Original paper

Differential roles of plagioclase and biotite in the early stage weathering of granite: a solid-sided approach combining laboratory experiment and surface analysis

Takaya, Yasuhiko; Hatta, Tamao; Matsukura, Yukinori

Abstract

This study elucidates the complicated mechanisms of granite weathering under acidic conditions. An experiment was carried out on uncrushed samples of granite and four minerals (single crystals of annite, albite, microcline, and quartz) in a closed system at room temperature for 56 days. The chemistry of the resulting surface was determined through XPS. The specific features of granite were confirmed by comparing them with the individual minerals. It was found that biotite played the central role in early stage weathering of the granite samples from a solid analytical viewpoint. Furthermore, plagioclase played the background role in near-neutral to acidic conditions. The increase of pH level due to plagioclase dissolution promoted Fe-oxide precipitation from biotite. Moreover, the precipitation of products including Al started slower, mainly because of its lower dissolution performance. This phenomenon led to an increase in the reactivity of biotite. Under more acidic conditions, the dissolution of biotite was dominant and constituent minerals had no influence on each other. The inter-mineral effect operated via pH fluctuation, as the weathering characteristics of the individual minerals depended on pH levels.

Keywords

biotitegranite weatheringplagioclasesecondary productssolution ph