Abstract

As part of the Shale Hills Critical Zone Observatory (SHCZO) research project, this study examined 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 N-S climate-transect that extends from New York to Puerto Rico. This study focused mainly on the weathering characteristics of the clay minerals of these deposits, some of which tend to undergo mineralogical transformations via mixed-layer intermediates. X-ray fluorescence (XRF) analysis of soil samples revealed a general trend of depletion of several elements, including Ca, Mg, Al, Fe and K toward the top of the soil profile. However, both Al and Fe were enriched at depth, and Si showed enrichment near the middle of the soil profile. Exchangeable base cations show similar trends, however Ca and Mg become slightly enriched at the top of the profile. X-ray diffraction analysis (XRD) of bulk shale samples reveals that the shale is composed of quartz, Fe-rich chlorite and illite. Some shale beds contain calcite or dolomite, or both, and feldspar is present in minor amounts. XRD analysis of the soil shows a similar mineral assemblage, however, the carbonates have weathered out of the soil. Modeling with NEWMODⓒ [1] suggests that illite is the most abundant phyllosilicate in the <2 mm clay-size fraction (~90%), with chlorite constituting
the remainder. XRD reveals that the <2 mm clay-size fraction of the soil contains illite, illite-vermiculite, chlorite (possibly chlorite-vermiculite) and vermiculite with hydroxyaluminum interlayers. Modeling with NEWMODⓒ reveals that the proportions of soil clays change with depth in the soil profile. For example, chlorite constitutes approximately 21% of the clay fraction in the 65-70 cm depth interval and decreases in
abundance to about 12%, with 8% vermiculite present in the 0-5 cm interval where near-surface weathering is more intense. Artificial weathering experiments with shale clays 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.

[1] Moore & Reynolds (1997) X-ray diffraction and the identification and analysis of clay minerals, Oxford, 378 p.

Citation

MacDonald, S. (2010): Clay Mineralogy, Geochemistry and Weathering Characteristics of Clinton Group Shales in Central New York . Bachelor of Arts Geology, Colgate University.

This Paper/Book acknowledges NSF CZO grant support.