The in-fall of meteorites, interstellar dust particles and other extraterrestrial material during the late accretion period on the early Earth is speculated to have contributed significant quantities of inorganic phosphorus (P) minerals, including in the form of the mineral schreibersite, (Fe, Ni)3P. This mineral readily corrodes in water to form various soluble inorganic P species including both reduced oxidation state and condensed P compounds. Hypophosphate ( \(\text{HP}_{2}{\text{O}}_{6}^{{3 - }}\) ) is a reduced P species produced as a consequence of this corrosion process with a nominal + 4 oxidation state. Yet hypophosphate is not observed in natural water nor have any signs of it been found in ancient rocks, despite observations of phosphite ( \({\text{HPO}}_{3}^{{2 - }}\) ). In the present work, we study the chemistry of hypophosphate under both aqueous and anhydrous geochemical conditions, including the effects of pH, oxic and anoxic environments, hydrothermal conditions, mild heating, oxidizing solutions, and exposure to UV light. We show that hypophosphate most commonly disproportionates into phosphite ( \({\text{HPO}}_{3}^{{2 - }}\) ) and phosphate ( \({\text{HPO}}_{4}^{{2 - }}\) ) under aqueous conditions, and these in turn form condensed phosphorus species such as pyrophosphite, pyrophosphate, and isohypophosphate. Under anhydrous and high-temperature conditions (e.g., 250–400 °C for 2–9 days), hypophosphate both oxidizes and disproportionates to species such as phosphite, phosphate, pyrophosphate, pyrophosphite, isohypophosphate, trimetaphosphate, and triphosphate. At elevated temperatures and under anhydrous conditions (350 °C and above), hypophosphate is completely oxidized, forming only phosphates. Thus, hypophosphate is lost through disproportionation reactions at low temperature, and by oxidation at high temperature, suggesting that it is not stable for geologic timescales, and hence any evidence of this molecule in early Earth materials has been lost.
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