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). Goethite is brittle and has a hardness of ~700 on the Vicker’s scale (where graphite is ~10, copper ~80, and pyrite ~1,500)(1). When scratched on a surface, it leaves a yellow/yellow-brown streak (1, 2). Goethite-rich yellow ochre pigments have been found on rock art in Argentina, dating back 9,000 years (4). They have also been identified in Old Kingdom Egyptian paintings, Minoan and Mycenean art, and across the Roman Empire (4). These pigments can be, and historically were, converted to red ochre by heating them above 250 degrees(C)(4, 5, 6)(i). The use of hematite-rich red ochre by humans stretches as far back as 300,000 years (4, 6).
As shown in the photos above, goethite is also found in more colorful forms. This specimen has a banded, metallic-like surface that looks iridescent (though the color doesn’t change with the viewing angle). Localities where these have been found include Spain, Greece, and the US (7, 8, 9). A scientific team recently studied the source of the iridescence and found that rod-shaped goethite nanocrystals were attached on the surface and stacked in fan-like shapes (10). The team also compared iridescent samples from five locations around the world against non-iridescent samples (10). They concluded that the iridescence was not caused by the nanocrystals, but rather the scattering of light from subsurface void layers that were only found in the iridescent samples (10).
i. In general, it is difficult to distinguish burnt ochers from non-burnt ochers (4, 5). Siddall reports that high purity goethite can be transformed into red-colored hematite in 45 minutes in a wood fire (4). When temperatures exceed ~450 degrees(C), the ochre will start to change into magnetite and turn black (11). Evidence of controlled and deliberate ocher burning has been found at the Les Maitreaux site in France (12). This site was a flint and pigment production workshop about 20,000 years ago, attributed to the Solutrean culture (12). Evidence of ochre burning has also been found at the Qafzeh Cave in Israel, indicating that heat treatment may have been used as far back as 100,000 years (11).
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. These are the handbook records for graphite, copper, and pyrite (Vicker’s hardness is abbreviated VHN). The full index is located here.
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)[Goethite]. 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. QueBul Minerals. (2019). Mineral specimens search results for [goethite]. Retrieved from QueBul Minerals.
10. 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.
11. Godfrey-Smith, D., & Shimon, I. (2004). Past thermal history of goethite and hematite fragments from Qafzeh Cave deduced from thermal activation characteristics of the 110°C TL peak of enclosed quartz grains. Revue d’Archéométrie, 28, pp. 185-190. Retrieved from ResearchGate.
12. Salomon, H., Vignaud, C., Lahlil, S., & Menguy, N. (2015). Solutrean and Magdalenian ferruginous rocks heat-treatment: Accidental and/or deliberate action? Journal of Archaeological Science, 55, pp. 100-112. Retrieved from the University of Liège Library Network.
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