Hydrolysis (Weathering)

Hydrolysis is the primary process by which silicate minerals decompose during chemical weathering. ^[1] It is an extremely important chemical reaction between silicate minerals and acids that leads to breakdown of the silicate minerals and the release of metal cations and silica - but crucially, it does not lead to complete dissolution of the minerals. ^[1] That incompleteness is the defining feature of hydrolysis and the reason it is so geologically significant: it does not simply destroy silicate minerals but transforms them into new minerals - principally clay minerals - that have very different physical and chemical properties from the originals.

Incongruent Dissolution

Hydrolysis is a form of incongruent dissolution. In a congruent dissolution reaction, all the ions of a dissolving mineral enter solution in exactly the proportions given by the mineral’s chemical formula. Incongruent dissolution means that the ions released into solution do not correspond to the formula of the original mineral - some fraction of the mineral’s components stays behind, recombining to form new solid phases rather than dissolving. ^[1] When aluminum is present in the original silicate mineral, those aluminum-bearing residues become clay minerals - kaolinite, illite, or smectite - depending on the intensity of the weathering environment and the chemistry of the parent mineral. ^[1]

The reactions in Table 1 of Boggs illustrate this clearly. Orthoclase feldspar (KAlSi₃O₈) reacts with H⁺ ions and water to produce kaolinite (H₄Al₂Si₂O₉), silicic acid (H₄SiO₄), and K⁺ ions in solution: 2KAlSi₃O₈ + 2H⁺ + 9H₂O → H₄Al₂Si₂O₉ + 4H₄SiO₄ + 2K⁺. ^[1] Albite (NaAlSi₃O₈) undergoes an analogous reaction, producing kaolinite, silicic acid, and Na⁺ ions: 2NaAlSi₃O₈ + 2H⁺ + 9H₂O → H₄Al₂Si₂O₉ + 4H₄SiO₄ + 2Na⁺. ^[1] In both reactions, the aluminum from the original feldspar ends up locked in kaolinite - a layered clay mineral with a very different structure and composition - while the potassium or sodium is released to solution and the silica largely leaves as silicic acid.

More specifically, the source states that orthoclase feldspar can break down to yield kaolinite or illite, and albite can decompose to kaolinite or smectite, depending on the weathering conditions. ^[1] The distinction between these products is significant: kaolinite is the product of more thorough leaching and represents a greater loss of cations from the weathering system, while smectite and illite retain more cations and reflect less intense weathering conditions.

Sources of H⁺ Ions

The H⁺ ions that drive hydrolysis are most commonly supplied by the dissociation of CO₂ dissolved in water. The more CO₂ dissolved in water, the more aggressive the hydrolysis reaction becomes. ^[1] Soil air typically has CO₂ concentrations far higher than the atmosphere because plant roots and decomposing organic matter generate it continuously. This means that water percolating through soil is much more acidic - and therefore more effective at driving hydrolysis - than simple rainwater.

However, hydrolysis can also take place in water containing little or no dissolved CO₂. In that case, H⁺ ions may be supplied either by clay minerals that have a high proportion of H⁺ ions in their cation exchange sites, or by living plants, which create an acid environment around their roots. ^[1] The availability of H⁺ from multiple sources means that hydrolysis is not confined to environments with high CO₂ - it is a nearly universal weathering process wherever silicate minerals are exposed to water.

Fate of Released Silica

Most of the silica set free during hydrolysis goes into solution as silicic acid (H₄SiO₄). ^[1] This silicic acid is transported away from the weathering site in solution and ultimately delivered to rivers and the ocean, where it may be extracted by silica-secreting organisms such as diatoms and radiolarians to form chert and siliceous ooze. However, some of the released silica does not escape in solution. It may instead separate as colloidal or amorphous SiO₂ that remains behind at the weathering site, where it can combine with aluminum to form clay minerals. ^[1] Whether silica escapes as silicic acid or stays behind as colloidal SiO₂ depends on the intensity of leaching, the pH of the weathering solution, and the rate at which water moves through the weathering profile.

pH and Weathering Intensity

The acidity of the weathering solution is expressed by its pH, defined as the negative logarithm to the base 10 of the approximate hydrogen-ion concentration in moles per liter. The pH scale extends from 0 to 14, corresponding to H⁺ concentrations ranging from 10⁰ to 10^-14. Solutions with a pH of 7 are considered neutral; acids have pH values lower than 7, and bases have values greater than 7. ^[1] The practical consequence for hydrolysis is that lower pH means more available H⁺ ions and faster, more aggressive breakdown of silicate minerals. This is why heavily vegetated tropical soils, where CO₂ production and organic acid release are high, produce the most intensely weathered mineral assemblages on Earth.

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