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.

Check out the web site www.dayoopers.com

WISCONSIN SULFIDES, MOONSTONES, QUARTZ & AZURITE (AND LUTEFISK)

Life in the fast lane, Wisconsin.

I recently had the opportunity to visit my ole stomping grounds along the Mississippi River in La Crosse, Wisconsin.  I was scheduled to present three geology/minerals talks at the Coulee Rock Club Gem and Mineral Show. The Club, although small in number, has an amazing cadre of volunteers who ran a smooth Show.

 

The beautiful Mississippi River at sunset with high bluffs capped by Ordovician carbonates.  "Normal" water stage with main channel in background.

Flood stage with water covering campgrounds and boat ramps submerged.

I also had planned to get in some good fishing on the River and hoped to search the shoreline for some Lakers (Lake Superior Agates).  Unfortunately, neither of those events took place as the River continued to be above flood level during my entire 10 day stay.  Boats could not get on the water as the ramps were completely covered and of course the shoreline gravels were nowhere to be seen.  But, the rock and mineral show was fun and I nabbed a few interesting minerals for my collection.

 

The doors opened at 9:00am Friday and the crowd wandered through.  The Show was held in the Onalaska Center.

The silent auction sold hundreds of specimens each day.

 

The big seller of the Show were these pieces of Ordovician carbonate with natural bugs planted with various kinds of  low growing "ground cover."  Items were made by members of the club as a "fund Raiser."

Southcentral and southwestern Wisconsin is the heart of the lead and zinc mining Tri State District (not to be confused with the Tri State District of Kansas, Missouri and Oklahoma) and was the first area populated by persons of European descent in the upper Mississippi River Valley.  Mining actually started in the 1700s with the last Wisconsin mine closing in the late 1970s.  In the 1830s and 1840s miners arrived from Cornwall, United Kingdom, and brought with them advanced mining technology.  These “Cousin Jacks” are fondly remembered in the area today and Wisconsin hosts many Cornish Festivals.  Cousin Jacks also brought along a vegetable-meat pie called a pasty that is loved by everyone in the State, and by most visitors.

Pasty from Warrens Bakery, Cornwall, UK.  Photo courtesy of David Johnson.

Although the major mineral products were lead and zinc extracted from galena and sphalerite, copper was present in many/most mines as the sulfide minerals bornite and chalcocite.  Secondary minerals in the oxide zone included azurite and malachite and although not common as collector minerals today, I was able to pick up a piece of azurite labeled “mine near Gratiot, Wisconsin.”  I suspect it is from an old collection.

Photomicrograph showing clusters of very small (submillimeter) crystals of azurite on matrix.

The Wisconsin lead-zinc ores are a Mississippi Valley Type (MVT) of deposit and the ores are hosted by Paleozoic limestone and/or dolomite.  Geologists now believe that MVT ores arrived in these carbonate basins via heated salt-water brine traveling in very deep “channels” in the bedrock (maybe as much as 13,000 feet deep). The source for the mineral laden brine was perhaps Arkansas along the leading edge of the structural front associated with the Ouachita Orogeny (part of the Appalachian mountain building event). As waters traveled north and reached near surface rocks (perhaps 1000 feet) the brine cooled, and precipitation of ore minerals started filling vugs, cracks and fractures in the Paleozoic carbonates.  Early mines in Wisconsin were quite shallow, about 100 feet or less, and local farmers called these prospectors badgers since they were always digging shallow pits looking for ore.  It was not until the Cousin Jacks and their technological improvements arrived did underground mining start to boom.  So, the Badger State was not named after furry little creatures but after early mom and pop prospectors. 

 

The Wausau Igneous Complex is a series of overlapping igneous intrusions of Precambrian age (Middle Proterozoic ~1.5 Ga) located in east central Wisconsin near the city of Wausau.  The exposed rocks are various granites, quartzite, and syenite (like granite but deficient in quartz).  I have visited the area a few times and the hard rock stratigraphy is very confusing to a softrocker like me.  At the Show I picked up a couple of specimens as I felt sorry for the dealer trying to push some not too pretty rocks.  Not much locality information except the crazy mass (looks like pickup sticks) of stained quartz crystals with some feldspar and goethite labeled “Wausau Pluton from the collection of Al Falster.”  I throw that tidbit in since Falster has published several articles on minerals of the Wausau Complex.

The second specimen is a group of better-looking quartz crystals, some clear, and all nicely terminated collected from Rib Mountain (also part of the complex). 

 

Mass (jumble) of hematite stained quartz crystals.  Width FOV ~4.3 cm.

Cluster of terminated quartz crystals.  Width FOV ~4.0 cm.

Perhaps the most famous rocks/minerals coming from the complex (Stettin Pluton) is Wisconsin Moonstone whose specimens are “hot” in the local mineral markets.  The specimens are a type of sodium-potassium silicate feldspar called anorthoclase [(Na,K)AlSi₃O₈].  It is an intermediate member of the albite (mostly sodium)—orthoclase (mostly potassium) solid solution series. According to Bill Cordua (Wisconsin’s go-to mineral man), homogenous anorthoclase splits on a fine scale into intergrown potassium feldspar and albite. Sometimes the bands of alternating minerals are coarse enough to see. Other times they are microscopic. If they are just the right size and spacing, they scatter the light that penetrates the various layers in the mineral – producing the moonstone effect, or schiller.

 

Tumbled Wisconsin Moonstone without schiller effect.

So, “moonstone” is not really a defined mineral but a combination of feldspar minerals.  Some, like anorthoclase, produce a “moonstone” schiller effect.  Most gemmy moonstone, as that from Myanmar, is more transparent with a strong blue sheen and schiller effect.  The specimens from Wisconsin are essentially opaque and take a very special skill in cutting and polishing to bring out the schiller effect.  This lapidary achievement has been perfected by the owner of Bill’s Moonstone and Crystals in Wausau, Wisconsin.

 

Unpolished/cut Wisconsin Moonstone showing a slight schiller effect (vertical near center of sample.  Width FOV ~4.1 cm.

I first saw Wisconsin Moonstone on the market several years ago when it was advertised and sold as a gemstone.  Today the marketing strategy has changed, and it now seems to be some sort of a magic rock.  According to information pushed at the show “Wisconsin Moonstone is very powerful…It is super calming and brings emotional balance…[and] provides protection while dreaming…[It] clears and opens all chakras…etc., etc., etc.”  And life goes on pretty well if you carry around a piece of Wisconsin Moonstone.  Try it! 

Epidote is a common low-grade metamorphic mineral found in the Precambrian metamorphosed basalts of the Mid Continent Rift System. In these rocks it usually replaces plagioclase, pyroxene and olivine during metamorphism. Most of the Keweenawan volcanic rocks in the northwestern part of the state in Polk, Ashland, Bayfield, Douglas, and Iron counties contain abundant epidote (Wisconsin Geological and Natural History Survey).  The volcanic basalt poured out of vents associated with the triple rifting of the late Precambrian continent about 1.1 Ma.  The Triple Junction of the three rifting arms was located about where Lake Superior is today, and the Lake lies in a basin created by the rift.  After about 20 million years of rifting and tearing apart the Precambrian stable craton, the rift failed, perhaps due to compressive forces to the east (in today’s directions) and started to heal.  Today the old rift system is filled with both volcanic rocks (from deep seated magma) and sedimentary rocks washed in from the surrounding highlands.  Rift system rocks in the western arm are only exposed as far south as Interstate State Park near St. Paul, Minnesota. However, subsurface high gravity anomalies (more dense basalt) have been traced to Kansas.  Probably the mineral best related to the Rift rocks are the native copper deposits of the Keweenawan Peninsula of upper Michigan (Yooper Country). 

 

Mass of green epidote crystals.  Width FOV ~3.4 cm.

Photomicrograph of radiating epidote crystals from above specimen.  Width FOV ~1.4 cm.

My specimen of epidote, a complex hydrated silicate with calcite, aluminum and iron, is essentially a mass of green, elongated crystals.  Small portions of the original basalt are present along with small amounts of quartz and unknown sub millimeter shiny black crystals along with dark prismatic crystals.  My level of skill does not allow me to identify these tiny crystals.  The location label simply said, “Epidote crystals, Wisconsin, Keweenawan volcanics.”

 

Pine River Pegmatite with numerous small crystals of elbaite, many colorless, some "green", others with a pink/red hue.  Width of specimen ~2.2 cm.

Photomicrograph of above specimen.  Arrow indicating prismatic crystal of elbaite with small areas of green coloring. Length of crystal ~ 8 mm.

Photomicrograph of above specimen.  Arrow indicating prismatic crystal of elbaite. Length of crystal ~4 mm. Faint red tint.

Section of "red" elbaite crystal. Width of crystal ~3 mm.

I also have a small specimen (thumbnail) labeled “Elbaite var. rubellite, Pine River Pegmatite, Florence County, Wisconsin.” Elbaite is a member of the tourmaline group of minerals and rubellite is a pink to red variety (usually).  The specimen I acquired has tourmaline crystals that are mostly colorless but also have a pink tint.  My color blindness did not allow me to pick out a very good sample.  Florence County is home of the Animikie Red Granite (red granite, several locations, is the State Rock of Wisconsin) and has a number of associated complex pegmatites.  The elbaite is rather uncommon but has produced some nice crystals according to photographs (mine is not one of them).  MinDat.org believes the pegmatite dikes are related to the 1.76 Ga Bush Lake Granite intrusion.  Florence County is in northeastern Wisconsin and is mostly covered with glacial drift.

When I moved to Wisconsin in 1998 the “big news” among geologists and environmentalists was the closing of the open pit Flambeau Mine near Ladysmith in the north central part of the state.  Kennecott Minerals Company had “discovered” copper in the area 30 years before.  In those days Wisconsinites, before the recent evisceration of the DNR, were proud and protective of their environment, especially of the woodlands and recreational waters of the northland.   This respect for nature led to a quarter century of lawsuits and permitting delays and planning documents before mining started in 1993.  The deposit was extremely rich, and ore was shipped directly to a smelter therefore avoiding any sort of a concentrator. The surface mine of 32 acres and ~220 feet in depth produced 181,000 tons of copper, 334,000 ounces of gold, and 3.3 million ounces of silver in a four-year period.  The mine closed in 1997 and was reclaimed by 1999.

MinDat.org described the deposit as an exhalative massive sulfide deposit deformed during the 1.80-1.85 billion-year-old Penokean orogeny [~1.86 to 1.81 Ga]. It was subjected to significant oxidation and supergene enrichment producing a zoned cap of, from the surface downward, gossan, chalcocite, bornite, and chalcopyrite. What this statement actually says is that during plate collision metal bearing solutions erupted via hot springs on the ocean seafloor, reacted with cold seawater, and “dropped their load.”  In todays parlance these springs are known as Black Smokers and they are responsible for some of the earth’s richest mineral deposits.

A Black Smoker on the ocean floor.  Photo courtesy of D. Kelley, University of Washington, Seattle, USA

The specimens I picked up at the show are: massive bornite with scattered crystals of chalcopyrite and perhaps some very tiny chalcocite crystals; and 2) some beautiful azurite crystals on the oxidized gossan (iron oxides goethite/limonite).  According to the Wisconsin Geological and Natural History Survey The major ore minerals at Flambeau are massive sulfides bornite [copper iron sulfide Cu₅FeS₄], chalcocite [copper sulfide Cu₂S], and chalcopyrite [also a copper iron sulfide CuFeS₂].  Both the bornite and chalcocite are sort of dark and gray and ugly while the chalcopyrite forms brassy colored crystals.  What made Flambeau famous (sort of like the beer that made Milwaukee famous) is the very high enrichment of the supergene strata where these minerals were mined.  The azurite [hydrated copper carbonate Cu₂(CO₃)₂(OH)₂] came from the highly oxidized zone very near surface.  According to MinDat.org the Flambeau Mine has yielded 67 valid mineral species.  Unfortunately, very few of these minerals appear on the market with the exception of chalcocite and bornite.  It appears that collectors were not allowed on site until later in the mining process and immediately after mining ceased reclamation started and the pit was filled and covered—no mine dump to sort through.

 

Massive bornite with scattered brassy chalcopyrite crystals.  Width FOV ~4.4 cm.

Azurite on goethite/limonite.  Width FOV ~2.5 cm.

Photomicrograph of azurite section from above specimen.  Width FOV ~1.0 cm.

In summary, I really have only a few minerals from Wisconsin considering I lived there for eight years.  But that was in my days of traveling across the country promoting undergraduate research, keeping a college of faculty members, staff and students reasonably “happy” and trying to locate sunnies and walleye in the mighty Mississippi River.  And don’t forget the pasties, smelt fries, lutefisk, pickled herring, 9,999 kinds of cheese, and smoked carp washed down with liquid from breweries such as the Lazy Monk, Dead Bird, Grumpy Troll and Pabst Milwaukee.