Chemical Weathering: How Rain Dissolves Mountains

The weak acid that does the work

Pure rain is not neutral. As droplets fall, they dissolve atmospheric CO₂ (now ≈ 420 ppm) to form a dilute solution of carbonic acid:

• CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)  (Henry's-law uptake)
• H₂CO₃ ⇌ H⁺ + HCO₃⁻  (pKa₁ ≈ 6.35 at 25 °C)
• HCO₃⁻ ⇌ H⁺ + CO₃²⁻  (pKa₂ ≈ 10.33)

That first proton release drops unpolluted rain to about pH 5.6. It is a feeble acid — most of the CO₂ stays as dissolved gas — but it is abundant and everywhere. In soil the effect is amplified: root respiration and microbial decay push soil-gas CO₂ to 10–100× atmospheric levels, so pore water can reach pH 4–5. Organic acids (oxalic, citric, humic and fulvic acids) add more protons and, crucially, chelate metal ions, stripping them from mineral surfaces. The proton, H⁺, is the universal reagent of weathering: nearly every reaction below is an acid attack.

Carbonation: how limestone simply dissolves

Calcite is the fast case because it dissolves congruently — the whole crystal goes into solution with nothing left behind. Carbonic acid supplies the proton that mobilizes solid CaCO₃:

• CaCO₃(s) + CO₂(aq) + H₂O ⇌ Ca²⁺ + 2 HCO₃⁻

Read the equation carefully: two bicarbonate ions form, but only one carried carbon from the atmosphere — the other came from the rock. This is why carbonate weathering is roughly CO₂-neutral over geologic time: when that Ca²⁺ and HCO₃⁻ reach the ocean and biology reprecipitates calcite (CaCO₃), exactly one CO₂ is released back to the air. The reaction is also reversible and controlled by Le Chatelier's principle: raise CO₂ pressure and more limestone dissolves; let CO₂ escape and calcite precipitates. That single equilibrium, run forward in a cave and backward on the ceiling, both dissolves karst caverns and grows stalactites. Dolomite behaves similarly but more sluggishly: CaMg(CO₃)₂ + 2 CO₂ + 2 H₂O ⇌ Ca²⁺ + Mg²⁺ + 4 HCO₃⁻.

Hydrolysis: dismantling a silicate mountain

Most of a mountain is not limestone — it is silicate rock (feldspar, mica, pyroxene, olivine). These weather by incongruent hydrolysis: acid attacks the framework, soluble cations and silica leave, and a new solid — clay — is left in place. The textbook case is the breakdown of K-feldspar to kaolinite clay:

• 2 KAlSi₃O₈ + 2 H₂CO₃ + 9 H₂O → Al₂Si₂O₅(OH)₄ + 2 K⁺ + 2 HCO₃⁻ + 4 H₄SiO₄

Here the H⁺ pries potassium out of the lattice, aluminium stays behind in kaolinite, and silicon leaves as dissolved silicic acid, H₄SiO₄. For fast-weathering olivine (the key mineral in enhanced-weathering schemes) the reaction consumes even more acid:

• Mg₂SiO₄ + 4 CO₂ + 4 H₂O → 2 Mg²⁺ + 4 HCO₃⁻ + H₄SiO₄

Notice the difference from limestone: all the bicarbonate here comes from atmospheric CO₂. When the resulting Mg²⁺/Ca²⁺ + HCO₃⁻ eventually make marine carbonate, only half the CO₂ returns — so silicate weathering is a net long-term carbon sink. That asymmetry, first written down by Urey, is the chemistry behind the silicate weathering thermostat.

Oxidation, and why iron rusts the landscape red

A third pathway is redox weathering, and it is why so much of Earth's surface is stained ochre and rust-red. Iron-bearing silicates and sulfides contain reduced Fe²⁺, which is unstable in oxygenated water. Pyrite (FeS₂, "fool's gold") is the aggressive example — it generates its own strong acid:

• 4 FeS₂ + 15 O₂ + 14 H₂O → 4 Fe(OH)₃ + 8 SO₄²⁻ + 16 H⁺

Each sulfur goes from oxidation state −1 to +6 and each iron from +2 to +3, releasing sulfuric acid that can crash stream pH below 3 — the chemistry behind acid mine drainage. Silicate iron oxidizes more gently to insoluble ferrihydrite/goethite (FeO(OH)) and hematite (Fe₂O₃), the pigments of tropical laterites and the Grand Canyon's red walls. Because O₂ is the oxidant, this weathering only became globally important after the Great Oxidation Event ~2.4 billion years ago; before that, dissolved Fe²⁺ traveled freely and built the banded iron formations.

What controls the rate

Weathering rate is a chemical-kinetics problem, and a handful of variables dominate:

• Temperature. Rates roughly follow the Arrhenius equation; with activation energies near 50–90 kJ/mol, a 10 °C rise can double or triple silicate dissolution. Hot, wet tropics weather fastest.
• Water flux. Runoff removes dissolved products and keeps the reaction far from equilibrium (Le Chatelier again). Rainforests weather; deserts barely do.
• Acidity / pCO₂. More soil CO₂ and organic acids mean more H⁺ and faster attack. This is the feedback lever climate pulls.
• Mineral reactivity. A rough kinetic ladder: olivine > Ca-plagioclase > pyroxene > K-feldspar > muscovite ≫ quartz. Quartz is nearly inert, which is why beaches are quartz sand.
• Surface area. Rate scales with exposed area, so physical weathering (frost, glaciers, roots) that shatters rock into fine grains massively accelerates the chemistry. The two processes feed each other.

Real bedrock develops a weathering rind: a leached, clay-rich outer skin that armors fresh mineral beneath, so field rates run orders of magnitude slower than pristine crystals ground up in a lab beaker.

The planetary payoff: a thermostat and a soil factory

Silicate weathering is Earth's slow, self-correcting climate regulator. The feedback loop: warmer climate → more rain and higher soil CO₂ → faster silicate weathering → more atmospheric CO₂ consumed → cooling. Cool the planet and weathering slows, letting volcanic CO₂ rebuild — the Walker feedback. It operates on a ~100,000-to-1,000,000-year timescale, far too slow to rescue us from a century of fossil-fuel emissions, but it is why Earth has stayed broadly habitable for billions of years. The steady-state balance is roughly the volcanic CO₂ input (~0.1–0.3 Gt C/yr) matched by weathering drawdown.

The same reactions are the soil factory: the clay minerals (kaolinite, smectite, illite) left by incongruent hydrolysis give soils their ion-holding capacity and structure. Weathering also liberates phosphorus, potassium, calcium, and iron from primary rock, the ultimate nutrient supply for terrestrial and, via rivers and dust, marine ecosystems. And it sets river and ocean chemistry — the Ca²⁺, HCO₃⁻, Na⁺, K⁺, Mg²⁺ and dissolved silica in every river are weathering products en route to becoming seawater.

Turning the thermostat up on purpose

Because silicate weathering permanently removes CO₂, engineers want to accelerate it as a carbon-removal tool — enhanced weathering. The idea: grind fast-reacting silicates (olivine, basalt) to fine dust and spread them on farmland, where soil acids attack them. In principle each tonne of pure olivine can sequester up to ≈ 1.25 tonnes of CO₂ as bicarbonate. Basalt on croplands is being trialed at field scale; the same chemistry underlies mineral carbonation of mine tailings, where CO₂ locks into stable carbonate:

• Mg₂SiO₄ + 2 CO₂ → 2 MgCO₃ + SiO₂

The catches are real: grinding rock costs energy, the drawdown takes years, olivine dust can release trace nickel and chromium, and monitoring the actual bicarbonate flux is hard. But it exploits a reaction the planet has run reliably for eons. Meanwhile the flip side is a warning — human SO₂ and NOₓ emissions created acid rain (pH 4–4.5, occasionally below 3), a supercharged version of natural carbonation that has visibly eaten the marble of the Parthenon and Taj Mahal within a single human lifetime.

Carbonate vs silicate weathering: same acid, very different consequences for atmospheric CO₂

Property	Carbonate weathering (limestone)	Silicate weathering (feldspar/basalt)
Typical rock	Calcite CaCO₃, dolomite CaMg(CO₃)₂	Feldspar, olivine, pyroxene, basalt
Rate	Fast — cave-visible in centuries	Slow — hundreds to millions of years
Reaction type	Congruent dissolution	Incongruent hydrolysis (leaves clay)
Net CO₂ effect	Neutral on 10⁴–10⁶ yr (returned when CaCO₃ reprecipitates)	Net long-term CO₂ sink
Sets water hardness?	Yes — main source of Ca²⁺, HCO₃⁻	Adds Na⁺, K⁺, dissolved SiO₂