Urey Reaction

The Urey reaction is the chemical process in which carbon dioxide reacts with calcium silicate minerals to form calcium carbonate and silica: CaSiO₃ + CO₂ → CaCO₃ + SiO₂. Named for the chemist Harold Urey, who described it in 1952, it summarizes how rock weathering removes CO₂ from the air over millions of years.

Rain absorbs CO₂ and becomes weakly acidic, forming carbonic acid. The acid dissolves silicate minerals on land, releasing calcium ions and bicarbonate that are carried by rivers to the sea. There, organisms such as plankton and corals build shells of calcium carbonate, the mineral calcite, and the carbon is buried in sediments. The overall reaction is the sum of these steps, as covered in silicate weathering.

Weathering is faster when it is warm and wet, so higher temperatures remove more CO₂ and cool the planet, while cold slows it down and lets volcanic CO₂ accumulate. This negative feedback in the carbonate-silicate cycle probably kept Earth's climate within habitable limits for billions of years. Carbon returns to the air through volcanoes at subduction zones, linked to plate tectonics.

The feedback is slow, acting over hundreds of thousands to millions of years, so it cannot absorb today's emissions quickly. Its role in the long-term carbon cycle means that carbon dioxide released now will eventually be removed by it. Some proposals for "enhanced weathering" seek to speed the reaction by spreading crushed silicate rock, while ocean acidification is a short-term effect of the same chemistry.

• Silicate Weathering — the process that drives it
• Carbonate-Silicate Cycle — the cycle it belongs to
• Carbon Cycle — the wider system