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

摘要

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.