Denver, Colo. -- The way that soil minerals break down and how they react with groundwater are important bits of information for anyone studying groundwater pollution and soil contamination, but sometimes the information gathered in the lab doesn't quite match what happens in the real world, according to Penn State geologists.
"It's fairly well known that weathering experiments in the lab don't match natural weathering mechanisms," says Melissa A. Nugent, graduate student in geosciences. "Laboratory rates are often several orders of magnitude faster than those observed in natural environments."
Nugent, Dr. Susan L. Brantley, associate professor of geosciences, and Dr. Yang Chen, a recent Penn State graduate, looked at samples of albite feldspar from Quebec that had been buried in central Pennsylvania soil and removed after six months, one year and two years to try to understand the mechanism of weathering.
When minerals weather in nature, molecules in the rock dissolve in groundwater, react to form coatings, form other minerals or simply wear away. In the laboratory, researchers are usually only looking at one process at a time, which may explain the discrepancies with the natural world.
"Our investigation of the surface of the feldspar after it was in the ground shows an initial slight decrease in aluminum on the surface and then an increase in the concentration of aluminum on the surface," Nugent told attendees today (Oct. 29) at the annual meeting of the Geological Society of America in Denver.
"Lab experiments usually show a depletion of aluminum and sodium," she added.
Feldspar is generally made up of sodium, aluminum, silicon and oxygen, and is the most abundant group of minerals on Earth.
"We haven't looked at the aluminum chemistry in the ground in detail yet," says Nugent. "So we don't really understand what is going on with this amorphous aluminum coating.
Other researchers have looked at the surface of feldspars from the ground and not reported enriched aluminum layers.
The geologists are also monitoring conditions in the field where the feldspar samples are buried, recording acidity, water volume and ion concentrations to try to understand the mechanisms underway in the soil.
"If we know the saturation state and other kinetic variables for both the laboratory and field experiments, then we should be able to compare the results of both and better understand what is happening in the soil," says Nugent.
Also working on this project are Dr. Carlo Pantano, professor of ceramic science, and Daniele Cherniak, department of earth and environmental science at Rensselear Polytechnic Institute.
Ms. Nugent may be reached at 814-865-3565 or Nugent@farallon.psu.edu
Dr. Brantley may be reached at 814-863-1739.