『Abstract
Comprehensive understanding of chemical and mineralogical changes
induced by weathering is valuable information when considering
the supply of nutrients and toxic elements from rocks. Here minerals
that release and fix major elements during progressive weathering
of a bed of Devonian New Albany Shale in eastern Kentucky are
documented. Samples were collected from unweathered core (parent
shale) and across an outcrop excavated into a hillside 40 year
prior to sampling. Quantitative X-ray diffraction mineralogical
data record progressive shale alteration across the outcrop. Mineral
compositional changes reflect subtle alteration processes such
as incongruent dissolution and cation exchange. Altered primary
minerals include K-feldspars, plagioclase, calcite, pyrite, and
chlorite. Secondary minerals include jarosite, gypsum, goethite,
amorphous Fe(III) oxides and Fe(II)-Al sulfate salt (efflorescence).
The mineralogy in weathered shale defines four weathered intervals
on the outcrop-Zones A-C and soil. Alteration of the weakly weathered
shale (Zone A) is attributed to the 40-a exposure of the shale.
In this zone, pyrite oxidization produces acid that dissolves
calcite and attacks chlorite, forming gypsum, jarosite, and minor
efflorescent salt. The pre-excavation, active weathering front
(Zone B) is where complete pyrite oxidation and alteration of
feldspar and organic matter result in increased permeability.
Acidic weathering solutions seep through the permeable shale and
evaporate on the surface forming abundant efflorescent salt, jarosite
and minor goethite. Intensely weathered shale (Zone C) is depleted
in feldspars, chlorite, gypsum, jarosite and efflorescent salts,
but has retained much of its primary quartz, illite and illite-smectite.
Goethite and amorphous Fe(III) oxides increase due to hydrolysis
of jarosite. Enhanced permeability in this zone is due to a 14%
loss of the original mass in parent shale. Denudation rates suggest
that characteristics of Zone C were acquired over 1 Ma. Compositional
differences between soil and zone C are largely attributed to
illuvial processes, formation of additional Fe(III) oxides and
incorporation of modern organic matter.』
1. Introduction
2. Study site
3. Methods
3.1. Sampling protocol
3.2. Chemical analyses
3.3. Mineralogical analyses
4. Results
4.1. X-ray diffraction data
4.2. Chemical data
4.3. Mineral chemistry
5. Discussion
5.1. Assessment of weathering effects on mineralogy and chemistry
5.1.1. Evolution of mineral composition during weathering
5.1.2. Absolute changes in chemical and mineral concentrations
5.2. Silicate and aluminosilicate weathering in the New Albany
Shale
5.3. Sulfide weathering in the New Albany Shale
5.4. Organic-matter weathering
6. Weathering zones in the New Albany Shale
7. Conclusions
Acknowledgements
References