Synthesis And Characterization Of Mixed-Valent Iron Layered Double Hydroxides ("Green Rust")

Mixed-valent iron-layered double hydroxides (LDHs), also known as "green rust" (GR), are increasingly becoming recognized as important chemically reduced minerals. GR is believed to have contributed to the accumulation of biomolecules in primordial oceans and to have played a role in the geochemical cycles of early Earth and Mars. The high oxygen sensitivity of these LDHs makes them challenging to prepare and study. Consequently, a synthesis is described herein that affords pure GR compounds over a range of Fe(III) mole fractions and with a variety of intercalated anions. Reliable synthesis of pure GR required two key considerations: (1) ensure accurate and controlled pH titrations and (2) rigorously exclude oxygen before, during, and after the target Fe(II)/Fe(III) ratio is achieved. A detailed protocol for the strictly anoxic synthesis, real-time reaction monitoring, product characterization, and transport of a variety of GR samples is described. Protection of the air-sensitive samples from traces of oxygen for postsynthesis measurements or experiments can be challenging, leading to the potential formation of oxidation product impurities. Analysis of GR specimens by electron microscopy with the described methods enables rapid quality control to confirm sample integrity, including elemental microanalysis. The experimental details in the preparation of pure GR samples provide an important foundation to the geochemical community studying this chemically versatile group of compounds.