Transformations of Biotite to Kaolinite During Saprolite‐Soil Weathering

Biotite transformations in 10 pedons representing a developmental sequence of soils formed from coarse‐grained regional metamorphic rocks in the Piedmont and lower elevations of the Blue Ridge provinces of North Carolina are reported. Kaolinite is common to abundant in sand fractions of six of the pedons and is found to some extent in nine of the 10 pedons. Most of the kaolinite is derived from the pseudomorphic (isomorphic and largely isovolumetric) alteration of biotite. Biotite first alters to interstratified biotite‐vermiculite in which the vermiculite is hydroxy‐Al interlayered. The kaolinization of biotite extends throughout the grain, the pseudomorphs retaining the morphology of the biotite precursor but having the optical and structural properties of kaolinite. The koalinite pseudomorphs can be considered as single crystals because unit cells within the individual grains are all aligned crystallographically. A transformation model is proposed that accounts for most of the properties observed in weathering biotite. The degree of kaolinization of biotite increases with nearness to the soil surface and with increased degree of soil development. In the saprolite of the shallowest Dystrochrept biotite is relatively unweathered. In the sola of deeper Dystrochrepts and fine‐loamy Hapludults most of the biotite is kaolinized. Sand‐sized kaolinite pseudomorphs are less elastic and tenacious than fresh biotite and begin to physically disintegrate in the upper sola of fine‐loamy soils under the influence of processes of pedoturbation. In deeply developed clayey soils, kaolinite pseudomorphs may be absent from the sola. The increased clay content of clayey soils over that of fine‐loamy soils is largely a result of claysized kaolinite added from the comminution of sand‐sized kaolinite pseudomorphs of biotite.