Fluorite with color zonation from Illinois, USA. (cut slice)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are about 5400 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The halides are the "salt minerals", and have one or more of the following anions: Cl-, F-, I-, Br-.

Fluorite is a calcium fluoride mineral (CaF2). The most diagnostic physical property of fluorite is its hardness (H≡4). Fluorite typically forms cubic crystals and, when broken, displays four cleavage planes (also quite diagnostic). When broken under controlled conditions, the broken pieces of fluorite form double pyramids. Fluorite is a good example of a mineral that can be any color. Common fluorite colors include clear, purple, blue, green, yellow, orange, and brown. The stereotypical color for fluorite is purple. Purple is the color fluorite "should be". A mineral collector doesn't have fluorite unless it's a purple fluorite (!).

Fluorite occurs in association with some active volcanoes. HF emitted from volcanoes can react with Ca-bearing rocks to form fluorite crystals. Many hydrothermal veins contain fluorite. Much fluorite occurs in the vicinity of southern Illinois (Mississippi Valley-type deposits).

The fluorite specimen shown above is from a Mississippi Valley-type deposit in southern Illinois. Commonly abbreviated "MVT", Mississippi Valley-type deposits are named for a series of mineral deposits that occur in non-deformed platform sedimentary rocks along the Upper Mississippi River Valley, USA. Many specific minerals occur in MVT deposits, but are dominated by galena, sphalerite, barite, and fluorite. These minerals occur in caves and karst, paleokarst structures, in collapse fabrics, in pull-apart structures, etc. MVT deposits in America are mined as important, large sources of lead ore and zinc ore. The classic areas for MVT deposits are southern Illinois, the tristate area of Oklahoma-Missouri-Kansas, northern Kentucky, southwestern Wisconsin, and southeastern Missouri. The minerals are hydrothermal in origin and were precipitated from basinal brines that were flushed out to the edges of large sedimentary basins (e.g., the Illinois Basin and the Black Warrior Basin). In basin edge areas, the brines came into contact with Mississippian-aged carbonate rocks (limestone and dolostone), which caused mineralization. The brines were 15% to 25% salinity with temperatures of 50 to 200 degrees Celsius (commonly 100 to 150 degrees C). MVT mineralization usually occurs in limestone and dolostone but can also be hosted in shales, siltstones, sandstones, and conglomerates. Gangue minerals include pyrite, marcasite, calcite, aragonite, dolomite, siderite, and quartz. Up to 40 or 50 pulses of brine fluids are recorded in banding of mineral suites in MVT deposits (for example, sphalerite coatings in veins have a stratigraphy - each layer represents a pulse event). Each pulse of water was probably expelled rapidly - overpressurization and friction likely caused the water to heat up. Some bitumen (crystallized organic matter) can occur, which is an indication of the basinal origin of the brines. The presence of asphalt-bitumen indicates some hydrocarbon migration occurred. Some petroleum inclusions are found within fluorite crystals and petroleum scum occurs on fluorite crystals. MVT deposits are associated with oil fields and the temperature of mineral precipitation matches the petroleum window. The brines may simply have accompanied hydrocarbon fluids as they migrated updip.

The high temperatures of these basin periphery deposits wasn't necessarily influenced by igneous hydrothermal activity. Hot fluids can occur in basins that are deep enough for the geothermal gradient to be ~100 to 150 degrees Celsius. If a permeable conduit horizon is present in a succession of interbedded siliciclastic sedimentary rocks, migration of hot, deep basinal brines may be quick enough to get MVT deposit conditions at basin margins.

MVT deposits occur in the Upper Mississippi Valley of America as well as in northern Africa, Scandinavia, northwestern Canada, at scattered sites in Europe, and at some sites in the American Cordillera. Some of these occurrences are in deformed host rocks. MVT deposits have little to no precious metals - maybe a little copper (Cu). Mineralization is usually associated with limestone or dolostone in fracture fillings and vugs. Little host rock alteration has occurred - usually only dolomitization of limestones.

The age of the host rocks in the Mississippi Valley area varies - it ranges from Cambrian to Mississippian. Dating of mineralization has been difficult, but published ages indicate a near-latest Paleozoic to Mesozoic timing.

MVT deposits in the Upper Mississippi River area are often divided into three subtypes based on the dominant mineral: 1) lead-rich (galena dominated); 2) zinc-rich (sphalerite dominated); and 3) fluorite-rich.

The fluorite shown here is from the Illinois-Kentucky Fluorspar District ("fluorspar" is a very old name for fluorite), which is an MVT fluoritic subtype. Fluorite and fluorite-rich rocks are mined for the fluorine, which is principally used by the chemical industry to make HF - hydrofluoric acid. Fluorite mineralization in this district occurred at about 277 Ma, during the Early Permian, according to one published study (Chesley et al., 1994). Another study concluded that fluorite mineralization was much later, during the Late Jurassic (see Symons, 1994).

Locality: unrecorded quarry near the town of Cave-in-Rock, far-southern Illinois, USA

Photo gallery of fluorite: <a href="http://www.mindat.org/gallery.php?min=1576" rel="nofollow">www.mindat.org/gallery.php?min=1576</a>

Some info. on Mississippi Valley-type deposits was synthesized from:

Chesley et al. (1994) - Direct dating of Mississippi Valley-type mineralization: use of Sm-Nd in fluorite. Economic Geology 89: 1192-1199.

Symons (1994) - Paleomagnetism and the Late Jurassic genesis of the Illinois-Kentucky fluorspar deposits. Economic Geology 89: 438-449.

Rakovan (2006) - Mississippi Valley-type deposits. Rocks & Minerals 81(January/February 2006): 69-71.