THE FATE OF DEGRADED MICAS IN THE DEEP CRITICAL ZONE: PATHWAYS TO THE FORMATION OF KAOLINITE AND OTHER SECONDARY MINERALS IN THE S.E. US PIEDMONT

The Calhoun Critical Zone observatory (CCZO) in SC provides insight into the interactions of biotic and abiotic components at the Earth’s surface. Biotite [K(Mg,Fe)₃AlSi₃O₁₀(OH)₂] and phengite [K(Mg,Al,Fe)₂(Al,Si)₄O₁₀(OH)₂] from a deep core (0-20m) in the CCZO influence the cycling of potassium at depth. As micas weather structural ferrous iron oxidizes and potassium is released in order to satisfy layer charge balance. Weathered biotite and phengite contain less potassium due to higher levels of oxidation of iron, where the 2:1 structure is maintained. As 2:1 layers become less abundant, layers are restructured to 1:1-layer kaolinite [(Al₂Si₂)O₅(OH)₄]. Sand-sized grains were analyzed in an electron microprobe for their elemental composition using energy dispersive spectroscopy. Whole grain mounts were also examined optically. Weathered grains displayed “frayed” ends when viewed perpendicular to the principle c-axis. Analysis of weathered grain ends in comparison to less weathered centers suggested potassium continued to be lost at the ends. Near the surface (<50 cm), more kaolinite grains were observed (also likely derived from feldspar dissolution). A continuous range of compositions between end member micas and kaolinite were found at all depths. Evidence for fixed potassium in biotite and phengite supports the notion that reservoirs are available to supply rooted zones as uplift and chemical erosion proceeds. Although the loss of potassium occurs, persistence of lower amounts of potassium in near-surface clays suggest that degraded micas can serve as a stock for nutrient cycling. As subsurface conditions become reducing in wetter conditions (lower-oxygen supply) the ferric iron in the 2:1 structure turns ferrous, necessitating uptake of interlayer ions such as potassium (and ammonium?) to compensate for the increase in negative layer charge. In turn, with dryer conditions (higher-oxygen supply) interlayer ions are released to compensate the decrease in negative layer charge. This implies that seasonal reducing and oxidizing cycles (fall/winter and spring/summer, respectively) create the potential for degraded biotite and phengite to act as refugia for nutrients in the subsurface CZ.