In the current Economist, there's a story called eating carbon about peridotite, which usually occurs only in the mantle of the Earth, typically about 20 km below the surface. But in some places, it occurs at the surface. And peridotite reacts with carbon dioxide to form limestone or marble.
This could be big.
More below the fold
The Economist article cites an article in the Proceedings of the National Academy of Sciences that found that the peridotite that rises to the surface in Oman absorbs tens of thousands of tons of carbon each year. But again per the Economist, the authors of that article Peter Kelemen and Juerg Matter, of Columbia University, think that it can be made to absorb about 4 billion tons a year of carbon dioxide, more than 10% of the total produced by man. And they think that carbon dioxide can be shipped there.
Peridotite doesn't show up at the surface in a lot of places, but it does show up in some: Greece, Croatia, some Pacific Islands (Papua New Guinea and New Caledonia), and some spots in the USA.
I decided to dig a little more.
First here is the abstract of the PNAS article. I decided to go crazy and buy access to the article itself, but I will be modest in quoting from it, since obviously it's copyrighted material.
Then I did some googling.
Wikipedia article
Peridotite is the dominant component of the Earth's mantle above 400 km; it is composed of various combinations of olivine and pyroxene. It reacts rapidly with both air and water.
Green car congress summarizes the PNAS article.
Main findings include:
The reaction between peridotite and carbon dioxide can occur underground
There is a lot more peridotite than people thought
More is being formed
and, most importantly (from the Green Car site):
The scientists say that the process of locking up carbon in the rocks could be speeded 100,000 times or more simply by boring down and injecting heated water containing pressurized CO2. Once jump-started in this way, the reaction would naturally generate heat. That heat would in turn hasten the reaction, fracturing large volumes of rock, exposing it to reaction with still more CO2-rich solution. Heat generated by the earth itself also would help. (The exposed Omani peridotite extends down some 5 kilometers.)
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The scientists say that such a chain reaction would need little energy input after it was started. Accounting for engineering challenges and other imperfections, they assert that Oman alone could probably absorb some 4 billion tons of atmospheric carbon a year—a substantial part of the 30 billion sent into the atmosphere by humans, mainly through burning of fuels. With large amounts of new solids forming underground, cracking and expansion would generate micro-earthquakes—but not enough to be readily perceptible to humans, says Kelemen.
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Some more, from the PNAS article:
It has long been known that this reaction took place, but it was uneconomical to use it on a large scale, especially of carbon dioxide that was not local.
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The two reactions of principal interest are:
Mg-olivine + carbon dioxide yields magnesite + quartz
and
Mg-olivine + CaMg Pyroxene + carbon dioxide + water =
serpentine + calcite + magnesite
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Ways of increasing the amount of carbon dioxide that could be absorbed involve drilling and fracturing the peridotite, heating it to 185 degrees C and then injecting fluids that are rich in carbon dioxide, or pure carbon dioxide. The initial heating would require energy, but the reactions then generate heat, making it self sustaining. The ideal temperature is about 185 degrees C.
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An alternative process could avoid prolonged pumping of fluid and use of purified CO2. In Oman, New Caledonia, and Papua New Guinea, peridotite is present beneath a thin veneer of sediment offshore. Here, peridotite could be drilled and fractured, and a volume could be heated. Again, little heating would be required if, for example, the initial temperature at the bottom of a 5-km bore hole is 100 °C (Fig. 8). Then, controlled convection of near-surface water through the rock volume could sustain high temperature via exothermic hydration of olivine at a flow rate of ≈4·10−6 m/s (as seen in Fig. 3 Right). The carbonation rate would be limited by supply of dissolved CO2 in convecting seawater—only ≈104 tons of CO2 per km3 of peridotite per year at a flow rate of 4·10−6 m/s—but the cost would be relatively low.
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The authors conclude:
Because these proposed methods of in situ mineral carbonation use the chemical potential energy inherent in tectonic exposure of mantle peridotite at the Earth's surface, the optimal temperature for carbonation can be maintained in a rock volume at little expense. Further, rock volumes at depth are, inherently, at relatively high pressure and elevated temperature. Thus, compared with engineered, mineral carbonation "at the smokestack," this method does not involve quarrying and transportation of peridotite, processing of solid reactants via grinding and heat treatment, or maintaining high temperature and pressure in a reaction vessel. Instead, the major energy investments in this method would be for drilling, hydraulic fracturing, pumping fluid, preheating fluid for the first heating step, and purification of CO2. Also, unlike ex situ mineral carbonation, this method may require on-site CO2 capture or transport of purified CO2 to the in situ carbonation locality.
Clearly, more elaborate models combined with field tests will be required to evaluate and optimize this method. For example, it is difficult to predict the consequences of hydraulic fracturing of peridotite, plus cracking associated with heating, hydration, and carbonation, in terms of permeability and reactive volume fraction. Such processes are all-but-impossible to simulate in the laboratory. Large-scale field tests should be conducted, because the proposed method of enhanced natural CO2 sequestration provides a promising potential alternative to storage of supercritical CO2 fluid in underground pore space, and to engineered, ex situ mineral carbonation.