OLD IS GOOD! HELP FOR FOSSIl IDENTIICATION

 

My geology/mineral/rockhound Blog (my only bow to social media) often receives a variety of questions from readers. I might write a post on a particular mineral but receive  questions like: Can you help me? I found this rock in my backyard, and it looks like a fossil. Do you know what it is? I like to collect fossils in the rocks around my farm. How may I learn to identify them? The questions usually require several more exchanges of information before I can give a best judgement call, but readers do get an answer.

Now, depending upon their interests and perhaps future collecting plans, I suggest publications that could be acquired and that would help with their identifications. Some of my suggestions are free on the Internet, most are available from used book dealers, and most are found in university libraries. As a last resort I will also scan some illustrations and send them via email if they request further help. Depending upon their collecting habits I might supply some information on collecting rules and regulations. No two questions, or answers, are the same but mostly all are enjoyable.

There are a ton of paleontological articles on the Internet. However, many are highly technical, and many are difficult to locate. Often a technical article only describes a particular taxon and that is not very useful to a kid trying to identify the clam she found in the roadcut.

I am an “old-fossil” type of guy so that qualifies me to talk about the “old days”. Well, in the old days, say 1900s in the U.S., research universities, the U.S. Geological Survey, and many state geological surveys, supported some “really smart” invertebrate paleontologists who published textbooks and research publications with hundreds/thousands of photographs or line drawings of fossils. Sometime in the late 1900s it became monetarily prohibitive to publish an article with 200 photographs of fossils. But readers should sometime visit the archives, or stacks, of university libraries and peruse the older USGS Professional Papers, the vast resources of the state geological surveys, and even publications of the state academies of science. In fact, one could enjoy large swaths of winter days bundled away in the library, forgetting the problems of the world,  and enjoying the company of Edward Drinker Cope.

As my article title indicated, Old is Good, and so my recommendations for identifying fossils, especially from states in my area of the country, are publications from the 1900s. All are packed with photographs or line drawings. Now there is a caveat to using older publications for fossil identifications—the fossil in question may have been switched from one taxon to another or changed its identification. But, one can come very close to the identification and then run that term through an Internet search.

I will focus on invertebrate fossils since these are of the most interest to rockhounds. And of course, my recommendations are simply my choices.

Index Fossils of North America

The best single book to acquire and have in your paleontological library is Index Fossils of North America, Hervey Shimer and Robert Shrock, 1944, John Wiley and Sons, Inc. (there may be later reprint editions). This tome is a revision of Grabau and Shimer’s 1915 classic North American Index Fossils. Known affectionally as the “Blue Book”, this amazing work describes and figures over 7,500 species with over 9,400 illustrations on 303 full page plates. It contains selected bibliographies for all the larger divisions and considers geographic as well as geologic distribution on 837 pages!

A page from Index Fossils (poorly copied).

Used copies are available from Internet used book dealers for somewhere around $75-$100 and is well worth the money if you are interested in collecting and identifying invertebrate fossils. If uninterested in shelling out a Benjamin, I have located a site to download a FREE copy. Use your browser and search for Internet Archive Index Fossils of North America. Sign up for free and download—an amazing gift.

Invertebrate Paleontology; More and others.

An older university textbook loved by all (well mostly loved) is entitled Invertebrate Fossils, 1952. by R.C. Moore, Lalicker, and Fisher and published by McGraw Hill. Moore, from the University of Kansas, was one of the best-known invertebrate paleontologists in the middle decades of the 1900s (check out his bio on Wikipedia). This was the standard “go-to” paleo textbook for college students for a quarter of a century as it is profusely illustrated with line drawings.

A page of illustrated fossils from Invertebrate Paleontology

Even today, fossil hounds look for, and are able to purchase, used copies for $30+. But again, FREE copies are available for download on Internet Archive (note instructions above).  

Kansas Geological Survey Bulletin 14.

Living in Kansas for several decades, I was always interested in collecting fossils from marine rocks of Pennsylvanian, Mississippian (Carboniferous) and Permian ages. These fossiliferous rocks are abundantly exposed in eastern Nebraska and Kansas over into Missouri, Arkansas, and Oklahoma. I fact, Carboniferous and Permian rocks crop out in many states but seem most fossiliferous in the middle of the nation. With that information I wish to recommend a paper by R.C. Moore    published in Bulletin 169 of the Kansas Geological Survey: Palaeoecological aspects of Kansas Pennsylvanian and Permian cyclothems, pp. 287-380. The paper has fantastic line drawings that will help you identify virtually all Pennsylvanian and Permian marine fossils you will ever collect in the central U.S. Many of these illustrated fossils may also be located in other states. The best part is that the Survey will allow you to download papers from Bulletin 169 for FREE. You can’t beat that. Head to:  KGS--Bulletin 169--Symposium on cyclic sedimentation

  Page of illustrated Pennsylvanian and Permian gastropods from Bulletin 169.

And finally, fossils of Cretaceous age are found in rocks cropping out in all central U.S states. Many/most of the marine rocks are associated with deposition in the Western Interior Seaway; therefore, the fossils are very similar in all outcrops. For example, although species of the Inoceramid bivalves may vary from Texas to Utah they are easily identified as the clam and can be keyed out.

Professional Papers of the USGS are great resources for determining and identifying fossils in specific stratigraphic units. However, it will take some internet sleuthing. My go-to book for identifying virtually every Cretaceous fossil that I have observed is Vol. 14, Nos. 3 & 4 of The Mountain Geologist, a peer reviewed publication of the Rocky Mountain Association of Geologists. Edited by Earle Kauffman, this 1977 classic served as a field trip guidebook for the North American Paleontology Convention II. Not only are Cretaceous fossils well illustrated with photographs, but field trip stops at various locations from Salt Lake City to central Kansas are described.

The Mountain Geologist, Vol. 14, Nos. 3-4.

A page of fossil illustrations from the Mountain Geologist.

Unfortunately, this publication is almost impossible to purchase. It periodically shows up on internet used book sites, but those buying chances seem rare. But there seems to be a solution from RMAG.org, move to publications, move to The Mountain Geologist, move to AAPG Datapages, verify you are human, fill in form: Full Text Bulletin 14, Year 1977 to 1977, Select 1977. You may then order any of the individual road logs or if interested in just the fossils, place an order for the Illustrated Guide…..at $24. Universities in the Mountain West region also may subscribe to RMAG publications that are available for examination in the library.

And finally for the hard-core fossil hounds, there is the Treatise on Invertebrate Paleontology, described by the University of Kansas, the publisher, as a definitive reference work that encompasses approximately 55 volumes published from 1953 to the present. It is authored by over 300 paleontologists and provides extensive information on every phylum, class, order, family, and genus of both fossil and extant invertebrate animals. The treatise includes detailed descriptions of prehistoric invertebrates, covering aspects such as taxonomy, morphology, paleoecology, and stratigraphic ranges.

An early Treatise volume.

The Treatise is available for open access, allowing users to download entire volumes or individual chapters in PDF format. This accessibility is facilitated through the University of Kansas website, where users can search for specific topics or browse through the available volumes. The content is provided under a Creative Commons Attribution 4.0 (CC BY) license, promoting its use in research and education.

In summary, I have tried to provide references for interested fossil hounds to help with identifications of collected specimens. The references noted provide line drawings or photographs that will help collectors of all ages or experiences. Good luck and if you run into problems just shoot me an email. 

IRON ORE. BOUNDARY WATERS, AND BIG FITZ

 How many roads must a person walk down

before you call them a rockhound?  Apologies to Bob Dylan

At one time in my geological past, I organized and led student field trips to the Boundary Waters Canoe Area in Northern Minnesota. Driving north from Hays, Kansas, our group observed and studied various rock units that were unavailable to see in the sedimentary section of western Kansas. For example, we took a good look (and camped) at exposures of the Proterozoic Precambrian Sioux Quartzite (~1.6-1.7 Ga) cropping out around Sioux Falls, South Dakota, and adjacent western Minnesota. These outcrops were a good eye opener for the kiddies since their previous observation of the quartzite was in a pasture in eastern Kansas where I had previously stopped during a paleontology field trip. At this cow pasture locality, the Sioux was observed as small cobbles and boulders, the result of riding down glacial ice during the Pleistocene. The glacier(s) plucked the distinctive pink quartzite from the South Dakota/ Minnesota outcrops and dropped them off in a terminal moraine in eastern Kansas. These two areas presented a great message to help students understand the power of  glaciers.

Sioux Quartzite exposed at the “falls” in the city of Sioux Falls.  Photo courtesy of Steve Dutch, University of Wisconsin.

   

Sources of identifiable glacial erratics found in northeastern Kansas. The dashed line represents the extent of Pleistocene glacier(s) in Kansas.  Map courtesy of Kansas Geological Survey.
 

Boulders and cobbles of Sioux Quartzite in terminal moraine near Wamego, Kansas. Photo courtesy of Kansas Geological Survey.

                 

 Protohistoric Catlinite pipe, probably late 17th century Ioway, from the Wanampito site in Iowa. Public Domain photo from Whittaker and Anderson, 2008, Wanampito: An Early Ioway Site: Newsletter of the Iowa Archeological Society 58(1):4-5.

Near Sioux Falls in southwestern Minnesota is Pipestone National Monument, a national treasure often overlooked by travelers zooming along on nearby I-90. At the Monument the Sioux is also exposed but contains significate layers of catlinite, AKA pipestone. Catlinite is not a formally recognized mineral, although it often appears as such in popular culture, but is a sedimentary rock named argillite. The rock at Pipestone represents tightly indurated “mudstone/claystone” that was formed between layers of the quartzite and subjected to deep burial where heat and compression lithified the clay into an argillite. At Pipestone there are specific minerals present in the argillite that give the rock a diagnostic chemical signature: kaolinite, muscovite, diaspore, hematite, and pyrophyllite. Although the sand quartzite is extremely hard at ~ 7.5 (Mohs), the catlinite is very soft at ~2.5. That softness then allows the argillite to be cut and carved into Native American pipes. The staff at Pipestone believe that “for over 3,000 years, Indigenous people have quarried the red stone at this site to make pipes used in prayer and ceremony - a tradition that continues to this day and makes this site sacred to many people.”

The town of Pipestone also offers an opportunity to observe numerous buildings constructed in the late 1800s and early 1900s of the pink quartzite. The architecture of these buildings is fantastic and well worth a walking tour. The Museum at Pipestone noted that quarried rock was used in building construction in Minneapolis, Sioux Falls, Detroit, Sioux City, Chicago, Kansas City, Omaha and other locations.

The Pipestone City Hall was constructed in 1896. Today it is home to the Pipestone Historical Society who furnished the photo.

North of the Twin Cities the group was able to observe, for the first time, the Mesoproterozoic Precambrian (~1.1 Ga) rocks of the Midcontinent Rift Zone (MRZ), the southern arm of a Triple Junction spreading center near Lake Superior. The MRZ is best known for the large-scale native copper deposits in the Keweenaw Peninsula of Michigan.  

The Midcontinent Rift System or Zone cuts across the North American Craton, the stable center of the continent, that begin to split apart (think East African Rift Zone) starting ~1.1 Ga. Around 20 Ma the rifting stopped and started to close, hence the geological term "failed rift."  The location of the rift south of Lake Superior in Minnesota (south of the Twin Cities), Michigan, Wisconsin, Iowa, Nebraska, and Kansas is inferred from gravity and magnetic data. Public Domain map and info from USGS.

Basalt flows associated with the MFZ exposed along the St. Croix River at Interstate State Park, near metro St. Paul.

Hull-Rust-Mahoning Open Pit Mine near Hibbing is a National Historic Landmark. Photo is Public Domain courtesy of  McGhiever.
 

Further north in Minnesota we took a gander at the giant Mahoning-Hull-Rust Iron Mine near Hibbing (home of Bob Dylan born Robert Zimmerman).  The Mine is located in  the Mesabi Range, one of four iron ranges in Minnesota, the others being the Cuyuna, Vermillion, and Gunflint. The term range refers to a linear feature rather than a topographic high. Rocks of the ranges are Proterozoic Precambrian in age and are the result of the erosion of older Precambrian rocks that were a part of the geologically complex Churchill Craton. Marine waters occupied a large, passive, continental margin of the Churchill  known as the Animikie Basin that accepted (2.5-1.8 Ga) erosional debris consisting of large amounts of silica (quartz etc.) and iron-rich minerals and happened to coincide with a massive change in the composition of the atmosphere known as the Great Oxidation Event (GOE).  The earth’s early atmosphere was a reducing atmosphere with little oxygen and consisting mostly of nitrogen and carbon dioxide that were probably derived from volcanic events. Somewhere in the Proterozoic, questionably as early as 3.5 Ga, photosynthetic cyanobacteria, with chlorophyl, begin replacing the anoxygenic life forms of the reducing environment. The GOE refers to the massive oxygenic time around 2.4 Ga when the chlorophyll-based photosynthesis of cyanobacteria released oxygen as a byproduct. At this same time massive amounts of silica and ferrous iron (Fe2++)was being transported into the Animikie Basin and hitting the oxygenated oceanic waters that oxidized the iron into insoluble ferric (Fe3+++) iron that combined with the silica to form the famous banded iron formations (BIF). Around 1.88 Ga supracrustal BIF rocks of the Animikie were thrust northward to their current localities in the Iron Ranges.The BIF were the sources of the ores fueling the massive iron mining industry. In the Mesabi Range the BIF were close to the surface and hence the concentration of large open pit mines.       

 

The Iron Ranges of Lake Superior. Photo Public Domain courtesy of W.F. Cannon (USGS).

As the high-grade BIF iron ores became depleted in the mid-20^th century mining engineers developed the “taconite process” whereby  low grade ore (termed taconite)   was crushed to a fine grain and the magnetite was removed by magnets, mixed with a bonding agent, usually bentonite, and then wetted, rolled and concentrated into marble size “balls” which were then hardened by subjection to high heat. At that point the taconite contains about 70% iron and heads to one of four taconite shipping ports in Minnesota (Duluth-Superior, Taconite Harbor, Two Harbors, Silver Bay) where it is shipped to the steel mills of Indiana and Ohio. Currently there are only about a half dozen iron ore mines operating in Minnesota; all are in the Mesabi Range. The taconite process is fascinating to observe, and I was pleased to get the class into a personal tour.

The most famous taconite freighter on the Lake was christened in 1958 as the Edmund Fitzgerald and was a monster: 729 feet long and weighing ~13,600 tons. On November 9, 1975, the Big Fitz, with a full load of taconite pellets and a crew of 29, left the Port of Superior, Wisconsin, headed to the steel mills near Detroit, Michigan. Unfortunately, the Big Fitz steamed into a stormy Lake Superior and on November 10 broke apart and capsized in winds at least 90 mph and wave swells of 25-30 feet. All aboard perished and today the ship and crew rest in ~530 feet of water not far from Whitefish Bay. The 1976 disaster was immortalized by Gordon Lightfoot’s recording of the immensely popular folk ballad, The Wreck of the Edmund Fitzgerald.

Upon leaving the iron ranges the class started experiencing a high level of excitement as we headed to Ely, home of the canoe outfitter. My mind begins to fade a little here and unfortunately my maps are still buried “somewhere” after my move. I do remember that we put in at Lake One heading counterclockwise, canoed to the Canadian border near Ensign Lake, canoed down the big Moose Lake, and finished after six days of paddling. Little did the kiddies, on that first trip, know that I had never been in the Boundary Waters previously and was sort of flying by the seat of my pants, my Brunton compass, and my maps. We did miss a few portages the first time, managed to escape most bears, brewed morning coffee that tasted fantastic, and had the time of our lives. Forty years later I periodically run into a student who canoed with me and has stories for their grandchildren. And the kiddies learned much about igneous and metamorphic rocks, how faults may define some lakes, and how other lakes are the result of Pleistocene glaciers scooping out large, and small, basins.

Boundary Waters: quiet and loud.

Now for the mineral, I have a specimen from the Thunderbird Mine in the Mesabi Range near the mining town of Eveleth. Today the property is owned by Cleveland-Cliffs Inc. and is named the United Taconite Mine and includes the former Thunderbird Mine North (TBN) and the Thunderbird Mine South (TBS), collectively the Thunderbird Mine of earlier literature. The Thunderbird is one of the few large open pits still operating in Minnesota. According to Cleveland Cliffs, magnetite-bearing taconite is currently the principal iron-bearing rock of economic interest on the property. In line with other Superior-type iron formations, magnetite-bearing intervals within the Biwabik Iron Formation occur as laterally extensive, stratiform intervals. Economically mineable magnetite occurs exclusively within granular iron-formation (cherty) units of the Biwabik. The ore is sent approximately 10 miles by rail to the concentrator at the Fairlane processing facility in Forbes, Minnesota, to produce a magnetite concentrate, which is then delivered to the on-site pellet plant. From the plant site, pellets are transported by rail to a ship loading port at Duluth, Minnesota.

The specimen I have for this posting is minnesotaite, an iron silicate and really not much to look at. I spent a very few bucks for the specimen due to the facts that: 1) the mineral was named for the State of Minnesota and that is not a common naming practice; and 2) J.W. Gruner (1944), the naming author, first described it as an “iron talc” Fe²⁺₃Si₄O₁₀(OH)_2. That tidbit intrigued me so I took it home and then noted that talc [Mg₃Si₄O₁₀(OH)₂] is a hydrated magnesium silicate while minnesotaite is a hydrated iron silicate and therefore they are isostructural with each other. As best I can tell, there is no substitution between minnesotaite and talc due to the differences in the ionic radii in the two cations (I "think" but don’t always trust my thinking).

Steel gray masses of amorphous Minnesotaite in matrix

Top width FOV ~9.0 mm; bottom FOV ~ 3.0 mm.

Minnesotaite is tough to physically describe since it often is a fine grained, greenish-gray (may appear almost black), massive “blob”. It really does not have much of a luster or shine although MinDat calls it resinous, waxy, or greasy. Some specimens evidently have tiny radiating crystals or platy forms. I might have expected an iron mineral to be quite hard; however, minnesotaite is very soft at maybe ~1.5 (Mohs), about the same as it’s isostructural relative, talc. It is always associated with the Banded Iron Formations.

In retrospect, canoeing the Boundary Waters was much more interesting than examining the iron minerals of northern Minnesota. My memories are quite valuable to me as they are lifelong mementos, something like treasured souvenirs of an interesting journey.

REFERENCE CITED

Gruner, John W., 1944, The composition and structure of minnesotaite, a common iron silicate in iron formations: American Mineralogist, vol. 29, nos. 9-10. Pgs. 363-372. 

RIP Big Fitz

In a musty old hall in Detroit they prayed
In the Maritime Sailors' Cathedral
The church bell chimed 'til it rang twenty-nine times
For each man on the Edmund Fitzgerald

 

The legend lives on from the Chippewa on down
Of the big lake they call, 'Gitche Gumee'
Superior, they said, never gives up her dead
When the gales of November come early

`                               Gordon Lightfoot.

MINERAL ABBREVIATIONS (SYMBOLS)

 

I presume that most rockhounds are serious users of Mineral Database and have noted that descriptions of minerals now include Mineral Symbols. MinDat states, “as of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams. Please only use the official IMA–CNMNC symbol.” For example, pyrite (FeS2) has a symbol of Py while the potassium feldspar microcline (K(AlSi3O8) has a symbol of Mcc. Warr (2021), in defining the naming process stated, “this contribution presents the first International Mineralogical Association (IMA) Commission on New Minerals, Nomenclature and Classification (CNMNC) approved collection of 5744 mineral name abbreviations…{that standardize} abbreviations by employing a system compatible with that used for symbolising the chemical elements.” The Commission also continues to approve symbols for new minerals as they are officially described. For example, in 2023 112 new minerals were approved. The American Mineralogist, on a regular basis, publishes articles listing, and briefly describing a few, new minerals appearing in the published record and approved by IMA. The latest listing that I could locate was a December 01, 2024 issue [American Mineralogist (2024) 109 (12): 2173–2175.] stating, “This issue of New Mineral Names provides a summary of the newly described minerals from May to August 2024, including karlseifertite, vegrandisite, touretite, auropolybasite, cuprozheshengite, calcioveatchite, and jianmuite.”  A total of 29 new minerals were approved by the IMA-CNMNC from May to August 2024.

The American Mineralogist is the flagship subscription journal of the Mineralogical Society of America; however, the issues containing New Mineral Names, is a free issue. Use your browser to search for New Mineral Names to find the appropriate articles. 

As of mid-July 2024, there are over 6000 valid minerals listed by the IMA! How are ole plugger rockhounds like me supposed to make sense of these thousands of minerals plus the new yearly additions? I have six suggestions/comments: 1) I don’t know the exact number but there are less than one hundred common rock forming minerals and learning about these is not an onerous proposition; 2) make MinDat your favorite web site; 3) most rockhounds will never have the opportunity to observe newly discovered  minerals. Almost all new minerals need electronic gizmos located in research laboratories to validate their existence and many are organic or post-mining minerals; 4) consider subscribing to professional journals, of which there are many. I certainly cannot afford several subscriptions but have found University libraries to be a fantastic reference source, and some public libraries subscribe to popular journals such as Rock and Minerals, Rocks and Gems, and Mineralogical Record; and 6) if your local rock club has a mineral study group, sign up and attend the meetings.

Finally, if you are interested in seeing the official 2021 mineral list with their symbols just scan this QR Code:

 

WORMS, A BLOOD MOON, AND MERCURY

 

As I write this article on Thursday the 13^th 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 20^th 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 was mined in Nevada from about 1907 (discovered then at Antelope Springs and with mining beginning in 1914) until the early 1990’s. The District mines produced from veins in Triassic limestone, dolomite, conglomerate, and shale (Gray and others, 1969). Evidently these veins were emplaced during the Miocene because of extensional magmatism (Noble and others, 1988).  That is, Miocene extensional tectonics involved the stretching of the earth’s crust producing what we know today as the Basin and Range physiographic province.

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 20^th 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 20^th 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.

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