Abstract
Electron microprobe analyses of fresh biotites, altered biotites and newly formed vermiculitic phases formed in two weathering profiles of the same calcalkaline granite indicate that the composition of the altering fluid can greatly influence the composition of the newly formed minerals. In one profile, calcic carbonate solutions produce a regularly interstratified mineral (hydrobiotite) which concentrates Fe and Si relative to Al and Mg while another profile gives a vermiculite-intergrade mineral with a higher Al content. Hydrobiotite (interstratified mineral) persists into later stages of weathering than does vermiculite in these profiles. In both cases the final assemblage is kaolinite + oxides which also begins to form in the early stages of weathering. Biotite breaks down to a multiphase assemblage in both of the profiles.
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