How can rocks absorb CO2?

Question

In the beginning of the earth, there was at a certain point so much $\ce{CO2}$ in the air that most of it was absorbed by rock. The consequence of that was the snowball earth, because the atmosphere was empty; all the heat of the earth couldn't be more absorbed by $\ce{CO2}$, so it was getting cold. But how can rock ($\ce{SiO2}$?) absorb $\ce{CO2}$?

Answer 1

rock ($\ce{SiO2}$)?

Rock is not strictly $\ce{SiO2}$. Rocks are composed mostly of silicates: minerals such as feldspars $\ce{CaAl2Si2O8}$ or $\ce{(Na,K)AlSi3O8}$, olivine $\ce{(Mg,Fe)2SiO4}$, pyroxene $\ce{CaMgSi2O6}$, etc.

There are two ways in which rocks can absorb carbonates, one being a more direct form of the other.

1. Weathering of rocks by carbonic acid. $\ce{CO2}$ reacts with $\ce{H2O}$ to form acid and bicarbonate: $\ce{CO2 + H2O -> H+ + HCO3^-}$. This acid would then trigger: $\ce{4KAlSi3O8 + 4H+ + 2H2O -> 4K+ Al4Si4O10(OH)8 + 8SiO2}$. This is the reaction of orthoclase (a type of feldspar, common in granites) to form kaolinite (a clay, aka dust or mud). All of the resulting cations ($\ce{Ca2+, Mg2+, K+, Na+}$, etc. and the bicarbonate end up washing to the oceans, where they precipitate carbonates: solid rocks composed of mostly calcite ($\ce{CaCO3}$) and dolomite ($\ce{CaMgCO3}$), thus overall removing $\ce{CO2}$ from the atmosphere.
2. Directly reacting of $\ce{CO2}$ with solid rock. All of the above basically happens in-situ, directly transforming solid silicate rocks to solid carbonate rock. When this happens, minerals such as magnesite ($\ce{MgCO3}$) or siderite ($\ce{FeCO3}$) also occur. This is actually one of the way to do industrial carbon sequestration: instead of putting $\ce{CO2}$ in the atmosphere, for example in power stations, you inject it down to the rock to lock it up in solid minerals.