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!
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.”
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.”
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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