In What Speeds Up Weathering you saw that warmth, water and surface area speed up the breakdown of rock. Now we look at the chemistry itself. Rain is not pure water: it dissolves a little carbon dioxide from the air and becomes weak carbonic acid, $\text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{H}_2\text{CO}_3$, with a pH near 5.6. Slowly, steadily, that acid takes rock apart.
- Hydrolysis: water and its $\text{H}^{+}$ ions attack feldspar, the most common mineral in the crust, and rebuild it as clay: $2\text{KAlSi}_3\text{O}_8 + 2\text{H}^{+} + 9\text{H}_2\text{O} \rightarrow \text{Al}_2\text{Si}_2\text{O}_5(\text{OH})_4 + 2\text{K}^{+} + 4\text{H}_4\text{SiO}_4$.
- Oxidation: iron in olivine, pyroxene and biotite loses an electron to oxygen, $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}$, and forms rusty iron oxides that stain soil red and brown.
- Dissolution: calcite dissolves completely in carbonic acid, $\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca}^{2+} + 2\text{HCO}_3^{-}$. Underground this carves caves and sinkholes, a landscape called karst.
Why it works
Minerals are most stable near the conditions where they formed. Bowen's series runs from olivine, formed at about $1,200^\circ\text{C}$ with no water, to quartz, formed at the cool end. At Earth's cool, wet surface, olivine is far from home and breaks down fastest; quartz, closest to surface conditions and with no iron to rust, barely changes. The weathering order (the Goldich series) is Bowen's series turned upside down.
Key idea
Weak carbonic acid turns feldspar into clay, rusts iron minerals and dissolves calcite. Minerals that crystallize first from magma weather fastest; quartz weathers slowest, so it piles up as sand.
Worked examples
Worked example: Balance a dissolution
Balance: $\square\,\text{CaCO}_3 + \square\,\text{H}_2\text{CO}_3 \rightarrow \square\,\text{Ca}^{2+} + \square\,\text{HCO}_3^{-}$.
- Calcium: one on each side, so start with $1\,\text{CaCO}_3$ and $1\,\text{Ca}^{2+}$.
- Carbon: $1 + 1 = 2$ on the left, so $2\,\text{HCO}_3^{-}$.
- Check hydrogen: 2 on each side. Oxygen: $3 + 3 = 6 = 2 \times 3$. Charge: $0 = +2 - 2$.
Answer: 1, 1, 1, 2.
Worked example: A fading gravestone
In a rainy city, a marble gravestone loses about 1.5 mm of its surface per century. How much after 250 years?
- Per year: $1.5 \div 100 = 0.015\,\text{mm}$.
- $0.015 \times 250 = 3.75\,\text{mm}$, enough to blur carved letters.
Answer: About $3.75\,\text{mm}$.
Common mistake
It seems logical that the minerals that crystallize first, at the highest temperatures, must be the toughest. At the surface it is the reverse: olivine weathers fastest and quartz lasts longest. Check with a beach: its sand is mostly quartz, even where the rocks upstream are full of feldspar and dark minerals.