Illinois Fluorite

Write By: tmmadmin Published In: Classic Mineral Localities Created Date: 2015-05-27 Hits: 24414 Comment: 0


5.1" Purple on Yellow Gem Phantom Fluorite - Crystal Mine ex-Ross Lillie

Some of the most beautiful fluorite specimens found anywhere in the world came from the fluorite mines in Southern Illinois. Since the 1950s and 60s, mines such as the Annabel Lee, Hardin, Denton, Rosiclare, Minerva #1, and Hill-Ledford became legends in the mineral world for the outpouring of their world-class fluorite specimens into the mineral marketplace. In 1993, the mining companies announced that they were going to shut down the mines, and the last mine closed in 1995. In the mineral world, when mines close, prices usually shoot up immediately, in anticipation of short supplies in the future. In the case of Illinois fluorite, the supply pipeline of fine fluorite specimens was very full. In fact it was so full, that it wasn't until 2008 that supplies began to dry up and prices began to rise. In 2009, the price for the really good Illinois fluorite specimens rose dramatically. In the years since, prices have continued to escalate very rapidly. Today, it will cost you 10-fold what you would have spent in 1993 to get a superb specimen.


2.5" Purple Phantom Fluorite Sharp Cubic Crystals - Denton Mine

HISTORY AND MINING
Fluorite was originally named fluorspar and was first discussed in print in 1530 book about the properties of minerals by Georgius Agricola. The written history of the earliest mining in Hardin County is scanty and confusing. Before settlers arrived, archeological finds in southern Illinois show that Mississippian Native Americans used fluorite to make jewelry, beads and ornaments as early as 900AD.

Fluorite has been mined commercially in southeastern Illinois since the early 19th century in a deposit that came to be called the Illinois-Kentucky Fluorspar Mining District, which spans southeastern Illinois and parts of northwestern Kentucky. The earliest reported mining in the area was in 1835 for lead at the Columbia Mine in Crittenden County. Settlers dug shallow pits to extract galena for its lead content and discarded the fluorite which had no market value. The first shipments of fluorspar were reportedly made in the early 1870s. The open-hearth steel furnace process, which was introduced in the 1880s, used fluorite for flux, increased demand considerably, and large-scale mining for fluorite began in Illinois soon after. In 1890 the Fairview Mining Company began mining fluorite, and in 1893, the property was purchased by the Rosiclare Lead & Fluorspar Mining Company.


Cave-In-Rock

Though fluorite from southern Illinois is often incorrectly labeled as coming from Cave-In-Rock, a village on the banks of the Ohio River. There are, however, no mines in this town; the workings are to the north and the west, in nearby villages such as Rosiclare, Elizabethtown, Eichorn, etc.


6.1" Purple Edge Yellow Fluorite Crystals - Victory Mine ex-University of Arizona

In 1911 the first fluorspar mill was built, which increased milling capacity greatly. In the 1920s, several new mining companies were started, including the Hillside Mining Company in 1929, and the Victory, Crystal and Benson mines were opened. The Minerva Oil Company started producing ore in the 1930s, along with the Ozark Mahoning Company. Around this time a concentrating plant was built by Ozark Mahoning in Rosiclare.

In the 20th century, mining was mostly underground, reaching depths of as much as 1,300 feet. Where large deposits of fluorite were found on the surface, open pit mining was more economical. In 1942 Illinois became the leading producer of fluorite in the U.S., and accounted for more than 50% of the country's production.

The fluorite ore in vertical vein mines was taken out by sinking shafts adjacent to the ore bodies, then and driving drifts into the ore body every 100 feet or so. Where possible, small rail cars pulled by mules transported the ore to the shaft to be hoisted to the surface. In smaller mines, loaded cars were pushed to the shaft by the miners. Later, motorized trams were used to move the loaded cars. In the bedded ore deposits, diesel powered trucks hauled ore to the shafts.

By 1988, the last two mines still open were the Denton and the Annabel Lee, both operated by the Ozark-Mahoning Company. These mines exploited the Harris Creek district, which was first opened in 1979. In 1995 the last mine closed, marking the end of commercial ore mining in southern Illinois.


Roadside marker

GEOLOGY
During the Jurassic Period, about 150 to 200 million years ago, hot water containing fluorine and other dissolved chemicals rose from deep in the earth. The mineral-laden water flowed through northeast-trending faults and fractures in limestones laid down earlier in the Mississippian Period, about 330 million years ago. When the hot brines reached the calcium-rich Mississippian rocks, the temperature and other conditions were just right for crystallizing fluorite along the walls of the faults and in flat-lying layers parallel to the beds of limestone. The host rocks dissolved and were replaced with the fluorite.


The Rosiclare Mine in the 1980s

Almost all Illinois fluorite was mined in Hardin and Pope Counties. The fluorite was primarily found in two types of occurrences: fissure-vein deposits in the Rosiclare district, and stratiform (bedding plane) deposits in the Cave in Rock district. Width of these fissure-vein deposits varied from a few feet to 30 feet or more, with depth in some mines reaching almost 1000 feet. The vein deposits also contained secondary mineralization of economic importance, including sphalerite (zinc), calcite, barite, and galena (lead).

The second type is stratabound or bedded deposits, also called mantos. These deposits primarily contained fluorite. Most of the bedded ore bodies were quite large; the biggest were 10-12 feet thick, 300-400 feet wide and a mile long. Stratiform deposits were the principal sources for mineral specimens, because they generated the open pockets that are necessary for crystal formation. Pockets are the result of volume shrinkage, and also by leaching and partial removal of the limestone host rocks.

 


3" Blue Phantom Fluorite Crystal with Leopard Spots and Multiple Violet & Purple Phantoms - Denton Mine

Here is a map of the region's fluorite mines and a detailed geological analysis from the Kentucky Geological Survey website:


Principal mining areas (in blue) in the southeastern Illinois part of the Illinois-Kentucky Fluorite Mining District.


"The vein and stratabound deposits were formed during the Permian Period by upward-migrating, fluorine-rich hydrothermal fluids that flowed along the fault planes of the northern New Madrid Fault System, extending north and east to Illinois and Indiana. Basinal hydrothermal fluids mixed with magmatic gases, and fluids generated by the Hicks Dome intrusive event were the dominant source of mineralizing fluids. Vein deposits occur along faulted areas, and most veins are lenticular and trend northeast in Mississippian carbonate rocks. Veins varying in width from 3 to 10 feet swell along strike for several hundred feet, and have been mined to a depth of 800 feet. Fluorite is the dominant mineral deposited in the district, but barite, sphalerite, and galena have been produced in smaller quantities. Some sphalerite mineralization is associated with mafic dikes at the Hutson Mine along the Big Sandy Creek Fault System, and smithsonite, a zinc oxide, occurs at the Old Jim Mine near the Crittenden Springs Fault System. Zinc has been produced for many years, and since 1940 the district has become a major source of zinc, some as a byproduct, some as a main product. Substantial quantities of barite have also been produced at the Mico, Ainsworth, and Pgymy Mines. Other major fault systems with associated fluorite are the Dyer Hill, Levias-Crittenden Springs, Commodore, Moore Hill, Claylick, and Tabb. Lead has been a minor byproduct in recent years, as have silver, cadmium, and germanium.


6.2" Butter Yellow Fluorite Sharp Cubic Crystals - Victory Mine ex-University of Arizona

"Bedded (manto) deposits occur along the Rock Creek Graben near Carrsville, and are generally 5 to 200 feet wide, and can be 20 feet thick. Lengths range from 200 feet to 2 miles, and ore averages 20 to 35 percent fluorite (Goldhaber and Eidel, 1992). Until the early 1930's, almost the entire production in the Illinois-Kentucky district was from vein ores. In Kentucky, practically all production has been from veins, although the bedded deposit near Joy and Carssville has been known since the 1950's and was put into production in 1970. Only minor production occurred because of milling problems, but there are still substantial reserves.


3.3" Sharp Magenta Purple Multiple Phantom Fluorite Crystal - Denton Mine

"The Coefield Ultramafic Complex was discovered by Billiton Minerals in the early 1980's. It is characterized by a large magnetic anomaly and has been identified as both a lamprophyre and kimberlite ultramafic intrusive. Numerous companies have examined the area for diamond potential, but no diamonds have been found. This intrusive is located southeast of Hicks Dome in northern Crittenden County, and there is another magnetic anomaly south of Coefield near Maple Lake in southern Crittenden County. Some exploration has been conducted on the site, but there has been no recent mining. These ultramafic and carbonatite complexes are characterized by igneous alkalic explosive breccia bodies and are discussed in Plumlee and others (1995) and Heck and others (2006).

"The vein and bedded deposits were formed by a release of fluorine-rich fluids from deep-seated alkalic intrusive rocks and by the intersection of these magmatic fluids with hydrothermal basinal fluids that were rich in base metals such as sphalerite, galena, and barite. These interactions of magmatic and hydrothermal fluids generated an acidic hydrothermal fluid and the fluorite mineral deposits (Plumlee and others, 1995). At the time of the initial Hicks Dome explosive magmatic event, northward-migrating hydrothermal brines intersected with hot, fluorine-rich magmatic gases, resulting in precipitation along isothermal reaction paths in the fault systems (Plumlee and others, 1995). Paragenetic studies by Hayes and Anderson (1992) suggest that these magmatic/hydrothermal fluids were part of a regional system of mineralization that correlates with the Central Tennessee Zinc District and the Central Kentucky Mineral District."


4.4" Purple Edge Fluorite Crystals Multiple Purple/Clear Phantoms - Minerva Mine

USES
Besides its role as flux in steel-making where it is used to decrease the viscosity of slags, fluorite has many other uses. It is employed by industry in producing aluminum, in the production of hydrofluoric acid, and in many other fluorine chemicals. In households, it is used to produce glassware and ceramics, as an additive to drinking water, and in toothpaste to prevent cavities. Fluorocarbon is used in cookware, lubricants, building materials, etc. Fluorite is a colorful mineral, both in visible and ultraviolet light, and the stone has ornamental and lapidary uses. Fluorite may be drilled into beads and used in jewelry, although due to its relative softness it is not widely used as a semiprecious stone.. Industrially, fluorite is used as a flux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride for hydrofluoric acid manufacture, which is the intermediate source of most fluorine-containing fine chemicals. Optically clear transparent fluorite lenses have low dispersion, so lenses made from it exhibit less chromatic aberration, making them valuable in microscopes and telescopes. Fluorite optics are also usable in the far-ultraviolet range where conventional glasses are too absorbent for use.

Native Americans carved fluorspar to make artifacts, but the first recorded use of Illinois' fluorite was in 1823, when fluorspar mined near Shawneetown in Gallatin County was used to manufacture hydrofluoric acid. Fluorite has also been used in jewelry, though due to its relative softness it is not widely used as a semiprecious stone. It does make fine ornamental carvings, as well.

Besides its industrial uses, fluorite is widely collected by mineral enthusiasts who appreciate its wide range of colors, sharp geometric formations, and lovely display esthetics. Southern Illinois became known for producing some of the world's finest fluorite specimens after the Minerva Mine was sunk in 1942, reaching a depth of 680 feet deep. In 1979 one of the biggest specimen producers, the Denton Mine, began operations, and in 1984 another great source, the Annabelle Lee Mine, opened, eventually reaching a depth of nearly 1,000 feet. Fluorite was declared the state mineral of Illinois in 1965.


6.1" Purple Phantom Fluorite Sharp Cubic Crystals - Annabel Lee Mine

MINERALOGY
Fluorite (also is a halide mineral made up of calcium and fluorine, CaF2. The word fluorite derives from the Latin noun fluo, meaning to flow. In 1852 fluorite gave its name to the phenomenon of fluorescence, which is prominent in fluorites from certain locations, due to certain impurities in the crystal. Ironically, fluorite from Illinois does not usually fluoresce.


4.6" Purple & Blue Phantom Fluorite Sharp See through Cubic Crystals - Denton Mine

Crystal Habits
Fluorite occurs commonly as cubic, octahedral and dodecahedral crystals. Crystals may be large and penetration twins are fairly common. Almost all fluorite from Illinois is cubic in habit.

Color
Fluorite comes in a wide range of colors and been called "the most colorful mineral in the world". The most common colors are purple, blue, green, yellow, or colorless. Less common are pink, red, white, brown, black, and nearly every shade in between. Color zoning or banding is commonly present. The color of the fluorite is determined by factors including impurities, exposure to radiation, and the size of the color centers. Fluorite's colors are caused by the absorption of certain wavelengths of light, while other wavelengths are allowed to pass through.


Blue with Purple Phantom Fluorite Translucent Cubic Crystals - Denton Mine

Phantoms
Illinois fluorite is renowned for its colorful phantom formation. Phantoms are common in fluorite, showing up as different color bands inside the crystal. The phantoms are visible as a sequence of larger and larger cubes visible through the outside layer of the crystal. The bands can be an inch or more thick, or only a fraction of a millimeter. They are most frequently caused by changes in the chemistry of the hydrothermal fluids that are deposited as fluorite. These fluids can come from great depths and are carrying a variety of elements in solution. Other elements can be added to the mix by the rock they are passing through, too.


3.7" Yellow with Purple Phantoms Cubic Fluorite Crystals - Minerva Mine

FUTURE VALUES
Often, when a mine closes which has produced classic mineral specimens during its operation, closing the prices for mineral specimens shoot up. For example, when the Red Cloud Mine closed, prices doubled overnight and for good specimens tripled or quadrupled. Similarly, when the Sweet Home Mine was shut down, prices immediately doubled, and then kept going up every year. Today, the $1500 specimen I lusted after at Collector's Edge in the fall of 2000 (but couldn't afford) would have a price tag of around $15,000 (a tenfold increase). But fluorite from the mines in Illinois took a different path. When the mines closed in 1995, there was no increase in prices, because there was so much material stockpiled in the "pipeline", which runs from the mine to the wholesaler to the dealer, that prices stayed about the same for years. Finally, in 2012, prices began to go up, and in the next 3 years reached shockingly high levels - especially for the really fabulous specimens, which are now up about 5 times what they were just 5 years ago. Will they continue to go up? Who knows! But if current trends in mineral collecting continue, prime Illinois fluorite will be a very good investment, indeed.


6.1" Imperial Purple Fluorite Penetrating Twin Crystals - Denton Mine

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