Mineral Of The Quarter: Cuprite

Senior Geologist, Senior Geologist, Borehole Image Specialists, P. O. Box 221724, Denver CO 80222 | ron@bhigeo.com

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Cuprite, cuprous (Cu+) oxide (Cu2O), is a common mineral in the oxidized zone of copper-bearing ore deposits and is a minor, though important, source of copper. Cuprite belongs to the isometric (cubic) crystal system and is commonly seen as well-formed cubic, octahedral and dodecahedral crystals, often as intergrown crystalline masses. An acicular variety, chalcotrichite, appears as felted masses of tiny red fibers. Although it is too soft for jewelry, the display of bright red internal reflections – like a glowing ember – makes this mineral a favorite of collectors. Cuprite is the first substance recognized as a semiconductor and was, therefore, an important material in the evolution of modern electronics.

Cuprite was named in 1845 by Wilhelm Karl von Haidinger for its copper content. The word copper, of course, comes from the Roman name for the island of Cyprus, which was among the most significant sources of copper to the Mediterranean world (Chaline, 2012; Still, 2016). Cuprite has other monikers, including: hydrocuprite, octahedral copper, oxydulated copper, red glassy copper ore, ruby copper and ruberite, most of which have fallen out of favor (Mindat, 2020). Cuprite has been noted as a common corrosion by-product on smelted copper (Korzhavyi and Johansson, 2011). It is a bit of inorganic irony that cuprite has been found as a secondary mineral growing on ancient human-crafted copper and bronze artifacts (Dana, 1922, p. 411).

Cuprite has a specific gravity of 6.1 and a hardness of ~3.5 to 4. Cuprite, like fluorite, has cleavage along {111} yielding 4 cleavage planes. Unlike fluorite, however, cleavage in cuprite is poor and “interrupted” – so cleavage faces are not often observed (Mineral Data Publishing, 2005). Cuprite, though soft, is brittle, which is reflected in conchoidal to uneven fracture. Cuprite crystallizes in the hexoctahedral (4/m, bar3, 2/m) cubic crystal class. Euhedral crystals are a common occurrence and are often highly symmetric, displaying cubes, octahedrons and dodecahedrons, frequently » CONTINUED ON PAGE 30

Bright, distorted octahedral cuprite crystals displaying a submetallic luster. In spite of the metallic sheen, these crystals exhibit a distinctive red translucent color when illuminated from behind. Near the city of Rubtzovsk, Altai Krai region, Siberia, Russia. When 1st discovered, these crystals were found “floating” in pockets of red or white clay attended by beautiful dendritic copper crystals (Levitskiy, 2009). Used wth permission from John Betts Fine Minerals.

Lustrous, red-black octahedral cuprite crystals parked atop native copper and calcite rhombohedra. These cuprites are small, up to 2 mm across. From Wheal Jewel, Crofthandy, Cornwall, England. Used with permission from John Betts Fine Minerals, Inc.

»CONTINUED FROM PAGE 29 as combined forms (Klein, 2002). Crystal aggregates and penetration twins are also a frequent occurrence (Cook, 2001). Cuprite also occurs in a massive or an earthy habit as well as hairlike crystals (chalcotrichite). Cuprite is differentiated from similar minerals (like cinnabar or hematite) by higher symmetry as shown by crystal forms, a distinctive brownish-red, sparkly streak and differences in hardness (Klein, 2002). Cuprite is softer than hematite or sphalerite and harder than cinnabar, chalcopyrite or copper (Johnsen, 2002).

Cuprite exhibits only small variation from stoichiometric Cu2O: isomorphic substitution of other cations appears to be uncommon (Klein, 2002). The mineral is 88% copper, which gives it the highest Cu yield of any copper ore (Other than elemental copper, of course). Cuprite appears in a variety of red-colored shades that can mimic red-colored gems. The carmine red of some translucent cuprite specimens is known as “ruby-copper” (Bonewitz, 2013). The dark red of many cuprite samples can sometimes appear to be almost black. Cuprite appears to change color due to extended exposure to light, slowly darkening from red to dark gray (Bonewitz, 2005). Cuprite has an adamantine (brilliant) to sub-metallic luster.

Cuprite displays one remarkable property that makes it a favorite of mineral enthusiasts: a tendency to glow a bright crimson-red when subjected to high-intensity transmitted light. This astonishing characteristic is the result of internal reflection inside the crystal. I was gobsmacked when I first witnessed a metallic looking octahedral chunk of cuprite transmogrify into a flaming scarlet ember under a small, but powerful, flashlight beam. If you haven’t seen this behavior in cuprite, you should put it on your bucket list. I decided to illustrate this glow for my article, yet found myself without a cuprite specimen. Luckily, at the time I was preparing this article, the 2020 Denver Gem and Mineral Show was running. Conducting “fieldwork” at the show, (with requisite facial mask and social distancing due to COVID-19), I discovered that cuprite was hard to find. Luckily, my quest was fulfilled by Rocky Krichbaum, proprietor of Rocky Houndenstein, LLC, who was in possession of some stellar cuprites from the Milpillas Mine in Sonora, Mexico. Rocky » CONTINUED ON PAGE 31 Dark-red mass of octahedral cuprite crystals with chrysocolla and quartz. From the Katanga (Shaba) Province, Democratic Republic of the Congo. 28 mm across. Used wth permission from John Betts Fine Minerals.

Large malachite pseudomorph after cuprite with matrix inclusions. Chessy-les-Mines, Rhone-Alpes (NW of Lyons), France. Used with permission from John Betts Fine Minerals.

»CONTINUED FROM PAGE 30 showed me several excellent cuprites (two of which I now possess). He graciously allowed me to photograph one eye-popping specimen and then he called forth the “scarlet fire” from the same sample with a bright flashlight. (The last 2 photographs in this article are from that encounter). Although I’ve not seen a detailed explanation for the intensity of the internal reflections, I’ll wager that this optical feature relates to the very high refractive index of cuprite (2.849), which is considerably higher than that of diamond (2.42) (Nesse, 2002).

Cuprite has a very distinctive appearance in thin section, characterized by a very high positive relief and red, orange-yellow or lemon-yellow color (Mineral Data Publishing, 2005). In reflected light, cuprite appears gray-blue with deep-red internal reflections that are diagnostic (Nesse, 2002). Although cuprite is isometric, it nevertheless is well-known to exhibit an unusual optical anisotropy with pale bluish-gray » CONTINUED ON PAGE 32

Malachite pseudomorph of an octahedral cuprite crystal from Chessy-les-Mines, Rhone-Alpes, France. Photo by Albert S. Wylie.

Felted mass of finely acicular chalcotrichite whiskers. From the Ray Mine, Mineral Creek District, Pinal County, Arizona. This locality was the source of the natural material studied by Veblen and Post (1983). Photo used with permission from John Betts Fine Minerals.

A large dark red cuprite crystal coated by a rind of fibrous green malachite microcrystals. From the Onganja Mine, Seeis, Khomas, Namibia. Used with permission from John Betts Fine Minerals.

An aggregate mass of cuprite crystals from the Milpillas Mine, Cuitaca, Santa Cruz, Sonora, Mexico. This specimen was for sale by Rocky Houndenstein, LLC at the 2020 Denver Gem and Mineral Show. Proprietor Rocky Krichbaum demonstrated the internal reflection-driven “scarlet fire” effect using this sample (next picture). Photo by Ronald L. Parker.

»CONTINUED FROM PAGE 31 to olive-green polarization colors. Libowitzky (1994) demonstrated that the apparent optical anisotropy is an artifact of thin-section preparation – specifically, mechanical alteration from surface polishing with diamond abrasives. A change in polishing materials and procedures was shown to retain isometric optical behavior.

Chalcotrichite is the fibrous variety of cuprite. It is composed of delicate, single-crystal hairs or whiskers of cuprite that grow with extreme aspect ratios (of 1000:1 or more) in reticulated, tufted or matted aggregates (Mineral Data Publishing, 2005). To explore fibrous cuprite, Veblen and Post (1983), studied synthetic chalcotrichite whiskers and natural specimens from the Ray Mine, Pinal County, Arizona, using transmission electron microscopy (TEM). (Note that the chalcotrichite photo in my article is from the Ray Mine). Veblen and Post identified that the most common fiber morphology (one with a with a square cross-section) is the result of propagation of a single screw-dislocation along one of the a-crystallographic axes.

Cuprite is formed by the interaction of near-surface, oxygen-bearing ground water with reduced, sulfidic, copper-bearing igneous intrusives. Supergene processes oxidize the primary sulfidic copper ores to create the suite of secondary minerals that are so beloved of mineral collectors, the oxides and carbonates (Cook, 2001). Cuprite is associated with many other minerals common to copper-ore deposits including native copper, malachite, azurite, chrysocolla, calcite, bronchianite, antlerite, atacamite, tenorite and iron oxides and clay minerals (Mineral Data Publishing, 2005).

Although cuprite is prized, it is not fit for jewelry. As Bonewitz (2013) observes, “Faceted stones are too soft to wear, but their exceptional brilliance and garnet-red color make them highly desirable as collector’s stones.” (p. 58). One element of this appeal is that large cuprite crystals – of a size that engenders faceting - are vanishingly rare. Most faceted cuprite crystals were collected from the unique discovery of large, gemmy » CONTINUED ON PAGE 33

»CONTINUED FROM PAGE 32 cuprite crystals from Onganja, Seeis, Namibia, which has reported crystals up to 6 inches across and up to 2 kg in mass (Arem, 1977). This amazing discovery has since been mined out. Joel Arem, former curator of the Smithsonian National Gem and Mineral Collection, elaborates, “…faceted cuprite… is considered one of the most collectible and spectacular gems in existence, with its deep garnet coloring and higher brilliance than a diamond. Only the gem’s soft nature prevents it from being among the most valuable jewelry stones.” (Wikipedia, 2020a).

Cuprite is a special mineral with special properties. Synthetic cuprous oxide is the earliest material demonstrated to have semiconductor properties. Rectifier diodes based on cuprous oxide have been in use since the 1920s. Cuprite has a foundational place in the history of the modern semiconductor industry. Cuprous oxide has been extensively investigated as a potential low-cost material for solar cells (Musa, 1998) and is now being studied as a photocatalyst for producing hydrogen from water. “New applications of Cu2O in nanotechnology, spintronics and photovoltaics are emerging” (Korzhavyi and Johansson, 2011, p.4). In addition to these high-tech applications, cuprous oxide is used industrially as a pigment, as a fungicide and as an anti-fouling agent in marine paints (Wikipedia, 2020b).

Important localities for cuprite include: Tsumeb and Emke regions of Namibia; the Ural Mountains and the Altai Krai region of Russia; Chessy, France; Cornwall, England; Sonora, Mexico, and Broken Hill, NSW, Australia. In the United States, cuprite occurrences are significant in Santa Rita, New Mexico and the Bisbee, Clifton, Morenci and Ray mines in Arizona.

Cuprite! A magical burning ember of a mineral.

• https://www.minerals.net/mineral/cuprite.aspx • https://en.wikipedia.org/wiki/Cuprite • https://en.wikipedia.org/wiki/Copper(I)_oxide • https://www.mindat.org/min-1172.html » CONTINUED ON PAGE 34

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»CONTINUED FROM PAGE 33 • http://www.handbookofmineralogy.org/pdfs/cuprite.pdf • http://webmineral.com/data/Cuprite.shtml#.X2GeNmhKiHs • http://copperoxides.altervista. org/ • https://meanings.crystalsandjewelry.com/cuprite/ • https://www.youtube.com/ watch?v=DVJzk6-J3MY (Excellent video describing space group symmetry of cuprite) • https://www.facebook.com/ collectorsedgeminerals/videos/1786845958289807 (Movie clip displaying internal reflection from a backlit cuprite specimen).

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Nature Guide: Gems, New York,

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Limited, 224 pp. Chaline, Eric, 2012, Copper, in Fifty

Minerals that Changed the Course of History, Buffalo, New York:

Firefly Books, Inc., pp. 12-15. Cook, Robert B., 2001, Cuprite:

Mashamba West Mine, Shaba,

Democratic Republic of Congo, and Red Dome Mine, Queensland,

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