As I write this article on Thursday the 13th of March, 2025, my mind keeps wandering to the celestial event of the month—the Worm Moon, a Full Moon. The name, according to the Old Farmer’s Almanac, is due to warming soil and the appearance of worm casts or even the appearance of earth worms. But hold on, up here in the Northland the temperature may be a tad too cool for earthworms in mid-March. Does 22 degrees F sound like earthworm nirvana? So, a couple of more appropriate names for the March moon is Crow Moon since Poe’s favorite bird is very busy cawing and telling the country that Spring is on the way. That certainly seems the case in our plethora of trees around here. In northern Wisconsin the native Ojibway refer to the March moon as Snow Crust Moon. Sounds good as 40 degrees during the day tends to melt snow while 22 degrees at night freezes it over and a crust forms.
No earthworms on the 19th!!
But the big event is that this a Full Moon is also a Blood Moon, a total lunar eclipse—and it was a dandy. In this arrangement the moon, earth, and sun are lined up and the earth’s shadow begins to creep across the moon until the moon is completely covered, but is still visible. What happens is that the earth’s atmosphere scatters some sunlight across the lunar surface and at “totality” the moon appears a copper red or even a blood red. Really spooky but also a spectacular event.
Here in the Northland, sometime shortly after 11:00 CDT the earth’s shadow started to creep across the moon, slowing getting larger and larger until totality was reached about 1:30 AM. I observed the process until about 2:00 AM but totality lasted until about 2:30 AM when the earth’s shadow started to withdraw. The “neat” thing about the event is that we were in a short term, major warming event in Wisconsin, so I was able plop myself on the porch rocking chair with only a light jacket and my binocs and experience this celestial event with a clear sky and quietness. Wow, and double wow, since Thursday the 20th is the Spring Equinox.
The copper red moon rang a little bell in my head that reminded me of a Perky box containing the mercury mineral corderoite that needed examination. The red connection is not copper related but because many mercury minerals display some sort of a red color.
Although mercury has not been legally mined in the U.S. since 1992, at one time our country had a substantial number of operating mines. Most of these mines were in the far western US (see map) although in terms of production per mine the Terlingua fields in the Big Bend area of west Texas was substantial. I could not easily locate past production figures before 1992. In winding down the lack of mine production, much of of the production in the early 1990s was from catching mercury as a byproduct in gold mining operations. The U.S. also imported mercury and recovered the metal from recycling efforts. Today the use of mercury in the US has greatly decreased due to its toxicity, environmental concerns, and human health conditions. As noted on the map, most mercury mines were located in California, Nevada, and Oregon.
Mercury mines in the U.S. None are active today. Map courtesy of Land Matters, a non-profit 501c3 charitable educational organization found at www.mylandmatters.org.
One of the best-known mercury mining locations of later years was the Cordero--McDermitt Mines, Opalite District, Humbolt County, Nevada. The McDermitt (including the Cordero and other smaller mines) is just one of several mines that are located in the Opalite Mercury Mining District that straddles the Nevada-Oregon State Line, The District is associated with a volcanic caldera complex where eruption centered around a Miocene age of ~16 Ma (Henry and others, 2016). The original mercury mineralization was in the Cordero Rhyolite, but later hydrothermal action deposited the metal into nearby lake or stream-deposited tuff (volcanic ejecta) (Henry and others, 2016). Mercury in the caldera complex was first mined in the 1970s and the operations ceased in 1990. The McDermitt complex was the most important mercury producer in the Americas during the 20th century producing 279,000 flasks to 1988 (geoconsultancy.com.au). The minerals cinnabar, ~50%, (mercury sulfide HgS) and corderoite, ~50%, (mercury sulfide chloride Hg3S2Cl2) yielded almost all the mercury. However, there are several other mercury minerals present in minor amounts.
Corderoite is another one of those mercury minerals that appeared “later in life” as Eugene Ford and others did not described it until 1974 from the Codero Mine. Of course, the Mine was very new at that point, but the mineral had not been noted at other mercury localities. In addition to corderoite, the Cordero Mine (McDermitt complex) is the Type Locality of these other mercury minerals: alexearlite, kenhsuite, mikecoxite, and radtkeite.
When many rockhounds think of a “standard” color for mercury minerals the bright cherry red of cinnabar probably comes to mind, at least it does for me. Cinnabar was the mineral we always studied in mineralogy courses and little did I know, until decades later, that less abundant (that we did not observe in class) mercury minerals are various shades of pink, orange, silvery-gray, brownish red, yellow, and even colorless. Interestingly, many of these mercury minerals begin to darken, some irreversible, when exposed to light sources as they are photosensitive. In the well-studied photosensitive cinnabar, the first reaction to light, moisture and chloride ions is the change to corderoite:
3HgS + 2Cl à Hg3S2Cl2 + S
Corderoite is also unstable when exposed to light and oxygen and will degrade to calomel:
Hg3S2Cl2 + 2Cl à Hg + 2S + Hg2Cl2
And finally, calomel will degrade into mercuric chloride and metallic mercury (Keune and Boon, 2005; Radepont and others, 2011). These reactions probably seem rather insignificant to rockhounds except to note that most corderoite in the rock record is the result of the degradation of cinnabar. However, to art historians and art conservators the chemistry behind this darkening is extremely important. Vermilion, the red coloring made from cinnabar perhaps as far back as 8000 B.C., was the primary red pigment during the Renaissance Era until the 20th Century. So, conservators were greatly concerned about beautiful red paints used by the masters (and others) in their majestic works of art darkening with age. Keune and Boon (2005) determined that chlorine salts in the atmosphere were the major culprits in darkening during the degradation of cinnabar into elemental mercury. Museums are now able to restrict moist air and chlorine from reaching the paintings and also to chose specific light frequencies for the gallery illumination (Wogan, 2013).
Cherry red cinnabar degrading to pink corderoite. Matrix is opalite with white quartz and glassy hyolite opal. Bottom width FOV ~ 7mm. Top width FOV ~ 4 mm.
Corderoite is an isometric mineral although individual cubic crystals are quite small, less than 2 mm, and quite rare. Most occurrences of the mineral is as tiny grains, druse-like, on degrading cinnabar. It usually is pink to pink red to orange pink color when fresh but as it also degrades in light and moisture to a gray and finally black color. Meanwhile it remains tough to identify except using the pink color and its relationship to the degrading cinnabar. As noted above, the McDermitt complex has produced corderoite (degrading cinnabar) from both the rhyolitic complex rocks and the original tuffaceous lake sediments which today have consolidated to “opalite” with angular fragments of rhyolite and tuff along with secondary amorphous silica and others. In other words, it usually is not a nice looking rockhound mineral but certainly was a critical mineral in the mining and production of mercury.
Want to know more about mercury? I would suggest the USGS Circular 1248, Geologic Studies of Mercury by the U.S. Geological Survey.
https://clu-in.org/download/contaminantfocus/mercury/geologic-studies-of-mercury-c-1248.pdf
REFERENCES CITED
Foord, E.E., Berendsen, P., and Storey, L.O. 1974, Corderoite, first natural occurrence of α-Hg3S2Cl2, from the Cordero mercury deposit, Humboldt County, Nevada. American Mineralogist, vol,.59, nos. 7-8.
Gray, J.E., M.G Adams, J.C. Crock, and P.M. Theodorakos, 1999, Geochemical Data for Environmental Studies of Mercury Mines in Nevada: U. S. Geological Survey Open-File Report 99-576.
Henry, C.D., Castor, S.B., Starkel, W.A., Ellis, B.S., Wolff, J.A., Laravie, J.A., McIntosh, W.C., and Heizler, M.T., 2017, Geology and evolution of the McDermitt caldera, northern Nevada and southeastern Oregon, western USA: Geosphere, v. 13, no. 4.
Keune, K. and Boon, J.J., 2005. Analytical imaging studies clarifying the process of the darkening of vermilion in paintings. Analytical Chemistry, vol. 77. No. 15.
Noble, D.C., J.K. McCormack, E.H McKee, M.L. Silberman, and A.B. Wallace, A.B., 1988, Time of mineralization in the evolution of the McDermitt Caldera Complex, Nevada-Oregon, and the relation of Middle Miocene mineralization in the Northern Great Basin to coeval regional basaltic magmatic activity: Economic Geology, vol. 83.
Radepont, M., De Nolf, W., Janssens, K., Van Der Snickt, G., Coquinot, Y., Klaassen, L., and Cotte, M., 2011. The use of microscopic X-ray diffraction for the study of HgS and its degradation products corderoite (α-Hg3S2Cl2), kenhsuite (γ-Hg3S2Cl2) and calomel (Hg2Cl2) in historical paintings: Journal of Analytical Atomic Spectrometry vol. 26, no. 5.
Wogen, T., 2013, Mercury’s dark influence on art: Chemistry World at: https://www.chemistryworld.com/news/mercurys-dark-influence-on-art/6735.article.