PAINTING A RAINBOW WITH OCHRE: IRIDESCENCE IN BOTRYOIDAL GOETHITE

Ochre is loosely defined as an aggregate of nanocrystalline goethite (FeOOH) and hematite (Fe₂O₃) mixed with aluminosilicate clay, and a recent study (Brooks et al. 2018) suggests that hominids have used ochre as a pigment for ~300,000 years. The minerals within ochre are idiochromatically colored red-brown-yellow due to ferric iron, but it is not uncommon to observe strikingly iridescent surfaces on samples with a botryoidal habit. Informally known as “turgite”, rainbow ochre exhibits an iridescence that does not vary with the viewing angle.

In this study, we explored the microstructures within turgite to determine the cause of its iridescence. Focused ion beam (FIB) milling and scanning electron microscopy (SEM) revealed that the botryoidal morphology reflects the epitaxial attachment of rod-shaped nanocrystals measuring ~20 nm in length and ~10 nm in diameter. Needles of goethite nanocrystals stack to create nested fans with a cauliflower-like fractal morphology. Selected-area electron diffraction of FIB-milled transmission electron microscopy (TEM) sections showed that surface attachment occurs along (001), the plane of closest packing in goethite (S.G. Pnma), and the nanorods attach to each other along (010) – the tunnel direction, to lengthen the needles.

Since the dimensions of these stacked needles fall below the diffraction limit for visible light, these ordered nanostructures cannot account for the iridescence. FIB cross-sections of iridescent and botryoidal goethite from 5 localities around the world have revealed subsurface void layers with quasi-periodicities of ~100 nm. FIB analysis of non-iridescent specimens disclosed no void layers. We attribute the angle-independent iridescence in turgite to light scattering from these void layers, and we are modeling observed reflectance spectra with gratings drawn from FIB and TEM images. The origin of the void layers remains uncertain. Possibilities include episodic surface poisoning, hydrofracturing as nanocrystals coalesce, or Kirkendall pore formation due to disparate diffusion rates of nanoparticles.