『Abstract
To evaluate the behavior of iron during weathering, colors of
four rhyolites with different degrees of weathering (eruption
ages: 1.1, 1.8, 26, and 52 ka) were studied. Because the initial
chemical compositions of the four lavas were similar to one another,
it is considered that the four lavas had similar whitish color
at the time of eruption. However, at present more weathered lavas
have more yellow-brownish color, indicating the formation of iron
(hydr)oxides or Fe-bearing clays. In order to characterize those
weathering products, spectro-colorimetry and differential dissolution
method were combined. The colors of 1.1- ad 1.8-ka lavas are close
to the color of ferrihydrite whereas those of 26- and 52-ka lavas
approach to the color of goethite. This indicates that the initial
stage of weathering is dominated by the formation of ferrihydrite
and with increasing weathering the transformation from ferrihydrite
to goethite proceeds. It is estimated from the change of rock
color that the half-life for the transformation reaction of ferrihydrite
to goethite in the field is >1100 years, which is at least 80
times longer than previously reported values based on laboratory
experiments. The presence of impurities such as Si and Al in ferrihydrite
can be a possible origin for retarding the transformation rate
in the field.
Keywords: Iron oxide; Clay; Color; Weathering; Rhyolite』
1. Introduction
2. Sample descriptions
3. Methods
4. Results
5. Discussion: behavior of iron during rhyolite weathering
6. Conclusions
Acknowledgements
References