Abandoned mine offers clues about permanent CO2 sequestration

We’ve been looking at the geologic structure and veining at Red Mountain to try and understand how hard ultramafic rock could be transformed into magnesite,” Maher said.

The Stanford team estimates that Red Mountain originally held nearly 1 million metric tons of magnesite, of which about 83 percent has been mined.

One million metric tons of magnesite is the equivalent of sequestering 140,000 metric tons of carbon in mineral form,” said graduate student Pablo Garcia del Real.

Our goal is to use the vast reservoirs of magnesium stored in ultramafic rocks to chemically bind with CO2 and form magnesite. But as we discovered at Red Mountain, breaking those rocks is one of the main engineering challenges that we face.”

San Andreas fault
After several field trips to Red Mountain and a series of laboratory tests, Maher and her co-workers concluded that tectonic forces played a crucial role in creating the magnesite deposits.

To unlock the secrets of these deposits, we needed to find clues about both the mineralization process and the geologic history of the area,” del Real said.

California’s infamous San Andreas fault lies less than forty miles west of Red Mountain. The fault formed about twenty-nine million years ago, creating a large gap between the Earth’s crust and the hot mantle below. The gap allowed heat to rise to the surface, raising the temperature of the water and liquid CO2 trapped in the ultramafic rocks.

When the temperature of a liquid increases, the volume increases,” del Real said. “We think that the CO2 enhanced the ability of the water to expand, adding enough pressure to break the ultramafic rock and cause the chemical reaction that formed the magnesite veins.”

The process was fast and furious, he added.

The magnesite veins are very white, homogenous and composed of very tiny crystals, so they probably formed quickly, perhaps instantaneously,” del Real explained. “The ultramafic rocks appear shattered and broken, which means that this was a violent event.”

Low-temperature process
The release notes that back at the lab, the Stanford scientists conducted an isotopic analysis of the magnesite samples collected at the mine. The results suggest that when the San Andreas fault opened, magnesite formed 1 kilometer below the surface as temperatures rose from about 53 degrees Fahrenheit (12 degrees Celsius) to 86 F (30 C). Such low temperatures should make it relatively easy for scientists to convert atmospheric CO2 into pure magnesite. Del Real and his colleagues, however, have yet to replicate the process experimentally.

If we inject CO2 from a power plant or other point source into ultramafic rock, we would expect it to form magnesite,” he said. “But when we try to make magnesite in the laboratory at low temperatures, it fails to form.”

For carbon sequestration to succeed, scientists will also have to figure out a way to make ultramafic rock permeable. “There is no way that CO2 or anything else will flow through these rocks,” del Real said. He discussed the problem of permeability at the AGU meeting.

In our research, we combine a big tectonics approach with the minute thermodynamic behavior of fluids,” del Real said. “So we go from the very large scale to the very small scale.”