Mineralogy of a burned soil compared with four anomalously red Quaternary deposits in Denmark

The Fe oxide mineralogy behind the reddish colour (up to Munsell 10R) of a soil and four Quaternary surface deposits from central Jutland (Denmark) was investigated by XRD, TEM , M össbauer spectroscopy and bulk saturation magnetization.

The red soil (Naesset) consists of a 1 m deep patch, ~10 m² in area. It is one of at least 5 - 10 isolated spots of limited size which contain hematite and maghemite, and are probably due to local fire.

The four more extended (several hundred m 2) red sediments (Salten Skov, Salten, Pot Molle and Laasby) all occur at the feet of slopes and formed from Fe²⁺-containing seepage water. They contain 25 - ~100% Fe oxides,originally consisting of ferrihydrite.The well-drained parts of the deposits are at present all dominated by goethite. Hematite and maghemite were also identified at two sites in the top ~25 cm, one site containing goethite and hematite and probably a little maghemite, and the other goethite and probably hematite. The presence of hematite and particularly maghemite in the surface sediments could be explained by heating of goethite. However, there is no historic or prehistoric evidence of heating activity, and the spatial extension is much wider than that of normal human events. In contrast to the burned soil site, goethite is still present in the upper layers together with a high content of organic matter. Thus, although the present belief is that the deposits formed by heating, site evidence is to the contrary.