Color development of iron oxides during rhyolite weathering over 52,000 years

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- and 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.