Abstract

Fossil fuels continue to provide major sources of energy to the modern world even though global emissions of CO₂ are presently at levels of ≥ 19 gigatons/yr. Future antipollution measures may include sequestering of waste CO₂ as magnesite (MgCO₃) by processing ultramafic rocks. Common ultramafic rocks react easily with HCl to form MgCl₂ which is hydrolyzed to form Mg(OH)₂. CO₂ would be transported by pipeline from a fossil fuel power plant to a sequestering site and then reacted with Mg(OH)₂ to produce thermodynamically stable magnesite. Huge ultramafic deposits consisting of relatively pure Mg-rich silicates exist throughout much of the world in ophiolites and, to a lesser extent, in layered intrusions. Peridotites and associated serpentinite are found in discontinuous ophiolite belts along both continental margins of North America. Serpentinites and dunites comprise the best ores because they contain the most Mg by weight (35 to 49 wt-% MgO) and are relatively reactive to hot acids such as HCl. Small ultramafic bodies (~1 km³) can potentially sequester ~1 gigatons of CO₂ or ~20% of annual U.S. emissions. A single large deposit of dunite (~30 km³) could dispose of nearly 20 years of current U.S. CO₂ emissions. The sequestering process could provide Mg, Si, Fe, Cr, Ni, and Mn as by products for other industrial and strategic uses. Because “white” asbestos (chyrsotile) is a serpentine mineral, CO₂ sequestering could dispose of some waste asbestos. The cost and environmental impact of exploiting ultramafic deposits must be weighed against the increased costs of energy and benefits to the atmosphere and climate.