File:Fluorite (Late Paleozoic; Elmwood Mine, near Carthage, Tennessee, USA) 2 (33750777303).jpg

Fluorite from the Late Paleozoic of Tennessee, USA.

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 over 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 Tennessee. 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.

This Tennessee fluorite occurs well east of the Mississippi River Valley, but it is from an MVT deposit. It is derived from the Central Tennessee Ba-F-Pb-Zn District (a.k.a. Central Tennessee Zinc District), which is an MVT deposit in the Nashville Dome (a large regional upwarped feature south of & along strike with the Lexington Dome and the Cincinnati Arch). Mineralization occurred as heavy metal-rich basinal brines migrated updip from the Illinois Basin or the Appalachian Basin during the Late Paleozoic. Host rocks are shallow marine dolostones of the Lower Ordovician Knox Group, principally dissolution collapse-brecciated and karstified, dolomitized limestones of the Mascot Dolomite. Mineralization in the Central Tennessee Mining District has been dated to the Early Mississippian or Late Permian, associated with the Allegheny/Ouachita Orogenies. Main-stage calcite dates to 260±42 Ma.

Locality: Elmwood Mine, south/southeast of the town of Carthage, south-central Smith County, north-central Tennessee, USA

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