Abstract

As part of the Shale Hills Critical Zone Observatory (SHCZO) research project, we studied the weathering characteristics of Clinton group (Silurian) shale and overlying soils in a research site in central New York State. This site is the northern-most of six SHCZO satellite sites making up a NS climate-transect that extends from New York to Puerto Rico. We were especially interested in the weathering characteristics of the clay minerals of these deposits, some of which tend to undergo mineralogical transformations via mixed-layer intermediates. X-ray diffraction analysis (XRD) of bulk shale samples reveals that the shale is
composed of quartz, Fe-rich chlorite and illite (mica). Some shale beds contain calcite or dolomite, or both, and feldspar is present in minor amounts. Modelling with NEWMOD© [1] suggests that illite is the most abundant phyllosilicate in the <2μm clay-size fraction (~90%), with chlorite constituting the remainder.  Both phyllosilicates are well crystallized.

Soils at our study site contain abundant fragments of Clinton group shale. [Most soils in central New York have developed from glacial sediments (e.g. till and stratified drift) comprising aggregates of sand, silt, clay, and rock fragments derived from ice-scoured bedrock units. The mineralogy of the glacial debris is often heavily influenced by the mineralogy of the local bedrock.] XRD reveals that the bulk soil contains abundant quartz with lesser amounts of feldspar, mica and chlorite. The <2μm clay-size fraction contains illite, illite-vermiculite, chlorite (possibly chlorite-vermiculite) and vermiculite with hydroxy-aluminum interlayers. Kaolinite may be present in minor amounts. Modelling with NEWMOD© reveals that the proportions of soil clays change with depth in the soil profile. For example, illite consitutes 60% of the clay fraction in the 65-70 cm depth interval and decreases in abundance to about 45% in the 0-5 cm interval where near-surface weathering is more intense. Artificial weathering experiments with the shale show that the Fe-rich chlorite is more susceptible to decomposition in a weak acid solution than illite. Clays exfoliated (by sonification) from the surface of shale fragments collected from the soil were analyzed by XRD
and show incipient weathering of clay minerals.

[1] Moore & Reynolds (1997) X-Ray Diffraction and the Identification & Analysis of Clay Minerals, Oxford, p.378.

Citation

MACDONALD, S., APRIL, R.H. and KELLER. D.M. (2010): Clay mineral weathering in shales and soils in the critical zone. Goldschmidt.

This Paper/Book acknowledges NSF CZO grant support.