HENDERSON CREEDITE,ANTERO MOLY, CHONDRODITE, AND WOODSTOCK

It has been one of those weeks where the hot weather has sort of fizzled my brain and sent me into memory lane.  Of course, 94 degrees with 10% humidity (Colorado) is not really “hot” compared to my former home in Kansas where my friends were suffering in 107 degrees.  My heat-addled mind often “remembers” in decades and so it was this time, 50 years ago—the summer of 69.  This time period was easy to remember since the country was in an uproar over events in southeast Asia, a great geology scenery movie starring Paul Newman as Butch Cassidy and introducing Robert Redford as the Sundance Kid was released, the U.S. put a “man on the moon” in late July, Charles Manson and his groupies went on a killing rampage in early August, and 400,000+ mostly young people converged on a farm in upstate New York to attend a music festival—Woodstock.  Lately I have been watching “documentaries” on the tube about Woodstock and it brought back many memories.  I liked much of the music (not a fan of Jimi Hendrix tho) but as a small-town kid from conservative Kansas I simply could not really identify with the drugs and seemingly open sex for everyone--was the latter really true?.  What I could identify with was the fear (and reality) of so many young men waiting for the county draft board to ship them off on a bus for a make-believe physical exam, and then off they went to southeast Asia.  If a male college student flunked just one class, it was, as Buffalo Springfield sang:   Paranoia strikes deep / Into your life it will creep, It starts when you're always afraid / Step out of line, the men come and take you away.

I lost friends in the conflict and the mind, intellect, reasoning and judgment of an entire generation was affected and remembered; our country has never healed. 

I’m fed up to the ears with old men dreaming up wars for young men to die in.     George S. McGovern

For the “hippie” pilgrims (the term of the press) of the day, Woodstock was a break in the reality of the war and a time where one, for three days, could just forget and kick back.  Let their mind rest. After the concert was over it was back to the reality of draft boards, physical exams and long plane rides:

Well, come on all of you, big strong men,

Uncle Sam needs your help again.

He's got himself in a terrible jam

Way down yonder in Vietnam

So put down your books and pick up a gun,

We're gonna have a whole lotta fun.

And it's one, two, three,

What are we fighting for ?

Don't ask me, I don't give a damn,

Next stop is Vietnam;

And it's five, six, seven,

Open up the pearly gates,

Well there ain't no time to wonder why,

Whoopee! we're all gonna die.

                                                         Country Joe and the Fish

But again, I was just the small-town kid, married for two years and working hard on a graduate degree at the University of Utah.  I had neither the time nor the money nor the inclination to tromp off to New York.  In fact, I just looked at my field book/notes and noticed that August 15-18 I was tromping around in rural southwestern Wyoming mapping some rock units and collecting vertebrate fossils.  I had been walking and searching all summer and had been concerned that a lack of fossils would not allow me to complete a dissertation and then I was in trouble, big trouble.  However, in mid-August the stars were aligned correctly, and I stumbled across an opening in the sagebrush north of Evanston and saw bone chips and part of a turtle scute. My heart fluttered as I sank to my knees and stuck my face to within six inches of the ground and started seeing small mammal teeth and bones.  I knew at that moment “things” were going to be OK and I could handle the rest of my graduate academics.  Maybe, as a popular 5^th Dimension 1969 song belted out, we were in the Age of Aquarius and Jupiter-Mars aligned with my fossil find!

When the moon is in the Seventh House
And Jupiter aligns with Mars
Then peace will guide the planets
And love will steer the stars
This is the dawning of the age of Aquarius
Age of Aquarius
Aquarius
Aquarius

So, the summer of 69 was an important time in my life.  Wow, 50 years ago.  The summer of 2019 (with some poor music) has not been as eventful but never-the-less has been interesting and I always look forward to the numerous rock and mineral shows scheduled for the summer months.  A week or so ago (I am still writing a month later) I was able to wander up to Woodland Park for the show and enjoyed visiting with many vendors and picking up a few specimens.  I mean how can one attend a mineral show and not come home with something?

I have a detailed posting on pyrrhotite (July 28, 2014) so will not repeat that information.  In fact, I really did not need another specimen of the iron sulfide (Fe₇S₈); however, this was a very nice piece, cheap, and available.  Pyrrhotite is sort of a strange mineral is that it is magnetic and easily attracts a magnet as a variable amount of iron vacancies in its crystal structure. The specimen was collected from the Potosi Mine, Santa Eulalia District, Mun. de Aquilies Serdan, Chiluahua, Mexico.

There are nice brassy crystals scattered on the specimen.  width FOV ~3.7 cm.

Photomicrograph of pyrrhotite crystals although light reflection does not allow for the brassy color to come through. Crystals are tabular in cross section but hexagonal when viewed down the C axis (as these are).  Width FOV ~1.2 cm.

Photomicrograph calcite crystals.  Width FOV ~1.2 cm.

Photomicrograph shiny black sphalerite crystals along with pyrrhotite and calcite .  Width FOV ~ 1.2 cm.

A second specimen is not a rare or uncommon mineral but is a new specimen (for me) from a favorite locality:  molybdenite (molybdenum sulfide: MoS₂) from the California Mine near Mt. Antero in Chaffee County, Colorado. The Mt. Antero gem mines (aquamarine mostly) were among the first high altitude localities that I explored upon moving to Colorado in 2006.  The geology of Mt. Antero/Mt. White has been described in numerous publications, the most comprehensive being Mark Jacobson’s book entitled Antero Aquamarines (1993).  In general, Mt. Antero/Mt. White is underlain by a rock unit termed the Mt. Antero Granite.  McIntosh and Chapin (2004) have assigned a date of 29.6 million years to the granite, or mid-Oligocene in age. Sharp (1976) described the granite as chiefly pinkish-orange, medium grained;… youngest of the plutons (intrusive igneous rocks) in the vicinity of Mt. Antero.  Miarolitic cavities (crystal lined cavities often containing unusual minerals) are common and are often filled with…beryl (including aquamarine), phenakite, and smoky quartz.

A flat plate-like matrix of beryl measuring ~5.7 cm. maximum width, and coated on both sides by bright flaky silver molybdenite crystals.  BTW, I cannot remove the photo of this specimen from the pyrrhoite photos--another Blogger quirk!

Photomicrograph from above specimen showing flaky crystals.  Width FOV ~1.0 cm.

My early trips to Mt. Antero were accessed by walking (twice) up the “Mt. Antero Road” through Baldwin Gulch off Chalk Creek Road west of Nathrop.  This is the major road that almost all rockhounds bounce and bump up. My initial walking treks were due to the fact that I wanted to summit (fairly) Mt. Antero at 14,276. Since the California Mine is a couple of miles southwest of the mountain on the flank of Carbonate Mountain at an elevation of ~12,500 feet, I decided one day (before several joint replacements) that a nice hike was in order and trudged up the Browns Creek Trail starting off US 285 south of Nathrop. 

The location of the California Mine is sort of obscured in the large talus slope on the mountain and an adit has been blasted shut. In 1953 Adams noted that “molybdenite occurs sparsely in small veinlets in the massive quartz outcrop of the vein…and is concentrated along surfaces that presumably represent the walls of open channelways or vugs. Away from such surfaces, it is in isolated flakes that in many places project into small voids between beryl and quartz crystals.”

I really did not find much of interest on the hike (except beauty and tired legs after a 12 mile in and out) and needed to retreat to a lower elevation as a summer monsoon storm was blowing in.  I do not enjoy electrical storms and quickly left the area never to return.  As best that I can tell, the California Mine had beryllium production from the beryl; however, I could not locate production figures.  MinDat also reported the presence of molybdenum and tungsten but again no production figures fell into my hands.  In fact, I am uncertain as to which mineral contained the tungsten.

Therefore, I was happy to locate the specimen shown below.  It originally came from the collection of coloradominerals.com, the owner/collector unknown since the domain site is now for sale.

Radiating aggregates of prismatic "soft" violet crystals of creedite from the Henderson molybdenite mine.Width FOV ~6.1 cm.

  

 Photomicrograph of a section from above specimen.  Width FOV ~1.4 cm.

A third specimen I acquired at Woodland Park is again a mineral that is not uncommon but certainly rather rare in Colorado. In a Posting on July 10, 2016, I described a specimen of creedite [Ca₃SO₄Al₂F₈(OH)₂-2H₂0], a hydroxyhalide mineral (minerals with a Halide Group anion—chlorine, fluorine, bromine, iodine--- plus the hydroxide radical) from the Cressen Pit at Cripple Creek (the famous gold mine).  Although the mineral creedite had been named (1916) from a locality in the San Juan Mountains near the community of Creed, it was not until 2001 that the second Colorado locality was recorded from the Cresson Pit.  And finally, in 2007, a third locality was reported from the Henderson molybdenum mine near Climax.  The Woodland Park show was the first time I had seen a specimen from this latter locality for sale so I “nabbed it.”

The final specimen brought back from Woodland Park is actually one that was not in my vocabulary (not a hard task to accomplish)---chondrodite (not to be confused with chondrite, stony meteorites) . As a softrocker I was totally unfamiliar with this metamorphic mineral, but it was cheap at four bucks and had some nice gemmy crystals.  Chondrodite is a somewhat rare neosilicate mineral [(Mg,Fe)₅(SiO₄)₂(F,OH,)₂] and is formed in a contact metamorphic environment, usually where hydrothermal solutions (created by heat from an intrusion) comes in contact with carbonate rocks (limestone or dolomite ormaybe even carbonatite) and where fluorine has been introduced by the fluids.  Most crystals of chondrodite are yellow, orange, red, red-orange, brown, red brown and occur as individuals or masses in remnants or the original carbonate, usually calcite or marble.  Changes in color intensity in the crystals is common.  Crystals are brittle and are easily fractured in a conchoidal manner.  Chondrodite is fairly hard at ~6.0-6.5 (Mohs), has a vitreous to greasy luster and a yellow to yellow-gray streak. It is transparent to translucent.  Some red crystals are quite gemmy and have been faceted.  In addition, a few crystals fluoresce an orange (SW) to yellow orange (LW).

 Crystals of chondrodite in a matrix of rhombohedral calcite.  Width FOV ~7.0 cm.

Photomicrograph of chondrodite crystals showing changes in color intensity.  Matrix of rhombohedral calcite.

In the United States the best specimens of chondrodite seem to be associated with intrusive rocks in New York and New Jersey, especially the Tilly Foster Mine located near Brewster, Town of Southeast, Putnam County, New York.  The mine was active from 1843 until 1897 with peak iron production in the 1870s, mainly from magnetite.  MinDat lists 120 minerals and stated the mine “is famous for its excellent brucite, chondrodite, clinochlore, titanite, and magnetite crystals and antigorite or lizardite (and other species) pseudomorphs after a wide variety of minerals.” 

REFERENCES CITED

Adams,  J.W., 1953, Beryllium deposits of the Mt. Antero Region, Chaffee County, Colorado: U.S. Geological Survey Bulletin 982-d.

Jacobson, M.L., 1993, Antero aquamarines: Minerals from the Mt. Antero-White Mountain Region, Chaffee County, Colorado: L R Ream Publishing.

McIntosh, W.C. and C.E. Chapin, 2004, Geochronology of Central Colorado Volcanic Fields: New Mexico Bureau of Mines and Mineral Resources.

Sharp, W.N., 1976, Geologic Map and Details of the Beryllium and Molybdenum Occurrences, Mount Antero, Chaffee County, Colorado. U.S. Geological Survey Misc. Field Studies Map MF-810.

If you want to see a little Woodstock live see if this will load:

 https://www.bing.com/videos/search?q=woodstock+live+videos&view=detail&mid=6DF53882817174785EFF6DF53882817174785EFF&FORM=VIRE

 

 

GOETHITE FROM THE PIKES PEAK BATHOLITH

The hardest thing to see is what is in front of your eyes.

In my long-ago undergraduate days in western Kansas students commonly worked deciphering the stratigraphy of the Cretaceous Dakota and/or Kiowa  formations.  If you did not want a project in the limestones or chalk beds, then the Dakota/Kiowa was about the only possibility within close driving distance.  In fact, my senior project was tying to map crossbeds in the Dakota and describing some sections.  I remember the rock colors of the Dakota were mostly red or orange (or so it seems).  We usually described the non-quartz and -calcite visible minerals as iron oxide or limonite and moved on from there. 

 

These large concretions (~10-12 feet) have eroded from the Dakota (maybe Kiowa) in Ottawa County, Kansas at Rock City.

As life progressed, I simply thought all red or orange “stuff” in these sandstones was limonite or some such iron oxide—who cared about trivialities?  

A piece of Dakota sandstone (~ 5 inches width) composed of microscopic quartz crystals/fragments cemented by calcite but with much iron oxide/hydroxide filling voids between the crystals, and coating the surface.  What do I call it--"limonite" or goethite or iron oxide/hydroxide?

Only later in life when my first teaching assignment included Sedimentary Geology did I “start to care,” at least a little!  BTW, teaching sedimentary geology was a joy since originally, I was assigned Structural Geology, a course that was not one of my strong points ( have you ever tried working with, and understanding, stereonets). 

A stereonet is a powerful method for displaying and manipulating the 3-dimensional geometry of lines and planes (www.sciencedirect.com)--or so they say, not so much for me!

Along with Sedimentary Geology I labored big time in Ground Water Resources in Western Kansas (I had never taken any sort of a ground water course), Invertebrate Paleontology, and Intro to Geology.  Yep, four different course preps for a kid who was trying to finish his Ph.D. dissertation and was soon to be a new father.  Spring semester was about the same with Historical Geology, Intro to Geology, Field Methods, and something else.  I finished the first good draft of my dissertation on a dark midnight in mid-February and my son was born the next day.  I have trouble, even today, remembering much of that first academic year, 1970-71, except the pay was $9000 for the academic year and no commitment for a second year.  However, things were going my way when a new tenure-track contract came in mid-May just as the three of us were heading to Dinosaur National Monument where I had a summer position.  Life was good and I did graduate that summer (although the Park Service would not let me miss a day to attend graduation in Salt Lake City).  After that hectic year I resumed remembering “things.”  

The reason my memory was recently jogged about iron oxide minerals is that Mr. Rockhounding the Rockies (rockhoundingkw.blogspot.com), one of the premier collectors of minerals from the Pikes Peak Batholith (age around 1.08 Ga) gifted me an absolutely gorgeous specimen of goethite, an iron oxide-hydroxide [FeO(OH)].  It reminded me, again, that not all iron oxides/hydroxides look like rust, appear as a coating of clay (as in limonite), attract a magnet (magnetite), are a critical ore of iron (hematite), nor do they all come from sedimentary rocks.

Goethite collected from rocks of the Pikes Peak Batholith near Lake George. Width FOV ~5.6 cm.  As with many dark, metallic luster, minerals photography is difficult with my equipment.  The specimen is much more attractive than depicted in the photo.

Photomicrograph of a 1.0 cm. width FOV section of above.  Individual prismatic crystals are east to observe.

In our basic chemistry/physical geology courses we learned that iron occurs in two different oxidation states: 1) ferrous iron has a plus 2 charge (written as Fe⁺⁺ or Iron II ) and needs to share two electrons with oxygen to form a neutral ion; 2) ferric iron has a plus 3 charge (written as Fe⁺⁺⁺ or Iron III) and needs to share three electrons with oxygen to form a neutral ion.  Ferric iron is more stable than ferrous iron, the latter then commonly is oxidized (adds more oxygen) and becomes ferric iron.

Ferrous oxide is rare as a mineral due to its lack of stability and about the only mineral is wustite, a rare oxide usually found in meteorites and man-made slag from smelters.  The cation iron has a ++ oxidation state (plus 2) and the anion oxygen has a - - (minus 2) oxidation charge so they balance out: one iron (++) combined with one oxygen (- -) = FeO.

The major ferric iron mineral is hematite, Fe₂O₃.  Here you can see two units of iron (charge of +++) X 2 = 6 combine with three oxygen units (charge of - -) X 3 = 6  or +++ X 2 irons = 6 and - - X 3 = 6 oxygens.  So, it balances.  

There also is a major iron oxide mineral termed magnetite, Fe₃O₄  that appears not to balance!  However, magnetite is actually composed of both ferric and ferrous iron and should be written as: FeO-Fe₂O₃, one part of each (one unit of ++iron (2) and two units of +++ iron (6) = 8.  One unit of - - oxygen (2) and three units of - - oxygen (6) = 8.  Wow, it balances.

Iron minerals become even more complicated when one considers the iron hydroxides where the OH ion with a charge - - (minus 2) combines with iron.  As far as I can tell, ferrous (Iron II) can combine with a hydroxide ion, but only as a solution in the lab: Fe⁺⁺(OH)_2. One iron ++ and two hydroxides - .  So, one iron ++(2) combines with two hydroxides - -(2) and it balances.

Ferric (+++ or Iron 3) iron may combine with hydroxide to form a really rare and complex mineral called bernalite [Fe(OH)₃]: One Fe+++ (3) combines with three hydroxides each with a charge of minus 1– to equal 3, and it balances.

Another major group of iron and oxygen minerals are the Ferric (Iron III) oxide-hydroxides: ferric iron plus the hydroxide ion plus oxygen.  The major mineral in this group is goethite, FeO(OH).  In goethite there is one unit of ferric iron with an oxidation state of +++ that combines with one oxygen (oxidation state of - -) and one hydroxide (oxidation state of -).  So, three of iron equals the two of oxygen plus the one of hydroxide, 3 = 3.

In reality, there are at least three named polymorphs of goethite---exact same chemical formula but crystallizing in different crystal systems: akageneite, lepidocrocite and feroxyhyte.

But, what about limonite, that rusty clay or black streak or “ironstone” or whatever that is common in the orange or red Dakota Sandstone of my youth.  Is the mineral ferric or ferrous iron and is it an oxide, or a hydroxide or an oxide-hydroxide? It turns out that limonite is not even a mineral [often written as Fe+³O(OH)-nH₂O] but a combination of several “real” minerals---goethite, lepidocrocite, akaganeite, maghemite, hematite, pitticite, and “jarosite group” minerals and the term is used “for unidentified massive hydroxides and oxides of iron, with no visible crystals, and a yellow-brown streak” (MinDat.org).  Commonly, limonite is composed of goethite.

I am still not certain that I can identify goethite from limonite in many orange to red sedimentary rocks since both have similar colors (red, reddish brown, yellow brown, brownish black), similar hardness (5.0-5.5 or 4.5-5.0 in limonite[Mohs]), dull to metallic to adamantine luster, and a yellowish brown to orange-yellow streak, and often massive.  However, the goethite from the rocks of the Pikes Peak Batholith is different in that it often forms spectacular crystals.

The Pike Peaks goethite is composed of slender, flattened crystals that are elongated along the C-Axis, vertically striated, and exhibit a  metallic luster.  They form “clumps” of radiating crystals and appear to be black or brownish black in color.  However, the streak is brown to brownish yellow to yellow orange.  The crystals are secondary in nature and are derived by weathering (an oxidizing environment) of many different iron-bearing minerals.  Mr. Rockhounding the Rockies has collected his goethite specimens from the same cavities that produce amazonite and smoky quartz (see his web site for many photos).

To learn more about goethite, and especially Goethe, check out my Blog posting on April 23, 2012: Goethite, Goethe, and Kaninchen.

And finally, words of advice from Johann Wolfgang von Goethe (1749-1832): Every day we should hear at least one little song, read one good poem, see one exquisite picture, and, if possible, speak a few sensible words.

The hardest thing to see is what is in front of your eyes. See top of article. Johann Wolfgang von Goethe

ARSENIDES FROM YOOPERLAND SHINING IN A BUCK MOON

The summer solstice was passed about six weeks ago, and my friends are upset when I causally mention that we have “lost” (and are continually losing) daylight here in Colorado Springs---about 45 minutes by July 31, another 66 in August!.  To emphasize, even more, that fall is on the way I noticed that some schools have started and conversation in the coffee shop is about the Broncos practicing up at Dove Valley in southeast Denver (the Rockies are out of fashion and no one is looking for a Rocktober).  One item that is sort of “out of whack” is the snowpack in the mountains---many ranges are still rather white and A Basin was still skiing on July 1.  Fishers are complaining about the high waters in virtually every stream with mountain runoff.  Unfortunately, the last count for drowning in the high-water streams was up to 12.  But fall is on its way and I can smell it when the morning temps are in the 50s here in the city, and cooler up in the mountains.  The “baby” birds have fledged and are out of the nest and my prairie grass has bloomed and started to cure.  Fruit and veggies are pouring into Farmer’s Market, especially from the truck farming area of Rocky Ford along the Arkansas River southeast of the city.  Bears seem everywhere in this part of the city and the cubs are growing, as are the fawns.  I am not ready for cold weather, but I do enjoy seasonal changes and fall/autumn is a wonderful time. We are finishing July with a Black Moon tonight, simply a rather uncommon second New Moon of the month.  It is similar to a Blue Moon, the second full moon of a month.


Did you know that neither a Black Moon or a Blue Moon are possible in the month of February since the cycle between these particular moons is ~29.5 days? Every 19 years February does not have a Full Moon!  We, at least in this part of the world, term the July Moon the Buck Moon due to the growth of antlers on deer, and boy are there some large racks around here.  I am certain that Mr. Rockhounding the Rockies (a meteorologist) is well aware of moon phases. 

All of my daydreaming about weather changes (along with decorations for sale in big box stores) reminds me that the holiday season is approaching down the road and along with that comes one of the classic Christmas songs—Grandma Got Run Over By A Reindeer (Randy Brooks).  That in turn reminds me of the Upper Peninsula of Michigan (UP) and the Da Yoopers who recorded the Christmas classic Rusty Chevrolet:  

Dashing through the snow
In my Rusty Chevrolet
Down the road I go
Sliding all the way
I need new piston rings
I need some new snow tires
My car is held together
By a piece of chicken wire

OK, I understand that I have a weird sense of humor, but it does not take much to make be smile.  And besides, Da Yoopers remind me that I purchased, at the La Crosse Show, a nifty arsenate mineral collected from the UP.


If you are not from the upper Midwest, at the mention of Michigan most rockhounds automatically think of the big to huge copper nuggets although some of my friends who did poorly in grade school geography class mix up the states of Michigan, Wisconsin and Minnesota.  These confused rockhounds shout out Lake Superior agates, and it is true that Lakers can be collected in all three states.

Yooperland (the UP) is connected to the Troll Land (lower Michigan) by the Big Mac (the Mackinac Bridge).

Big Mac is a suspension bridge about 5 miles in length.  Public Domaine photo courtesy of Justin Billau.

The original source of the Lakers, and the copper nuggets, is from the basalts (several different layers) located in the Midcontinent Rift System (MRS).  This geological rift (think about the great East African Rift Zone) begin to form in the Precambrian (Proterozoic Era) perhaps 1.1 Ga splitting the stable part of the North American “continent” or plate (referred to by geologists as the craton).  The Rift is nearly 1400 miles long extending from northeast Kansas to Lake Superior with an eastern arm curving around and heading toward Ohio and a shorter arm trending west along the Minnesota-Ontario border.  Hugh amounts of lava erupted along faults while adjacent rivers from the uplands dumped thousands of feet of sediments (later sedimentary sandstones and conglomerates) into the lowlands of the Rift.  For some reason the Rift “stopped splitting” (a failed rift in geological jargon) and the continent healed. To fix this image in your mind, just imagine the top crust of a pie and how triple cracks develop during baking (but magnify it by zillions!). Perhaps the compression stopping the rifting was the result of orogenic activity (mountain building) on what we now know as the east coast of North America.  

The MRS is centered in Lake Superior with two well-defined arms and one sort of trending west.  Map Public Domaine (I think). 

Most of the rocks in the rift are buried below the surface of the earth and are only known from geophysical studies and drill holes.  For example, the Midcontinent Geophysical Anomaly (MGA) in Kansas delineates the rift since the concentration of magnetite in the Rift rocks creates a magnetic “high” that is picked up by geophysical instrumentation. However, rocks of the Rift become exposed around Lake Superior and the amygdaloidal agates erode from the basalts.  Since the Rift rocks include substantial amounts of iron, the agates have some sort of a red or orange color---oxidized iron.  Most likely the agates formed post-deposition of the basalt and are the result of percolating silica-rich groundwater filling the many vugs or vesicles in the basalt.


The UP copper is also found in rocks associated with the Midcontinent Rift System.  Scientists at Michigan State University have described the (www.geo.msu.edu/geogmich/copper.html ) formation of UP copper as follows: most of the native copper occurs at the top of the MRS basalt in a unit known as the Portage Lake Volcanics/Lavas.  However, this series actually contains over 200 individual lava flows (now basalts and some rhyolite), and 20 discreet conglomerate beds, that collectively have produced over 11 billion pounds of copper.  Over one billion pounds of copper have been extracted from copper sulfides (mostly chalcocite, CuS) in the overlying Nonesuck Shale.  The original source of the copper was from secondary deep-seated hydrothermal solutions percolating toward the surface with native copper crystallizing in the open vugs and pore spaces of older Rift rocks.  Most of these native copper deposits are found in the Keweenaw Peninsula “sticking out of the UP into Lake Superior” and home of Michigan Technological University with the fabulous A.E. Seaman Mineral Museum.

An exhibit case in the Seaman Mineral Museum.  Photo courtesy of the Museum.

South of the Keweenaw, but still in the UP (and adjacent Wisconsin), are the Precambrian Iron Ranges where the original sedimentary rocks have been subjected to metamorphism creating the ores.  The mining of various iron ores “has had a long and significant on the socio-economic development of the Northern Peninsula, beginning in September, 1844…” (Heinrich and others, 2004).

The iron mining districts of Michigan in Yooperland.  Photo courtesy of Michigan Mining History Association.

So, although the copper and Lakers are the most familiar specimens from Yooper Country, the MRS and Iron Ranges have produced an amazing number of collectable minerals.  Interested readers need to consult Mineralogy of Michigan (Heinrich and others, 2004), and if ever traveling near the UP visit the A.E. Seaman Mineral Museum. 


I am always interested in arsenic minerals and so was able to secure a couple of specimens containing copper and arsenic, but they are a bugger to identify. There are two principle copper arsenides found in the Keweenawan rocks (Heinrich and others, 2004)—domeykite [Cu₃As] and algodonite [Cu₆As] and they appear, at least to an ole clunker like me, to be very, very similar in appearance—silver shiny, steel gray, massive, tarnishing to bronze to iridescent to dull dark (black)!  Both are soft at ~3.5-4.0 (Mohs).  They occur with other “silver-shiny” minerals such as skutterudite, nickel skutterudite, and silver in addition to brass-shiny arsenian copper.  However, there are differences between the two minerals: 1) algodonite is in the Hexagonal mineral system and the chemical formula is “officially” written as Cu_1-xAs_x where x=~0.15.  In my understanding the mineral contains 83.58% copper and 16.42% arsenic. The density averages ~8.5; 2) domeykite, Cu₃AS, belongs to the Isometric crystal system and contains 71.79% copper and 28.21% arsenic.  The density averages ~7.65.  So, the additional arsenic (density ~5.7) in domeykite compared to less copper (density ~8.94) would reduce its density.  However, since the minerals are usually massive crystals cannot be observed visually, and both minerals are often found with other metallic minerals, so separation is often impossible to determine density. And finally, stuck in the middle of this mess is a rock called mohawkite that is a mixture of domeykite, algodonite, arsenian copper, skutterudite, silver and perhaps nickel.  

I believe this is domeykite in a calcite matrix.  The top view is of a "sawed" surface with large dark blob below the surface of the calcite.  The silver shiny streak is where the saw cut through the metal.  The middle view is a reverse side where the surface has been covered with some sort of protectorate.  The lower photo shows either bronze tarnished domeykite (I think) or perhaps arsenian copper. Width FOV ~2.9 cm.

A broken edge of the specimen showing "fresh" silver shiny domeykite with very minor quartz (Q) and the calcite matrix (C).  Width FOV ~ 2.0 cm.

The second specimen, a bright shiny-silver metallic color that causes major reflections with the camera.  The middle photo is with a greatly reduced light source that allows the iridescent tarnish to show.  There are streaks of what may be arsenian copper. The lower photo indicates to me the specimen is composed of arsenian copper, a dark, almost black, arsenide and then a much lighter silver colored arsenide.  This would make it mohawkite.  Width FOV top photo ~ 2.0 cm., bottom photo ~1.3 cm.

Both of my specimens are labeled as coming from the Mohawk Mine.  This is not surprising since early collectors often labeled any Yooperland arsenide as coming from the best-known location, the Mohawk Mine.  Heinrich and others (2004) reported that in 1900 and 1901, 105 metric tons of “mohawkite” was taken from the Mohawk Mine.  It is hard to believe that a number of arsenic minerals were not included in this figure!


I am uncertain what all of this means.  In the days of wet chemistry analyses (no electronic gizmos) geologists and chemists identified several other arsenide minerals; however, their favorite was mohawkite.  In todays world most of these early identified minerals are no longer considered valid minerals, including mohawkite.  The only thing certain to me is that arsenic and copper form numerous shiny specimens that are probably algodonite, domeykite, or mohawkite so rockhounds should be prepared to see any of these labeled as such!


So, I have two specimens.  One has a silver metallic mineral (on a fresh break but tarnished to a dull black on an “older” surface) in a calcite matrix with a few small quartz crystals.  I call this domeykite.  The second is a smaller specimen without matrix consisting of a shiny sliver mass with some arsenian copper (I think), a few small quartz crystals, some black mineral and some iridescence. I would like to call it mohawkite but perhaps I am pushing my luck!  While pouring over the photos in MinDat it appears, to me, that rather identical looking specimens are one person’s domeykite, another's algodonite, and another’s mohawkite!


I have also noticed that the arsenic must be tightly tied to the copper since there are numerous cabochons of these copper arsenides “for sale” on certain web sites.  Any "loose" arsenic would not be good to breath in!

REFERENCES CITED

Heinrich, E.W. updated and revised by G.W. Robinson, 2004, Mineralogy of Michigan: A.E. Seaman Mineral Museum, Michigan Technological Museum, Houghton.

This small posting is dedicated to Yoopers Pete and Jane probably relaxing on the front porch of their UP lake cabin.

ADDENDUM: October 23, 2019

I recently acquired, from a German dealer, a small specimen labeled "Algodonit [algodonite], Mohawk Mine, MI, USA." Again, it is really tough to distinguish between the copper arsenides. 

Black dendrites of algodonite on calcite matrix.  Note tiny "flaky" brass-colored material scattered around on dendrites.  Near the center of the dendrites notice the purple-blue iridescence. Width FOV of photomicrograph ~7 mm.

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