Thursday, January 5, 2023

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.

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