text: Any reaction in the domain of photogeochemistry, either observed in the environment or studied in the laboratory, may be broadly classified according to the nature of the materials involved.
  # Reactions among naturally occurring compounds. Photogeochemistry, both observational and exploratory, is concerned with reactions among materials known to occur naturally, as this reflects what happens or may happen on Earth.  
  # Reactions in which one or more of the reactants are not known to occur naturally. Studies of reactions among materials related to naturally occurring materials may contribute to understanding of natural processes. These complementary studies are relevant to photogeochemistry in that they illustrate reactions that may have a natural counterpart. For example, it has been shown that soils, when irradiated, can generate reactive oxygen species and that clay minerals present in soils can accelerate the degradation of synthetic chemicals; it may therefore be postulated that naturally occurring compounds are similarly affected by sunlight acting on soil. The conversion of N to NH has been observed upon irradiation in the presence of the iron titanate FeTiO. While such a compound is not known to occur naturally, it is related to ilmenite (FeTiO) and pseudobrookite (FeTiO), and can form upon heating of ilmenite; this may imply a similar reaction with N for the naturally occurring minerals.