This is a photo of iridescent goethite from Spain. The specimen features hemispherical (botryoidal) shapes with a colorful, iridescent surface. Some people apparently call these “unicorn poop.” This particular piece also has small, round, pink forms of some sort (?), that are a little easier to see on the detail view.
Approximate Photo Location (Side 1)
Field of view: ~1/4” x 3/8“ (6.2mm x 9.2mm)
Images in focus stack: 80
Goethite (named in honor of Johann Wolfgang von Goethe) is a common and widely distributed mineral (1, 2). It is often red-, black-, or yellow-brown in color; opaque; and found as masses that are hemispherical (like a cluster of grapes), kidney-shaped, or cylindrical (like stalactites)(1, 2). It can also form as long slender crystals, flat blades, or radiating crystals and be a pseudomorph of some minerals (i.e., it replaces the properties of the mineral while retaining the original shape)(2, 3). When scratched on a surface, goethite leaves a yellow/yellow-brown streak (1, 2). Goethite-rich yellow ochre pigments have been identified in Old Kingdom Egyptian paintings, Minoan and Mycenean art, and across the Roman Empire (4, 5). These pigments can be, and historically were, converted to red ochre by heating them above 250 degrees(C)(4, 5, 6).
Rainbow-like varieties of goethite (like the one in the photos above) have been found in Spain, Greece, and the US (7, 8). A scientific team recently compared iridescent samples from five localities around the world against non-iridescent samples. They attribute the color to light scattering from subsurface void layers (9).
1. Anthony, J.W., Bideaux, R. A., Bladh, K. W., & Nichols, M.C. (Eds.). (2001). Goethite. In The Handbook of Mineralogy. Retrieved from the Mineralogical Society of America.
2. The mineral goethite. (n.d.). Retrieved from Minerals.net.
3. Pseudomorphs (2016). Retrieved from Mindat.
4. Siddall, R. (2018). Mineral pigments in archaeology: Their analysis and the range of available materials. Minerals, 8(5). Retrieved from MDPI.
5. Faivre, D. & Frankel, R. B. (2016). Iron oxides: From nature to applications. Wiley. Retrieved from Google Books.
6. Tarlach, G. (2018). Prehistoric use of ochre can tell us about the evolution of humans’ cognitive development. Discover Magazine, April 2018. Retrieved from Discover.
7. Cook, R. B. (1978). Minerals of Georgia: Their properties and occurrences (Bulletin 92). State of Georgia, Department of Natural Resources. Retrieved from the State of Georgia’s Environmental Protection Division.
8. Fabre Minerals. (2019). Mineral specimens search results for [goethite]. Retrieved from Fabre Minerals.
9. Heaney, P. J., Post, J. E., Chen, S. A., Clark, T., Wenzel, T., Jacucci, G., & Vignolini, S. (2018, November). Painting a rainbow with ochre: Iridescence in botryoidal goethite. Paper presented at the Geological Society of America’s (GSA) Annual Meeting in Indianapolis, Indiana, USA. Summary retrieved from the GSA.
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