Introduction
Carbon mineralization, also called carbonation, is the process whereby CO₂ molecules chemically bind with metal ions in silicate or oxide minerals—most commonly calcium (Ca²⁺) or magnesium (Mg²⁺)—to form rock-like carbonate solids. Unlike many sequestration methods that store carbon in a volatile form, the bonds created during mineralization are thermodynamically stable for millions—even billions—of years. In essence, we’re tapping into nature’s oldest carbon vault.
1. Fundamental Chemistry
At its core, mineralization is a set of acid–base reactions:
- CO₂ dissolves in water to form carbonic acid: CO₂ + H₂O ⇌ H₂CO₃
- Carbonic acid dissociates to bicarbonate and hydrogen ions: H₂CO₃ ⇌ HCO₃⁻ + H⁺
- Bicarbonate reacts with metal ions (Mg²⁺/Ca²⁺) from rock: M²⁺ + HCO₃⁻ → MCO₃(s) + H⁺ (where M = Ca or Mg)
The result is a solid carbonate mineral—calcite (CaCO₃) or magnesite (MgCO₃)—and a free proton. In natural settings, these protons are buffered by additional rock or fluid, keeping the cycle running.
2. Natural vs. Engineered Pathways
2.1 Natural Weathering
Over geological time, silicate rocks exposed to rain and groundwater slowly weather, releasing Mg²⁺ and Ca²⁺ that bind with atmospheric CO₂. This is how Earth regulated its climate for hundreds of millions of years—though at rates too slow to address today’s emissions surge.
2.2 In-Situ Basalt Carbonation
Projects like Carbfix in Iceland pump CO₂-charged water into subsurface basalt formations. Within two years, over 95% of the injected CO₂ has mineralized, thanks to basalt’s high magnesium-silicate content and pervasive fracture networks.
2.3 Ex-Situ Mineral Reactors
On the industrial side, we grind mine tailings or olivine-rich rock into powder, mix it with captured CO₂ and water in reactors, and accelerate carbonation with heat or catalysts. These systems can turn fluid CO₂ into solid rock within hours, rather than years.
3. Kinetics & Acceleration Strategies
Untreated mineralization can take decades or centuries. To scale up rapidly, engineers employ:
- Particle size reduction: Finer rock powder increases surface area and reaction rate.
- Elevated temperature & pressure: Boosts dissolution of CO₂ and metal ions.
- Chemical additives: Lowers activation energy (e.g., amine catalysts, pH buffers).
- Ultrasound & mechanical agitation: Keeps reactants in suspension and disrupts passivating layers.
When combined, these techniques can shorten turnover time from centuries to a few hours—unlocking high-throughput mineralization reactors.
4. Global Mineral Resources & Capacity
Earth’s crust holds trillions of tons of suitable silicate rock. Basalt alone covers 5% of the continental surface, with the theoretical capacity to lock away over 1,000 years’ worth of current emissions. Ex-situ feedstocks like mine tailings add another multi-century buffer. With smart site selection, we have more than enough material to scale mineralization to gigaton annual throughput.
Conclusion
Mineralization stands out as the only truly permanent carbon removal method—leveraging chemistry that Earth perfected over eons. By combining in-situ injections with fast ex-situ reactors, we can turn CO₂ from a climate liability into a benign rock resource. Next up, we’ll explore where on the planet these mineral vaults belong.