Iron-rich Clay Minerals on Mars: Potential Sources or Sinks for Hydrogen and Indicators of Hydrogen Loss over Time

Abstract

Although direct evidence is lacking, indirect evidence suggests that iron-rich clay minerals or poorly ordered chemical equivalents ("palagonite") are widespread on the martian surface. Such clays (or other Fe and OH-bearing phases) can act as sources or sinks for hydrogen ("hydrogen sponges"). Ferrous clays can lose hydrogen, and ferric clays (especially dehydrogenated "oxy-clays") gain it by the coupled substitution Fe3+o(Fe2+0H)1, equivalent to minus atomic H. This "oxy-clay'' substitution (H2-loss) involves only proton and electron migration through the crystal structure, and therefore occurs nondestructively and reversibly, at relatively low temperatures. The reversible, low-temperature nature of this reaction contrasts with the irreversible nature of destructive dehydroxylation (H20-loss) suffered by clays heated to high temperatures.