Geology And Geophysics

Igneous Rocks Peridotite

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Peridotite is an igneous rock, which means that it was formed after molten lava cooled and solidified.  It is composed mostly of olivine (a mineral which, when it is gem quality, is the beautiful peridot found in jewelry) and pyroxene, a silicate mineral.  It is dense and coarse-grained, and is ultramafic, meaning that it is composed of dark colored elements such as iron and magnesium, with less silica content.  It is believed to be the primary rock of the exterior of the earth’s mantle, which is the layer between the inner core and the outer crust.  Diamonds from the mines of South Africa are obtained from a mica-rich form of peridotite known as kimberlite. 

Peridotite seldom appears on land and is, instead, usually underground, beginning about twenty kilometers or more below the surface. Tectonic plate collisions that brought molten material to the earth’s exterior are responsible for its appearance on land, when it does happen.  It can be found in the Appalachian Mountains, and in New Zealand, Oman, Greece, and Japan.  

When peridotite is exposed to the air, it reacts with carbon dioxide (CO2) to form a solid carbonate, such as limestone (calcium carbonate) or magnesium carbonate.  This interesting property is being researched as a possible application for the reduction of greenhouse gases in the atmosphere.  CO2 is produced by the burning of fossil fuels, such as oil and gas.  Excessive CO2 in the air is believed to contribute to global warming.  Peter Keleman and Juerg Matter, researchers at Columbia University’s Lamont-Doherty Earth Observatory, are studying the feasibility of locking up some of the excess CO2 in the earth’s atmosphere in solid calcium carbonate by speeding up the reaction that occurs naturally.   By boring holes in areas of the crust and injecting water containing pressurized CO2, the reaction could be sped up 100,000 times or more.  The reaction, which generates heat, would speed up even more due to the higher temperature.  This would then cause fractures in the rock, which would create even more exposure to CO2, and further reaction.  If power plants could be built in areas that were near formations of peridotite, the CO2 emissions from the plants could be directly pumped into the rock, trapping it in solid calcium carbonate.  Trapping gases in rock in this manner is sometimes referred to as sequestration.

Another approach that has been proposed is to take advantage of the ocean that lies above peridotite formations off the coast of Oman and the western United States.  At the surface, ocean water absorbs CO2 from the air.  The CO2 content of the surface water is in equilibrium with the air.  If the peridotite beneath the seabed were heated and injected, the seawater would react with it, forming calcium carbonate and trapping the CO2 gas.  The seawater, then depleted of CO2, would return to the atmosphere and pick up more of the gas, allowing the cycle to continue.  With the vast supply of seawater and air, this may remove a great deal of CO2 from the atmosphere.

One study that is being done to get more specifics about how to safely and effectively operate this kind of mineral sequestration is the Carb-Fix Experiment.  This pilot project, a collaboration between Reykjavik Energy of Iceland, France, and the University of Iceland and Columbia University, is being done to show that mineral sequestration actually works, and is safe. The project’s site is the Hellisheidi Geothermal Plant in Iceland.  The rock being used in the project which is present in abundance at the site is basalt.  Basalt is very similar to peridotite in that it is igneous and mafic and has similar reactive properties.  At Hellisheidi, basalt makes up about 90 percent of the bedrock, and CO2 is taken from the plant’s geothermal wells.   The CO2 gas is then dissolved in water and injected into the ground to depths of from 400 to 800 meters.  The experiment data has shown that the site does not leak, and project monitoring continues with wells that are downstream from the injection well and in the soil around the injection site and wells.  Monitoring this is important to ensure that the groundwater will not be contaminated.

Klaus S. Lackner, Director of the Lenfest Center for Sustainable Energy at Columbia University in New York, has written a paper entitled “Air Capture and Mineral Sequestration:  Tools for Fighting Climate Change”.  In it, he states, “We could theoretically reduce the atmospheric level of carbon dioxide to the pre-industrial level (280 ppm, or parts per million) while continuing to use fossil fuels.”  He also points out that without CO2 capture and storage, the only way to stabilize the concentration of CO2 in the atmosphere is to abandon the use of coal, oil and natural gas.  Of course, renewable sources of energy such as solar, geothermal and wind are still being researched and may prove to be more and more useful in future years.  But even though this is the case, mineral sequestration with rock like peridotite deserves more research, beyond pilot projects and out into the field.

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