the structure of
agates is published in the Sept. 15 issue of Science magazine,
whose cover pictures one of Heaney's microscopic views of an agate
from Antelope, Ore.
Why study agates? The colored ones especially are beautiful. But
what causes the color--tiny impurities of iron or manganese
oxides--turns out to be less interesting than tiny bands that are
not visible to the naked eye. This iris banding can be seen with
an electron microscope as dark and light striations. The pattern
they create is the result of a natural chemical process that is
self-regulating. The idea is to understand how nature does it in
order to take advantage of self-regulating or smart processing in
making materials.
Agates, a type of quartz or silicon dioxide, form inside cavities
caused by gaseous bubbles in lava from volcanic eruptions. The
lava hardens to the rock basalt, and over time water with silican
trickles into the cavities, and silicon dioxide is deposited first
around the rim of the cavity. The crystal grows in a helically
twisting fiber from the rim of the cavity to the center. The
fibers, which grow from rim to center like bicycle spokes, run
perpendicular to the banding.
The first thing Heaney did was to design an experiment to test a
hypothesis advanced by Clifford Frondel, now emeritus at Harvard,
that the dark bands contain more hydrogen as an impurity than the
light bands. Heaney enlisted the aid of Andrew M. Davis of the
Enrico Fermi Institute at the University of Chicago. Davis, co-
author of the Science paper, is an expert at operating a
sophisticated machine called an ion microprobe, which is capable
of detecting hydrogen. The ion probe detected hydrogen, but not
much of an oscillation of hydrogen, so Frondel's hypothesis
doesn't explain the agate banding.
Next Heaney used transmission electron microscopy at the Princeton
Materials Institute (PMI) to get highly magnified pictures of
agates. He discovered that iris banding is related to the two
crystal configurations of quartz--one in which the silicon-oxygen
groups spiral in a right-handed sense and the other in a left-
handed fashion. The light bands are made up of big crystals (one
micron-sized), all of which spiral one way, all either left or all
either right in a given light band. In the dark band made up of
smaller crystals (0.05 micron), right and left-handed spirals grow
next to each other, indeed mostly alternating, but with lots of
defects in the crystal structure. "This pattern of high and low
defect concentration," says Heaney, "repeats itself thousands of
times in these agates."
Heaney has proposed a model for the process that gives rise to the
banding pattern in agates. "In these water solutions," he says,
"the silicon groups, which we represent schematically as a silicon
surrounded by four oxygens, form a single isolated stand-alone
unit called a monomer." So the light bands with big crystals
formed from a solution of monomers. Heaney has suggested that the
dark bands formed from solutions in which silica monomers are
linked to form a chain or polymer. "I've argued that the presence
of polymer within the solution will lend itself to the change of
handedness as the material crystallizes," he says.
So in effect the key determinant of the banding pattern in agates
is the way the silicon dissolves in the water that trickles into
the cavity in the volcanic rock. Says Heaney, "Low concentrations
of silica at the crystal's tip lead to the formation of the coarse
grained quartz. It takes thousands of years, and the amount of
silica in the fluid begins to increase over time because the
crystallization is slow. As the amount of silica within this
reservoir begins to increase over time, the monomer changes to a
polymer. Then, since this polymerization is a fast type of
crystal growth process, the amount of silica in the solution is
depleted. And the drop in concentration causes the switch back to
the regime where only monomer silica is present. This is a
process that can repeat and repeat thousands of times."
Isolated systems can form patterns in and of themselves without
any kind of template if there is a process that involves a
feedback mechanism such as the fluctuating levels of silica in
Heaney's model.
As a geologist Heaney is interested in working "from the rock back
to the environment of deposition--what were the conditions that
formed it."
As a material scientist (he's a member of the Princeton Materials
Institute), what's interesting, he says, is "the idea that you can
mimic this type of process, can create a process that will develop
this type of patterning. That's probably way down the road.
You've read about smart materials (i.e., that sense fatigue and
correct themselves in some type of way, for use in bridge
structures). Those types of materials are just being developed.
But in addition to smart materials, there's the concept of smart
processing: how can you create a system that will in and of
itself generate a material with a desired pattern? It's analogous
to having a cake bake itself into layers."
One other finding that Heaney reports in the Science paper is that
the overall texture of the agate exhibits fractal symmetry.
That's when apparent textures are unchanged with changing
magnification. "Agates have a fractal quality," says Heaney. "That surprised me. But it turns out that fractal quality is not an uncommon characteristic to these self-organized systems."
The National Science Foundation (NSF) has supported Heaney's
research.
Note: Photos are available through the Princeton University
Communications Office at (609) 258-3601.