Red Beds - Primary Red Beds

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. A fundamental problem with this hypothesis is the relative scarcity of Permian red colored source sediments to the south of Cheshire. 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. In fact, much of this material may be the mineral ferrihydrite (Fe₂O₃ H₂O).

This dehydration or "aging" process is now known 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

Gibbs Free Energy (G) is defined as some reactions are spontaneous because they give off energy in the form of heat (H < 0). Others are spontaneous because they lead to an increase in the disorder of the system (S > 0). Calculations of H and S can be used to probe the driving force behind a particular reaction. The Gibbs free energy of a system at any moment in time is defined as the enthalpy of the system minus the product of the temperature times the entropy of the system.

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 older desert dune sands are more intensely reddened than their younger equivalents.