Mineral Carbonation — Definition and GHG Accounting Context
Left alone, the planet already removes CO₂ by turning it into rock — silicate minerals weather over millions of years and lock carbon into stable carbonate stone. The problem is the timescale.
Mineral carbonation is the engineering of that natural reaction to run in years instead of aeons — and it produces the most permanent carbon store we know of.
Mineral carbonation is a carbon removal and storage pathway in which CO₂ reacts with calcium- and magnesium-rich minerals to form stable solid carbonates such as calcite and magnesite. Because the carbonate is the carbon’s lowest-energy state, the storage is effectively permanent — thousands to millions of years.
What mineral carbonation is
Mineral carbonation is a carbon dioxide removal and storage pathway in which CO₂ reacts with metal-oxide-bearing minerals — chiefly calcium and magnesium silicates — to form solid carbonate minerals. The carbon is not contained; it is chemically converted into new rock.
This is the same reaction that drives natural silicate weathering, the slow geological process that has regulated atmospheric CO₂ over Earth’s history. Mineral carbonation accelerates it deliberately — by injecting CO₂ into reactive rock, or by grinding suitable minerals and reacting them above ground — so that a process which naturally takes millennia completes in months or years.
What sets it apart from every other removal method is where the carbon ends up. A forest holds carbon in living biomass that can burn or be cut; a geological CO₂ store holds a pressurised fluid that must be contained. Mineral carbonation ends with the carbon as a thermodynamically stable solid — the form carbon reaches when it has nowhere lower to go.
CO₂ + (Ca,Mg) silicate → solid carbonate + silica. The product — calcite (CaCO₃) or magnesite (MgCO₃) — is the carbon’s lowest-energy state, so the reaction releases heat and does not reverse under ambient conditions. Net removal is the CO₂ mineralised minus the emissions of running the process.
The chemistry
The reactions are simple to write and thermodynamically favourable — carbonate formation is exothermic, so once carbon is in the mineral it stays there. The three most-cited feedstock reactions:
| Feedstock | Reaction |
|---|---|
| Olivine (forsterite) | Mg₂SiO₄ + 2 CO₂ → 2 MgCO₃ + SiO₂ |
| Wollastonite | CaSiO₃ + CO₂ → CaCO₃ + SiO₂ |
| Serpentine | Mg₃Si₂O₅(OH)₄ + 3 CO₂ → 3 MgCO₃ + 2 SiO₂ + 2 H₂O |
Suitable feedstocks fall into two groups: mined silicate rock — olivine, serpentine, wollastonite, and the calcium- and magnesium-rich basalts and peridotites — and alkaline industrial residues such as steel slag, cement kiln dust, red mud, mine tailings, and recycled concrete fines. The industrial residues are attractive because they are already crushed, chemically reactive, and often a disposal liability.
The catch is kinetics. Left to weather naturally, the reaction is glacially slow; the engineering problem is speeding it up without spending more carbon than it stores. In-situ methods use naturally reactive rock and hot, deep formations to drive the reaction; ex-situ methods grind the mineral to a fine powder and may add heat, pressure, or pre-treatment — all of which cost energy that must be counted against the CO₂ removed.
In-situ, ex-situ and enhanced weathering
Three approaches share the same chemistry but differ in where the reaction happens and how the carbon is ultimately stored.
In-situ (geological)
CO₂, often dissolved in water, is injected into reactive rock formations — basalt or peridotite — where it mineralises underground in the pore space. Field projects have shown the bulk of injected CO₂ turning to carbonate within roughly two years.
Ex-situ (industrial)
Minerals or alkaline wastes are reacted with CO₂ in engineered reactors or piles above ground. The solid carbonate product can be used — as aggregate, fill, or in building materials — turning storage into a material rather than a disposal problem.
Enhanced weathering
Finely crushed silicate rock is spread on land or coasts to speed natural weathering. This is a related pathway, but the carbon is stored mostly as dissolved bicarbonate carried to the ocean — a different storage form covered under enhanced weathering.
The first two are what the term “mineral carbonation” most precisely denotes: pathways that end in a solid carbonate. Enhanced weathering sits in the same family but stores carbon as ocean alkalinity rather than rock, which changes how it is measured and monitored.
Why it matters for permanence
Removal pathways are judged not only on the tonnes they capture but on how long those tonnes stay out of the atmosphere. This is where mineral carbonation is in a class of its own.
Because the carbonate is thermodynamically stable, mineral carbonation offers storage durability measured in tens of thousands of years or more — with no active containment required. That is a categorically different permanence profile from biological sinks like forests, biochar, or BECCS, whose stores carry reversal risk, and even from conventional carbon capture and storage, which keeps CO₂ as a fluid that must be physically confined and monitored for leakage. Mineral carbonation and direct air capture paired with mineral storage are therefore the pathways most often described as delivering effectively irreversible removal.
Accounting and crediting
A tonne of CO₂ turned to rock is only a tonne of removal if the process that put it there did not emit as much getting the job done. Crediting mineral carbonation as a removal rests on a life-cycle net calculation, not a gross one.
The measured quantity is the CO₂ chemically bound as carbonate — verified through feedstock mass balance, the carbonate actually formed, and tracking of the calcium and magnesium cations. From that gross figure, the emissions of mining, grinding, transport, and any heat or pressure are subtracted to give the net removal. Projects are accounted under ISO 14064-2 and quantified with tools like the carbon removal (CDR) calculator; the ICVCM Core Carbon Principles set the durability and integrity bar a resulting carbon credit must clear. Where an ex-situ process is powered by fossil energy, the net removal can be far smaller than the gross — and, in the worst case, negative.
Worked example
The stoichiometry sets the theoretical ceiling; the real net figure is that ceiling minus the process emissions of preparing and reacting the rock.
| Step | Figure |
|---|---|
| Reaction | Mg₂SiO₄ + 2 CO₂ → 2 MgCO₃ + SiO₂ |
| Feedstock — pure forsterite olivine | 1 tonne |
| Theoretical CO₂ bound (88 g CO₂ per 141 g forsterite) | ≈ 0.63 tCO₂ |
| Process emissions — mining, grinding, transport (illustrative) | − 0.10 tCO₂ |
| Net CO₂ removed | ≈ 0.53 tCO₂ |
Illustrative figures. The theoretical ratio is fixed by chemistry (~0.63 t CO₂ per tonne of pure forsterite); real-world capture is lower because reaction is incomplete, and the process-emission deduction depends entirely on how the grinding and heat are powered. Factor data across GreenCalculus reads live from the MasterBrain v2026.110.
Common mistakes
- Confusing it with enhanced weathering. Both react silicates with CO₂, but mineral carbonation ends in solid carbonate rock while enhanced weathering stores carbon mostly as dissolved ocean bicarbonate — a different storage form with different monitoring.
- Confusing it with conventional CCS. CCS stores CO₂ as a physically trapped fluid that must be contained; mineral carbonation chemically converts it to stable stone that needs no containment.
- Quoting gross, not net. The removal is the CO₂ mineralised minus the emissions of mining, grinding, transport and heat — a fossil-powered ex-situ plant can erase most of its own benefit.
- Assuming it is instant. The reaction is thermodynamically favoured but kinetically slow; the entire engineering challenge is accelerating it affordably.
- Double-counting industrial residues. Carbonating slag or concrete that is already accounted for elsewhere risks claiming the same tonne twice.
Frequently asked questions
Mineral carbonation is a carbon removal and storage pathway in which CO₂ reacts with calcium- and magnesium-rich minerals to form stable solid carbonates such as calcite and magnesite. It accelerates the natural silicate-weathering reaction that turns CO₂ into rock, either by injecting CO₂ into reactive geological formations or by reacting crushed minerals with CO₂ above ground. Because the carbonate is the carbon’s lowest-energy state, the storage is effectively permanent.
Effectively permanent — storage durability is measured in tens of thousands of years or more, with no active containment required. The carbon is chemically bound in a thermodynamically stable carbonate mineral, so it does not reverse under ambient conditions. This is a stronger permanence profile than biological sinks (which can burn or be cleared) or conventional geological CO₂ storage (which keeps CO₂ as a fluid that must be contained and monitored).
They share the same underlying reaction but differ in where the carbon ends up. Mineral carbonation produces solid carbonate rock — either underground (in-situ) or in a reactor (ex-situ). Enhanced weathering spreads crushed silicate on land or coasts, where the carbon is stored mostly as dissolved bicarbonate that flows to the ocean as added alkalinity. The storage form drives different measurement and monitoring, which is why they are treated as distinct pathways.
Conventional carbon capture and storage (CCS) stores CO₂ as a physically trapped fluid in a geological formation, relying on cap rock to contain it and requiring long-term leakage monitoring. Mineral carbonation chemically converts the CO₂ into a solid carbonate mineral, so the carbon is bound into stable rock and needs no containment. In-situ mineral carbonation is sometimes described as a form of CCS with mineral trapping — but the storage mechanism is chemical, not physical.
The theoretical ceiling is set by chemistry: about 0.63 tonnes of CO₂ per tonne of pure forsterite olivine, and broadly comparable figures for other magnesium and calcium silicates. Real-world capture is lower because the reaction is incomplete, and the net removal is smaller still once the emissions of mining, grinding, transport and any heat are subtracted. Net removal — not the stoichiometric maximum — is what can be credited.
It can be either, depending on the CO₂ source. Paired with atmospheric CO₂ — for example from direct air capture or ambient reaction of spread minerals — it is a genuine carbon dioxide removal. Applied to a concentrated industrial CO₂ stream, it functions as durable storage that avoids emissions rather than removing historical CO₂. The accounting distinction matters: only atmospheric or biogenic CO₂ turned to rock counts as removal.