A TALE OF TRACKING DOWN CORDIERITE VAR. IOLITE FROM SOUTH DAKOTA AND COLORADO

In the end, it's not the years in your life that count. It's the life in your years. A. Lincoln

I really was not very knowledgeable about the mineral cordierite until I looked at some gemstones labeled water sapphire and listened intently to the jeweler’s long-winded description of these blue-violet stones. That little experience caused me to start reading about these colored stones and trying to better understand them. Soon I discovered that water sapphire, also known as iolite, was not an “official” mineral but was the gemmy variety of cordierite. However, the only time I remember seeing cordierite was in my optical petrology course as we studied metamorphic rocks. I really don’t remember observing the mineral in hand sample but only in petrographic slides.

Later in life, after my move to Colorado, I was reading Dan Hausel’s book (2009) on Wyoming minerals and was fascinated by his descriptions of cordierite and gemmy iolite from the Laramie Range west of Wheatland. Hausel noted large deposits of cordierite gneiss that produced such gems as the Palmer Canyon Blue Star (1,750 Carats) and the 24,150 carat Grizzley Creek Blue Giant, the latter a specimen he believed was the largest iolite gemstone in the world. Hausel also located other iolite deposits in the Laramie Anorthosite cropping out near Sherman Mountain. Additionally, he noted that perhaps millions of gemmy iolites remained in these Precambrian rocks but lamented the fact that these gemstones were mostly untouched and off the market.

Since reading Hausel’s descriptions I have looked in many rock/mineral shows, and on the internet, trying to locate specimens of the Wyoming iolite. But Hausel also stated that most/all gemstone localities were off limits to “average” collectors and rockhounds. I even tried examining a few roadcuts of the Laramie Anorthosite, but no luck for any gemmy material. I have seen a few cabs of Wyoming iolite (at least noted as such) for sale on internet sites but could not locate jewels at shows or stores. It is my understanding that imported iolite is of better quality and less expensive than the Wyoming variety; however, it would be nice to have some local material!

Pendants faceted from Palmer Canyon, Wyoming, cordierite/iolite. Offered by Etsy seller Jane Reneau.  Now they are "Out of stock."    

After my failed search for Wyoming iolite, I decided to try Colorado localities and was somewhat more successful. Successful indicating that cordierite occurs in tens of localities across Colorado in metamorphic rocks, commonly in some relationship with sillimanite and/or staurolite and is often altered (?ugly). The less successful part of the equation is that few cordierite exposures seem to exhibit the nice blue to violet variety, iolite (Eckel and others, 1997). One somewhat major exception is the Grape Creek locality in Fremont County where glassy, clear, blue corundum (sapphire?) was noted by Finlay over a century ago (1907). Mark Jacobson (1988) later described the blue masses as cordierite, “essentially unaltered, usually less than 1.5 cm in diameter.”  Unfortunately, I could not locate photos on MinDat and did not have access to some older publications that could contain photographs. Never-the-less the hunt was on for Grape Creek iolite.

Since the collecting locality was only relocated in 1987 “with some difficulty” (Eckle and others, 1997), and by that time in my life bone joint replacements prevented serious hiking, I started looking at shows and asking dealers for information. Not much luck until about four years ago when I discovered a specimen at the Denver fall show. I consider myself lucky as I have not observed another “for sale” specimen.

Cordierite from Grape Creek locality, Fremont County, Colorado, Width FOV ~7 mm.

My next attempt at locating cordierite var. iolite was to explore the Precambrian rocks of the Black Hills of South Dakota, one of my favorite places to wander. I remembered: 1) that Roberts and Rapp (1965) had stated that “cordierite occurs chiefly as a microscopic constituent of highly aluminous metamorphic rocks.” They also noted a couple of localities west of Custer; and 2) several years ago I was “exploring” the metamorphic rocks west of Custer trying to figure out what sort of a rock was described as amphibolite. Although at that time in my life I was hot into sedimentary rocks and vertebrate fossils, my curiosity had popped up while reading USGS papers describing the geology of the Four Mile and Berne Quadrangles immediately west of Custer and noting the large number of times “amphibolite” was mentioned. So off I went to explore, and to try and understand.

Life seems a quick succession of busy nothings. J. Austen

If I remember correctly, I located the amphibolite unit as it is exposed over several square miles. There were also “lots of” other rock units that I noted were really “gneiss” (pun intended). I collected a few hand samples (why???) because I was practicing being a geologist. Most were later discarded in one of my many rock gardens although a few were retained including one that I thought might be amethyst. But before you giggle, remember I have never claimed to be a mineralogist or petrologist!

So today I have a “hunk” (~4 x 5 cm) of metamorphic rock that appears to be part gneiss and part schist with layers of glassy blue or blue-violet cordierite var. iolite collected, as my label states, “west of Custer.”

Cordierite from "west of Custer County, South Dakota, near the amphibolite unit."  Width FOV ~7 mm.

By-the-way, I never really completely understood amphibolite. As defined by Wikipedia (retrieved 3 January 2023): “Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure.

Amphibolite frequently forms by metamorphism of mafic igneous rocks, such as basalt. However, because metamorphism creates minerals entirely based upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may also metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolite (tremolite-schist, grunerite-schist, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses.”

Cordierite collected in "Madagascar" (top and middle figures) purchased 2022 from Geofossiles in Colorado Springs. Bottom figure: Purchased, but collected Eminiminy (Anbinany), Androy Madagascar. Width FOV ~8 mm.

Thin, glassy, translucent blue-violet fragment of cordierite without matrix, etching is natural. Maximum width ~1.0 cm. Purchased, but collected by Luiz Menezes, 2001, Coroaci, Minis Gerias, Brazil.

Cordierite [(Mg,Fe)₂Al₃(AlSi₅O₁₈)] occurs in a variety of colors: gray, yellow-brown, greenish, colorless, blue, and bluish violet. It has a hardness of ~7.0+ and a vitreous luster while thinner crystals are translucent to transparent while the massive material seems rather opaque. Cordierite belongs to the Orthorhombic Crystal System although some twins resemble pseudo-hexagonal prismatic crystals; other material appears as massive to embedded grains. It has a white streak and a subconchoidal fracture.

Cordierite/iolite is also quite pleochroic, that is there are changes in color depending on the angle at which you view the specimen. Gemmy iolite may have pale blue color or a violet color or even a pale-yellow color. This pleochroism is quite easy to observe in my specimens as the mineral is rotated. In some case the blue color almost disappears into a gray-blue color.

The variety iolite/water sapphire is a blue to blue-violet to a blue-gray color and can be quite gemmy. I assume that lapidaries are experts in cutting the gems correctly so that the stones bring out the brightest blue color. It is softer than natural sapphire and has a lower refractive index (less brilliance). However, the cost of using iolite in jewelry is substantially less than mounting sapphire and most casual observers of a well-cut stone (cabs or faceted) would likely not notice the difference.

So, that is my tale of tracking down a mineral that was of interest to me but without the chance to tromp through the mountains.  It just took a little sleuthing while remembering the words of Dr. Suess: You have brains in your head. You have feet in your shoes. You can steer yourself any direction you choose.

REFERENCES CITED

Eckel, E.B., 1997, updated and revised by R.R. Cobban and others, Minerals of Colorado: sponsored by Friends of Mineralogy. Colorado Chapter, Denver Museum of Natural History, Fulcrum Publishing, Golden CO.

Finlay, G.I., 1907, On an occurrence of corundum and dumortierite in pegmatite in Colorado (near Canon City): Journal of Geology, vol. 15, no. 5.

Hausel, W.D, 2009, Gems, Minerals & Rocks of Wyoming: Private Publication, Gilbert, Arizona.

Jacobson, M.I., Part II: 1988, Corundum in pegmatite, or is it?, Rocky Mountain Boy claim, Grape Creek, Fremont County, Colorado: Mineral News, vol. 4, no. 2.

VIVIANITE FROM FLORIDA SEDIMENTARY ROCKS AND BLUE DEAD BODIES

 

Vivianite is a hydrated iron phosphate mineral [Fe₃⁺⁺ (PO₄)₂-8H₂0] that is a crystal of many colors, and in fact, can change color over its lifetime. Freshly exposed vivianite is generally colorless but with time oxidizes to green to bluish green to blue crystals.  Continued oxidation of the iron from Fe⁺⁺ (ferric) to Fe+++ (ferrous)  will produce crystals so dark blue they appear black.  Many crystals have a vitreous luster although they can grade into pearly or dull specimens.  Colorless crystals are transparent while lighter colored specimens become translucent and massive specimens generally are rather opaque. As with the color, mineral streak ranges from colorless to various shades of blue. Vivianite is quite soft, ~2.0 or less (Mohs).  The best “showy” specimens have prismatic (elongated along the C Axis) or flattened/bladed (along the B Axis) crystals and often form in stellate cluster; however, there are a variety of other morphological forms.

Vivianite is thought to occur as: 1) as a secondary mineral in metallic ore deposits; 2) in pegmatites as an alteration product of primary phosphate minerals; or 3) as a mineral associated with the phosphate found in sedimentary deposits. However, Petrov (2008) noted the mineral is not characteristic of the oxidized zone but of “deep unoxidized levels of ore deposits.” Most vivianite specimens collected in the western states, or observed in rock and mineral shows, are secondary in nature or from the phosphate minerals in  pegmatites.  Collected specimens, when first exposed to sunlight and oxygen, are often a beautiful blue and prismatic along the C-axis.

Very rarely do Colorado rockhounds come upon vivianite collected from organically rich unconsolidated clays and other sediments/rocks (mostly Cenozoic in ages). In the U.S. most of these sedimentary vivianites come from the Central Florida Phosphate District (AKA Florida Platform) where iron and water, along with original phosphatic material, has allowed vivianite to form: Fe₃⁺⁺ (PO₄)₂-8H₂0. The original phosphorus is thought to have been derived from precipitation in marine waters, and from the skeletons/shells and the waste products  of animals living in these waters.

The basement rocks of the Florida Platform are a fragment of the African Plate that remained attached to the North American Plate when rifting occurred in the Jurassic and range in age from late Precambrian-early Cambrian to mid-Jurassic (Barnett 1975).

A sedimentary sequence rests uncomfortably on top of the basement rocks, and is composed of Middle Jurassic to Holocene evaporite, carbonate, and siliciclastic sediments. This sedimentary sequence is the result of deposition on the relatively stable, passive margin of the North American Plate (Scott, 1989, 2016).

During the Cenozoic concentrations of silt to sand-sized phosphate pellets, mixed with carbonates and clastic sediments, were deposited in shallow water environments over much of the Florida Platform, in a broad range of carbonate and clastic sediments. During the Miocene and Pliocene phosphate was particularly concentrated in several basins in the Central Florida Phosphate District and these were the areas where a major phosphate industry begin development in the late 1800s. By 1893, production had expanded to 1.25 million tons and Florida became the world's leading producer of phosphate for the next century. By 2015/2016 the U,S, had dropped to the 3^rd largest producer of phosphate behind China and Morocco and production had expanded from the “Eastern Phosphate Fields”  of Florida and North Carolina to the “Western Phosphate Fields” of Utah and Idaho. However, in 2021 Florida still produced ~75% of the U.S. production of ~24 million tons. The Eastern Field operations use open pit mining to extract the ore from Miocene and Pliocene sediments/rocks. The Western Fields mine phosphate from limestone in the Permian Phosphoria Formation (Scott, 2016).

A well-formed, terminated crystal of glassy and gemmy, blue-green, vivianite, ~4 mm in length, collected from Clear Spring Mine, Homeland, Central Florida Phosphate Mining District, Polk Co., Florida. Light patch is carbonate matrix. A backlight would show transparency. Collection of Art Smith 1980.

REFERENCES CITED

Barnett, R. S., 1975, Basement structure of Florida and its tectonic implications: Gulf Coast Association of Geological Societies Transactions, Vol. 25.

Hurst, M. V. (Ed.), 2016, Central Florida Phosphate District Third Edition: Southeastern Geological Society Field Trip Guidebook No. 67.

Scott, T.M., 1989, The Geology of Central and Northern Florida with Emphasis on the Hawthorn Group, in Scott, T.M., and Cathcart, J.B., AGU 28th International Geological Congress, Field Trip Guidebook T178.

Scott, T. M., 2016,  Geologic overview of Florida in Hurst, M. V. (Ed.), Central Florida Phosphate District Third Edition: Southeastern Geological Society Field Trip Guidebook No. 67.

Virtually everything you might want to know about Florida phosphate may be found in the Hurst guidebook referenced above and available as a PDF file: http://www.segs.org/wp-content/uploads/2010/01/SEGS-Guidebook-67.pdf

 

AND NOW FOR THE REALLY INTERESTING STORY FROM CHRIS DRUDGE October 25, 2016 at: The Vivid Blue Mineral That Grows on Buried Bodies and Confuses Archaeologists - Atlas Obscura

IN 1861, a railway engineer by the name of John White passed away, was buried in a cast iron coffin, and began a slow transformation from White to blue.

The explanation for this spooky color change, which has occurred on numerous occasions all over the world, lies in the composition of the human body. Among the molecules contained within us is phosphate, a central phosphorus atom bound on four sides to atoms of oxygen. Phosphate is present in the hard bits of bones and teeth (as part of the mineral hydroxylapatite), helps hold together strands of DNA and RNA, and is used by cells to store and move energy around as well as to organize their many protein-driven activities.

If a dead person ends up buried somewhere waterlogged, lacking in oxygen, and loaded with iron, the phosphate leaking from their decaying remains can slowly combine with the iron and water to form a mineral called vivianite. It starts out clear and colorless, but will rapidly turn progressively darker shades of blue upon exposure to air as the iron within it reacts with oxygen. The formation of vivianite (also known as blue ironstone) is helped along by bacteria which act to dissolve iron out of soil and phosphate out of bodies while also directing the growth of the blue crystals. 

In the case of Mr. White, in keeping with the styles of the time, his coffin had a glass window installed in the front so his face could be seen by mourners when the lid was shut. At some point after burial, the glass had broken, allowing groundwater to seep inside and react with the cast iron coffin and phosphate-rich body. The end result was a corpse surround by blue vivianite crystals, revealed when the coffin was exhumed as part of an archaeological rescue excavation over a century after being buried.

 

 

THEISITE: Just because you don't understand it doesn't mean it isn't so.

I am always on the lookout for funky and sort of quirky uncommon to rare minerals. My small mineral collection has numerous specimens that I purchased due to the facts that I did not recognize the name, and it was cheap. Anything coming off of a dusty shelf with an older label was a bonus. An extra, extra bonus was created if the new mineral was collected in Colorado. And a three-level bonus appeared if the mineral came from near Tuckerville, Colorado, a town, well really a former town now inhabited by ghosts. After purchasing a three-level bonus mineral I didn’t have the slightest idea about the location of Tuckerville until I noticed it was close to Vallecito Reservoir. That particular body of water, on the Pine River, is located about 18 miles northeast of Durango. I have camped there twice in absolutely beautiful USFS campgrounds and spent the time fishing rather than exploring back roads. Tuckerville is then located about 12 miles northeast of the Reservoir on FS 2274 or Middle Mountain Road, a winding road that feels better in a 4-wheel drive pickup than a low-slung passenger car. The elevation is ~ 10,600 feet. After speaking to the ghosts of “almost nothing left” Tuckerville, rockhounds must take a hike (400 feet elevation gain) on the established “jeep trail” to reach the old mines of Tuckers Tunnel or Tuckerville Prospects. Evidently the tunnel has collapsed as have other adits. Mineral collectors who made it this far have zeroed in on what remains of the Tunnel dump. MinDat lists 31 valid mineral species collected in the dump and noted the Tunnel dump as the Type Locality of theisite [Cu₅Zn₅(AsO₄,SbO₄)₂(OH)₁₄].

WORDS OF THE DAY

Funky Mineral: Theisite

Ghost Town:  Tuckerville

New Geological Term to Learn:  Fahlore Deposits

There is not much to say about the Tuckerville Prospects except I regret not being able to visit the locality (at the time of camping at Vallecito I was chasing fossils). Very little information, at least that I could locate, is published on the mining area. Certainly, the best publication is a Rocks and Minerals article by Haynes and Paul Hlava (1998). In addition, Williams (1982) described theisite as a new mineral in Mineralogical Record; however, I could not locate a copy of that article but did find the abstract.

Google Earth© image of the location of Tuckerville (nothing to see on image) and the Tuckerville Prospects.  The winding road comes north from Vallecito Reservoir.

According to Haynes and Hlava (1998) the Tuckerville Prospects are part of the Cave Basin Mining District where in 1913-1914 speculators and investors were overly optimistic about future production of copper, silver, and gold—many claims were filed but production was minimal. As best I can determine in adding figures together, the Cave Basin mines, mostly the Mary Murphy, Holbrook, and Silver Reef, produced a tad less than 100 ounces of gold, ~270 ounces of silver, ~2900 pounds of copper, and ~1700 pounds of lead---all from ~120 tons of ore shipped (via animal drawn wagons or mule trains????) down the mountain to an unknown processing plant. Mining was sporadic from 1913 to 1936. The above production figures are from Schmitt and Raymond (1977) and Steven and others (1969).

I may be missing something but have been unable to locate base metal production figures for the Tuckerville Prospects (the above production figures are from the entire District).  However, it must have been minuscule. The minerals listed by MinDat do not include silver or gold; however, some minerals do include copper, mercury, zinc and maybe a grain or two of galena (lead). Theisite was discovered in 1980 at Tucker Tunnel by two geologists, N.J. Theis and Michael Madsen (Theis and others, 1981), as they tromped through the area looking for uranium—evidently there are some rocks that excite a Geiger Counter, perhaps uraninite and/or zeunerite, as Haynes and Hlava stated, “the prospect [Tuckerville] is radioactively anomalous (up to 700 gamma counts per second).” Steven and others (1969) noted that “base and precious metals in the Cave Basin District were replacement deposits in lower Paleozoic sedimentary rocks”—perhaps the Ouray of Leadville formations.

Now down to the new mineral from Tucker Tunnel—theisite, a copper zinc arsenate antimonate. We know the chemical makeup of the mineral due to analyses by X-ray Powder Diffraction and Electron Microprobe studies. Unfortunately, I don’t have either gizmo in my office.

Visually (with a microscope) theisite is very difficult to identify, especially for an ole plugger like me. First of all, specimens are usually quite small (mine are really, really tiny) and rarely occur as crystals but as crusts, spherical aggregates or simply individual spheres, cleavage plates, micaceous plates, or just plain globs. The color is some sort of blue + green: greenish blue, turquoise-green, turquoise-blue, pale green, or pale blue. Specimens have a pearly luster and are very soft (MinDat states 1.5 Mohs).  I found it very difficult to identify my small specimens and if not for the collection and identification by David Shannon I could have guessed any number of copper zinc minerals.

Two dark green spheres of theisite and two lighter green spheres of "your guess." The dark mineral is a manganese oxide. Width FOV ~4.0 mm. 

Green to blue green theisite spheres or cluster of spheres in and around a vug. Width FOV ~4.0 mm.

Scattered green to blue green theisite spheres or cluster of spheres. Width FOV ~3.0 mm. 

Clear gemmy crystals of platy hemimorphite with black manganese oxide. Width FOV ~2.0 mm. 

MinDat pointed out that theisite was a rare secondary mineral in fahlore deposits. Well, that piece of info rattled my brain and sent me scrambling. The best I could do with an understanding definition of fahlore was from Wikipedia: Fahlore refers to an ore consisting of complex sulfosalt minerals (a metal + semi-metal + sulfur) and in the case of theisite the mineral is formed due to oxidation of a mineral(s) in the tennantite--tetrahedrite solid solution series.

Just because you don't understand it doesn't mean it isn't so.

            Lemony Snicket

REFERENCES CITED

Haynes, P.E. and P.F. Hlava, 1998, Mineralogy of Tuckers Tunnel: Tuckerville, Hinsdale County, Colorado: Rocks & Minerals, vol. 73, no. 5.   

Schmitt, L. J. and W. H. Raymond, 1977, Geology and mineral deposits of the Needle Mountains district, southwestern Colorado: U.S. Geological Survey Bulletin 1434.

Steven, T. A., L. J. Schmitt Jr., M. J. Sheridan, and F. E. Williams, 1969, Mineral resources of the Sun Juan primitive area, Colorado: U.S. Geological Survey Bulletin 1261-F.

Theis, N. J., M. E. Madsen, G. C. Rosenlund, W. R. Reinhart, and H. A. Gardner, 1981, National uranium evaluation, Durango quadrangle, Colorado, Grand Junction, Colorado: Bendix Field Engineering Corp.

Williams, S. A. 1982, Theisite, a new mineral from Colorado: Mineralogical Magazine, vol. 46.