Primary red beds
Krynine (1950) suggested that the red beds were primarily formed by the erosion and redeposition of red soils or older red beds, but a fundamental problem with this hypothesis is the relative scarcity of red-colored source sediments of suitable age close to an area of red bed sediments in Cheshire, England. Van Houten (1961) developed the idea to include the in situ (early diagenetic) reddening of the sediment by the dehydration of brown or drab colored ferric hydroxides. These ferric hydroxides commonly include goethite (FeO-OH) and so-called "amorphous ferric hydroxide" or limonite. Much of this material may be the mineral ferrihydrite (Fe2O3 H2O).
This dehydration or "aging" process has been found to be intimately associated with pedogenesis in alluvial floodplains and desert environments. Berner (1969) showed that goethite (ferric hydroxide) is normally unstable relative to hematite and, in the absence of water or at elevated temperature, will readily dehydrate according to the reaction:
- 2FeOOH (goethite)→ Fe2O3 (hematite) +H2O
The Gibbs Free Energy for the reaction goethite → hematite (at 250 °C) is −2.76kJ/mol and Langmuir (1971) showed that G becomes increasingly negative with smaller particle size. Thus detrital ferric hydroxides, including goethite and ferrihydrite, will spontaneously transform into red-colored hematite pigment with time. This process not only accounts for the progressive reddening of alluvium but also the fact that older desert dune sands are more intensely reddened than their younger equivalents.