Scientists from Trinity College in Dublin and their Italian colleagues from the National Institute of Geophysics and Volcanology for the first time received a detailed answer to the question: what are the conditions for carbon dioxide to turn into minerals inside basalt rock? For these purposes, they studied Sverrefjellet volcano on Spitzbergen Archipelago, where thousands of years ago magmatic CO₂ interacted with glacial melt water resulting in forming carbonates of calcium, magnesium and iron in the rock. This natural storage of carbon became an open-air lab to study the ways to cope with excessive carbon dioxide with the help of nature.
It is commonly known that global warming requires not only CO₂ emissions reduction, but also the technologies for its safe and long-term removal. Mineralization is believed to be the most high-potential method: carbon dioxide interacts with nature, and natural calcium, magnesium and iron form carbonate materials. This process is irreversible and environmentally safe, but it goes on extremely slowly in artificial conditions, especially when we are talking about magnesium carbonate and dolomite, which provide for the most stable storage of carbon.
Irish researchers collected basalt sample from Sverrefjellet volcano and studied them with the help of X-ray diffraction and electron microscopy. The analysis showed that the rock was practically permeated with carbonate veinlets and cement. And these minerals were formed in strict sequence. At first, proto-dolomites rich with calcium were formed close to basalt surface, then, while the calcium reserves were depleting, magnesium and iron got involved into the process forming magnesium carbonate and ferrous carbonate. This serial transition from one mineral to another one reflects the change of the chemical composition of fluids in the process of their interaction with rock.
The researchers even succeeded in assessing the speed of this process. The measured the carbonate layers thickness, which varied from 40 to 320 micrometers, and compared it with the assumed duration of hot hydrothermal waters circulating in the rock – from dozens to hundreds of years. The calculations showed that the average speed of crystals growing was from 10⁻¹⁴ to 10⁻¹¹ meters per second. To check the reliability of these calculations, the obtained data was compared with the ones received during the lab tests of magnesium precipitation at different temperatures. The concurrence turned out to be absolutely striking: at 100 °C the speed of mineral formation grew million times vs the surface conditions. The scientists identified that forming a millimetric layer of magnesite in hydrothermal system of Sverrefjellet volcano took place for decades only, while as at room temperature it would have required hundreds of thousands of years. This explains, why the engineering projects working at 25–50 °C do not allow for forming stable magnesium carbonates.
The research also showed fundamental discrepancy in stability of the minerals. Calcium carbonates turned out to be the least reliable: they may dissolve with changes of acidity or with inflow of new portions of calcium-stripped water. Magnesium carbonate and dolomite demonstrated absolutely different behavior: they are practically insoluble and are capable of being preserved in the rock for millions of years. These minerals are key for long-term capturing of carbon dioxide in subsoil. Ferrous carbonates, on the contrary, turned out to be unstable: they are disintegrated with oxygenation, releasing iron and leaving voids inside the rock. However, this feature has a positive effect: this secondary porosity provides for inflow of fresh solutions and supports further banding of CO₂.
Hence, the study of Irish and Italian geologists demonstrated for the first time: magnesium carbonate and dolomite, the most valuable minerals for the climate strategy, are naturally formed in moderately hot hydrothermal systems at 60-220 °C and in the light acidity (pH 5-6) solutions. This result directly prompts to the engineers: for assuring efficient long-term storage of carbon dioxide, they need to focus on active geothermal zones or artificially create similar parameters when injecting CO₂ into subsoil.
