CHARACTERIZING SURFACE PROPERTIES OF NATURAL IRIDESCENT IRON (HYDR)OXIDE MINERALS

Iridescent minerals are visually spectacular. Naturally occurring hematite, iris quartz, opal, and labradorite are among the more sought after iridescent minerals because of their intense, and often broad, rainbow-like spectrum of colors. Iridescence in minerals can be caused by a multitude of mechanisms: diffraction grating interfering with visible light; thin-layer interference adhering to Bragg diffraction; coherence; absorption; scattering; multi-layer reflection. A recent study using SEM, AFM and XRD show that these phenomena explain iridescence in hematite and several other minerals (Lin, Xiayang, MSc Thesis. Penn State, 2015). Hematite iridescence, in this case, was attributed to the interference of light caused by an ordered microstructure of hematite nanorods on a freshly fractured mineral surface.

In this project, we are using SEM, XRD, EDSX, XPS and TGA to evaluate the origin of iridescence in the natural (unaltered) surfaces of two botryoidal turgite (goethite and hematite) samples. Unlike this previous study, which used fractured hematite samples, we are interested in understanding whether there are other types of surface features that cause iridescence. Bulk XRD patterns show that both samples consist of hematite and/or goethite. SEM imaging and chemical analysis by EDSX and XPS were performed on small pieces broken off the iridescent surfaces. Imaging showed no evidence of ordered microstructures reported previously for fractured hematite. Rather, we observed random aggregates of pseudo-spherical structures with dimensions of a few hundreds of nanometers. EDS chemical analysis of these spherical structures few trace elements however, the overall chemical compositions of these structures were consistent with that of goethite/hematite. We are using these observations to hypothesize that iridescence from botryoidal iron (hydr)oxide minerals is caused by light interfering with a surface coating consisting of disordered and randomly aggregated nanospheres. Future experiments will further characterize the size distributions of the nanospheres as well as attempt to correlate differences in iridescence color with variations in the size/distributions at the surfaces. This study hopes to identify a possible combination of multiple mechanisms in varying degrees of significance.