Fluorite

Many people mistakenly assume that the mineral fluorite got its name from the element fluorine, but the reverse is true. The element fluorine actually derives its name from fluorite and is a deadly poison that killed or permanently injured a number of scientists involved in its original isolation and identification.

From improved dental health via fluoridated water and toothpaste to the development of nuclear energy and atomic bombs, fluorite’s legacy spans the spectrum of fluorine use in our society across medical, chemical and industrial fields. Although fluorine occurs in a variety of minerals, fluorite is our main source of fluorine simply because of its abundance and high fluorine content.

Description and Identifying Characteristics

Although fluorite is not particularly abundant, it is one of the more familiar minerals because it forms well-developed crystals in a variety of colors that are widely collected and traded. These crystals are transparent to translucent and form distinctive cubic or octahedral crystals (similar in shape to 6-sided and 8-sided dice). Fluorite that forms from higher temperature fluids tends to occur as octahedral crystals, while fluorite that forms from lower temperature fluids tends to occur as cubic crystals. Pure fluorite crystals are clear, so the many colors fluorite crystals come in simply reflect a variety of minor impurities in the crystal’s chemical composition that tint the crystal. Some fluorite crystals display bands of color in complex patterns that record subtle changes in the chemistry of the fluids from which the crystal formed. Although fluorite crystals occur in a range of colors, dark purple fluorite crystals are probably the best-known fluorite variety as they are often prominently displayed in museum galleries and mineral book photos. These dark purple or violet crystals are easily confused with amethyst (purple quartz), although fluorite’s distinctive crystal shape, four planes of cleavage, and its relative softness can distinguish the two.

Fluorite exhibits perfect cleavage in four directions to form octahedrons. It is most often found as a pore-filling void or vein deposit, so the fluorite crystals tend to occur as a distinct layer or series of layers lining the walls of what was originally a void in the rock.

One of fluorite’s more unusual properties is that it will fluoresce, or glow, under ultraviolet light. It was one of the first materials in which the phenomena of fluorescence was recognized and studied, and the term ‘fluorescence’ actually comes from the mineral name ‘fluorite’.

Fluorite primarily occurs as a pore-filling mineral in carbonate rocks (limestones and dolostones), as low temperature hydrothermal vein deposits associated with lead and silver ores, and less often as an accessory mineral in pegmatites and granites. It is sometimes even precipitated at hot springs.

Limestone and dolostone rocks through which low temperature hydrothermal fluids have moved may be particularly rich in fluorite.  In the past, carbonate rocks of the Mississippi River Valley hosted some of the world’s richest fluorite deposits, but most of these have since been mined away. When fluorite occurs as a cavity fill in carbonate rocks it is usually associated with calcite, dolomite, anhydrite, gypsum and sulfur. In hydrothermal vein deposits, fluorite may be found with calcite, dolomite, barite, galena, sphalerite, and even silver ores.

The name ‘fluorite’ comes from the Latin word ‘fluere’ that means “to flow.” Fluorite melts easily and is used as a flux in the smelting of metallic ores. A ‘flux’ is a material used to help remove impurities from ore as it is refined into metal. As such, large amounts of fluorite are extensively used in open-hearth steel and steel enamelware production, the smelting of aluminum, as well as lead and antimony refining. It is also used in the manufacture of some types of glass, enamel, and porcelains.

The element fluorine has many uses in our industrial society, and fluorite is our major source of fluorine. Consequently large amounts of fluorite are used in the production of hydrofluoric acid and the manufacture of high-octane fuels, as well as the manufacture of flat panel displays and semiconductors. The first large scale manufacture of fluorine occurred during WWII, when it was used to separate and enrich the uranium needed for the Manhattan Project’s atomic bomb program. For better or worse, fluorine helped usher us into an atomic age and is still used to enrich uranium for nuclear power plants and weapons. Fluorine is also used to create a variety of non-reactive fluorocarbon resins that are used to line pipes, tanks and even cooking utensils. One of the latter is best known under the trade name of Teflon.

At one time, fluorite was also used in the manufacture of chlorofluorocarbons (CFC’s) that were used as refrigerants and in aerosol sprays, but more recently this use has been curtailed because of concerns over the role of CFC’s in the reduction of the Earth’s ozone layer. On a more beneficial side, fluorite is the source of fluoridated water and the fluoride compounds used in toothpaste and mouthwash that help to reduce dental cavities.

You might wonder exactly how fluoride treatments help prevent tooth decay and the answer lies in the effect of fluorine on another mineral called apatite. Apatite is a phosphate mineral that makes up much of our bones and teeth.  When the bacteria that live in plaque on teeth are exposed to sugar, they produce weak acids that can dissolve some of the apatite present in enamel to create a porous tooth surface. If this process continues, eventually a ‘cavity’ forms. Apatite occurs in three main varieties and the one that naturally makes up the enamel layer of our teeth is called hydroxyapatite. If fluorine is present in your saliva, which it is after using fluoride toothpaste or drinking fluoridated water, then the apatite of the tooth enamel can alter to fluorapatite. Fluorapatite crystals are larger than hydroxyapatite crystals, so their relative surface area is less and they do not dissolve as easily as hydroxyapatite crystals. Hence teeth with enamel composed largely of fluorapatite will have far fewer cavities!

Fluorite is too soft to be used as jewelry, but the Romans mined a massive crystalline variety of fluorite, later known as Blue John or Derbyshire spar, that exhibited a distinctive color banding of blues, violets and purples. This banded fluorite was carved to create bowls, vases and ornamental objects that were extensively traded across Europe and the Mediterranean area. Although the Derbyshire deposits are now largely mined out, other varieties of fluorite are still among the most actively traded and collected mineral samples, as they occur in a wide range of colors and forms. As an example, for centuries the Chinese mined a green variety of fluorite, erroneously called ‘green quartz’, for carvings and figurines.

Clear fluorite is among the rarest of the many fluorite varieties and was once used in the manufacture of optical lenses. Modern fluorite lenses, though, are usually from commercially manufactured fluorite, rather than natural clear crystals.

Fluorine taken from fluorite is an extremely reactive chemical. In small quantities it can provide medical benefits such as reduced dental cavities, but in larger quantities fluorine can be quite dangerous. Fluorine gas is a deadly poison. Combined with hydrogen, fluorine makes hydrofluoric acid, an acid so powerful that it is used to etch glass.

Perhaps fluorine’s most potent threat though, comes from its use in compounds that can be handled with little to no risk. Fluorine is used to create chlorofluorocarbons, known as CFC’s. CFC’s were once widely used as the cooling fluid of refrigerators and air-conditioners, as well as the propellant for aerosol cans of paint and hairspray. They are inert compounds that do not react with other substances, are non-toxic and initially appeared to pose little risk to human populations. Consequently they were widely used before researchers realized that CFC’s released into the atmosphere pose a serious threat to the atmospheric ozone layer that protects the Earth’s land life from harmful ultraviolet radiation that can cause cancer and mutations. As ozone naturally forms and breaks apart, it absorbs ultraviolet radiation from the sun, thus shielding the Earth’s surface (and much of its life) from this harmful radiation. However, CFC’s in the high atmosphere help to speed up the process of ozone breaking down, so that much less ultraviolet radiation is absorbed. The radiation that is no longer absorbed can reach the Earth’s surface and poses a particular threat to populations living in high latitude areas, where this process of ‘ozone depletion’ is the most effective. International agreements have now banned the manufacture and use of CFC’s in many nations, but unfortunately this ban is not global so CFC’s still pose a potent, long-term environmental risk.

Thick widespread carbonate rock units of the Upper Mississippi River Valley once hosted large amounts of fluorite. These deposits formed as hydrothermal fluids moved along fractures in the carbonate rock, altering or dissolving the carbonate minerals and precipitating a wide variety of metallic ores and pore-filling minerals, including fluorite. Most of the richest fluorite deposits have now been mined out, but some fluorite can still be found, especially in Illinois’ Hardin and Pope Counties.

Fluorite commonly forms transparent to translucent crystals that can initially be mistaken for a variety of other minerals. Misidentifications may also arise due to the wide range of colors that fluorite can exhibit. However, fluorite’s intermediate hardness, perfect cleavage in four directions, and common fluorescence should allow it to be easily distinguished from other minerals that share a similar appearance.

Quartz:

At first glance, fluorite crystals may be mistaken for quartz crystals, especially the purple variety of quartz known as amethyst. However, quartz is much harder than fluorite. It will scratch glass and, unlike fluorite, it cannot be easily scratched by metal. Quartz also lacks any cleavage and breaks in a conchoidal manner, while fluorite has four perfect cleavage directions. In addition, fluorite will often fluoresce under ultraviolet light.

Halite:

Halite is another halide mineral that also shares a similar crystal form with fluorite. The two can be distinguished, however, by their different cleavages, their taste, and their different hardness. Fluorite cleaves in four directions to form octahedrons, while halite has perfect cleavage in three directions to form cubes, so the cleavage planes of halite always form at right angles to one another. Halite has a distinctive salty taste and is softer than fluorite, being easily scratched by a fingernail. With a hardness of 4, fluorite is more resistant to being scratched, although it is still much softer than glass or metal.

Calcite:

Calcite and fluorite can be distinguished by differences in cleavage. Fluorite cleaves in four directions to form octahedrons, while calcite cleaves in three directions to form rhombohedra. The two share a similar hardness, but calcite will readily bubble, or effervesce, in dilute acid, while fluorite does not react to dilute acid.