Fig. 5: Geologic context and properties of natural and synthetic ferrihydrites.
From: Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
a Ferrihydrite deposit on a lava cave floor in Gruta Dos Balcões, Terceira island, Azores, Portugal. Iron-rich water was percolating from the basaltic cave ceiling, forming stalactites and depositing on the ground. b Ferrihydrite precipitates in a stream in Block Island, Rhode Island, USA. c X-ray diffraction patterns of natural and synthetic ferrihydrites used in this study. The peaks at 2.56 Å and 1.5 Å are characteristic of 2-line ferrihydrite, demonstrating its poorly crystalline structure4. Ferrihydrite from Block Island is impure and contains quartz (SiO2), some illite, chlorite and traces of other minerals. d VNIR spectra showing the variability of natural and synthetic ferrihydrites in the 6A1 → 4T2 band position (dashed black line), compared to ChemCam LIBS spectrum of dust47 and the resulting spectral shoulder at the 600–650 nm wavelength range. Spectra normalized at 800 nm. Source data are provided as a Source Data file.
