Kaolinite and gibbsite weathering of biotite within saprolites and soils of central Virginia

The mineralogical and chemical characteristics of saprolites and soils deve loped from granulitic, monzonitic, and charnockitic gneisses and mylonites of the Blue Ridge Mountains and western Piedmont foothills of central Virgi nia, were studied. Micromorphological, xray diffraction (XRD), scanning ele ctron microscopy (SEM), and microprobe analysis showed that the mineralogic al and geochemical evolution of biotite in profiles developed on gneisses i s consistent with pseudomorphic weathering of mica to kaolinite and halloys ite, with or without a mica-vermiculite intermediate phase. On mylonitic re eks, saprolites and soils also contain multimineral pseudomorphs after biot ite, in which gibbsite crystals eventually fill the whole volume. There is evidence of topotaxial formation of halloysite after biotite. Although gibb site is present at the first stages of the weathering of biotite, it is not clear if gibbsite forms directly from the mica or if it is a weathering pr oduct of kaolinite and/or halloysite. The source of aluminium may also be l ocated outside the original biotite crystal. These multimineral assemblages suggest that microenvironments of weathering are controlling the formation of secondary products from the parent biotite rather than the so called an ti-gibbsite effect, at least at this scale of investigation. It is suggeste d that the mylonitic fabric and subvertical foliation planes of these rocks are responsible for this mineralogical and geochemical evolution.