Listwanite
Listwanite (also sometimes spelled listvenite, listvanite, or listwaenite) is unusual rock type that forms when ultramafic rocks (most commonly mantle peridotites) are completely carbonated. The pyroxene and olivine minerals found in peridotite commonly alter to form carbonate and serpentine minerals. However, peridotites are usually not completely carbonated. Rather, they typically contain carbonate veins (primarily magnesite; also calcite, dolomite, and other carbonates). Complete carbonation of peridotite means that every single atom of magnesium and calcium as well as some of the iron atoms has combined with CO2 to form secondary carbonate minerals such a magnesite and calcite. The silica atoms in listwanite are found in quartz. Thus, liswanites consist of quartz (a rusty red colour) and carbonate and also sometimes talc and Cr-muscovite (a mineral known as mariposite/fuchsite).
Formation
Geologists are still studying how listwanites form, but they likely form through the interaction of CO2-rich fluids with peridotites at higher than ambient temperatures up to ~200 degrees Celsius. Structural controls (faults and fractures) permit the percolation of the CO2-rich fluids through peridotite, so the formation of listwanites is generally structurally controlled.
CO2 content
Listwanites are important rocks to study for a number of reasons. First of all, listwanites contain large amounts of CO2 which originated from fluids that are now stored in solid mineral form. Recently, geologists and other scientists have been investigating the potential of storing CO2 in solid minerals (which are more stable than CO2 stored as a liquid or gas) through carbonation of mafic and ultramafic rocks (ref1). Mafic and ultramafic rocks take up significant CO2 through their natural alteration processes. However, the natural carbonation rates of these rocks are too slow to significantly offset anthropogenic CO2 emissions. Therefore, scientists are currently investigating if it is possible to geoengineer CO2 uptake in mafic and ultramafic rocks so that this CO2 uptake happens more quickly. This could be done, perhaps, by fracturing and heating and injection of CO2-rich fluids. This is already being tested in mafic basalts through the CarbFix Project in Iceland.
In addition to the recent interest in listwanites for carbon sequestration efforts, listwanites are also important because they are often associated with economic mineral deposits, particularly gold deposits.
 |
Wikimedia Commons has media related to Listwanite. |
References
- (Nature Geoscience Progress Article by Matter and Kelemen, 2009) http://www.nature.com/ngeo/journal/v2/n12/full/ngeo683.html