Which constituent mineral is dominant in granite weathering? A solution-sided approach through a laboratory experiment

• The influence of constituent minerals in granite weathering is elucidated.
• A laboratory experiment is carried out and adapts a solution analytical method.
• The dominant mineral is albite at a slightly acidic pH.
• The increase of pH due to plagioclase dissolution made Fe(OH)3 to precipitate easily.
• The dominant mineral is biotite under more acidic condition.

To elucidate the influence of constituent minerals in weathering of whole rock granite under acidic conditions, an artificial weathering experiment was carried out in a closed system using uncrushed samples of a piece of granite and four single crystals (annite, albite, microcline, and quartz). Thermodynamic analysis of rock (mineral)–solution equilibria was performed using pH–Eh and stability diagrams. Based on the solution analytical viewpoint, dominant mineral which played a central role depended on the pH of solutions in the early stage of weathering: the mineral was plagioclase under a slightly acidic condition (pH 5), and biotite under more acidic conditions (pH 4–1). In near-neutral to slightly acidic conditions (pH ≥ 5), pH increased remarkably due to plagioclase dissolution. As a result, the solution characteristics made Fe(OH)3 to precipitate easily. This H+ consumption enhanced an acid buffer capacity of the aqueous solution with granite.