FLUORITE [ Halides ]
CaF2, calcium Fluoride
As a flux (hence the name) in iron smelting, a rare gemstone, a source of fluorine, as special optical lenses and a popular mineral specimen
Fluorite is a mineral with a veritable bouquet of brilliant colours. Fluorite is well known and prized for its glassy luster and rich variety of colours. The range of common colours for fluorite starting from the hallmark colour purple, then blue, green, yellow, colourless, brown, pink, black and reddish orange is amazing and is only rivaled in colour range by quartz. Intermediate pastels between the previously mentioned colours are also possible. It is easy to see why fluorite earns the reputation as “The Most Colorful Mineral in the World”.
The many colours of fluorite are truly wonderful. The rich purple colour is by far fluorite’s most famous and popular colour. It easily competes with the beautiful purple of amethyst. Often specimens of fluorite and amethyst with similar shades of purple are used in mineral identification classes to illustrate the folly of using colour as the sole means to identify minerals.
The blue, green and yellow varieties of fluorite are also deeply coloured, popular and attractive. The colourless variety is not as well received as the coloured varieties, but their rarity still makes them sought after by collectors. A brown variety found in Ohio and elsewhere has a distinctive iridescence that improves an otherwise poor colour for fluorite. The rarer colours of pink, reddish orange (rose) and even black are usually very attractive and in demand.
Most specimens of fluorite have a single colour, but a significant percentage of fluorites have multiple colours and the colours are arranged in bands or zones that correspond to the shapes of fluorite’s crystals. In other words, the typical habit of fluorite is a cube and the colour zones are often in cubic arrangement. The effect is similar to phantomed crystals that appear to have crystals within crystals that are of differing colours. A fluorite crystal could have a clear outer zone allowing a cube of purple fluorite to be seen inside. Sometimes the less common habits such as a coloured octahedron are seen inside of a colourless cube. One crystal of fluorite could potentially have four or five different colour zones or bands.
To top it all off, fluorite is frequently fluorescent and, like its normal light colours, its fluorescent colours are extremely variable. Typically it fluoresces blue but other fluorescent colours include yellow, green, red, white and purple. Some specimens have the added effect of simultaniously having a different colour under longwave UV light from its colour under shortwave UV light. And some will even demonstrate phosphorescence in a third colour. That’s four possible colour luminescence in one specimen. If you count the normal light colour too. The blue fluorescence has been attributed to the presence of europium ions (Eu+2). Yttrium is the activator for the yellow fluorescence. Green and red fluorescent activation is not exactly pinned down as of yet, but may be due to the elements already mentioned as well as other rare earth metals; also manganese, uranium or a combination of these. Even unbonded fluorine trapped in the structure has been suggested. The word fluorescent was derived from fluorite since specimens of fluorite were some of the first fluorescent specimens ever studied. The naming followed the naming precedence set by opalescence from opal; ergo fluorescence from fluorite.
Another unique luminescent property of fluorite is its thermoluminescence. Thermoluminescence is the ability to glow when heated. Not all fluorites do this, in fact it is quite a rare phenomenon. A variety of fluorite known as “chlorophane” can demonstrate this property very well and will even thermoluminesce while the specimen is held in a person’s hand activated by the person’s own body heat (of course in a dark room, as it is not bright enough to be seen in daylight). The thermoluminescence is green to blue-green and can be produced on the coils of a heater or electric stove top. Once seen, the glow will fade away and can no longer by seen in the same specimen again. It is a one shot deal. Chlorophane (which means to show green) is found in very limited quantities at Amelia Court House, Virginia; Franklin, New Jersey and the Bluebird Mine, Arizona, USA; Gilgit, Pakistan; Mont Saint-Hilaire, Quebec, Canada and at Nerchinsk in the Ural Mountains, Russia.
Fluorite has other qualities besides its great colour assortments that make it a popular mineral. It has several different crystal habits that always produce well formed, good, clean crystals. The cube is by far the most recognized habit of fluorite followed by the octahedron which is believed to form at higher temperatures than the cube. Although the cleavage of fluorite can produce an octahedral shape and these cleaved octahedrons are popular in rock shops the world over, the natural (e.g. uncleaved) octahedrons are harder to find.
A rarer habit variety is the twelve sided dodecahedron however it is never seen by itself and usually modifies the cubic crystals by replacing the edges of the cube with one flat face of a dodecahedron. The tetrahexahedron is a twenty four sided habit that is also seen modifying the cubic habit. But instead of one face replacing each cubic edge, two faces modify the cube’s edges. Occasionally combinations of a cube, dodecahedron and tetrahexahedron are seen producing an overall cubic crystal with no less that three minor parallel faces replacing each cubic edge. A fifth form is the hexoctahedron which modifies the cube by placing six very minor faces at each corner of the cube. Twinning is also common in fluorite and symmetrical penetration twins, especially from Cumberland England are much sought after by collectors.
Fluorite, as mention above, has octahedral cleavage. This means that it has four identical directions of cleavage and when cleaved in the right ways can produce a perfect octahedral shape. Many thousands of octahedrons are produced from massive or large undesirable crystals of fluorite (hopefully.) and are sold in rock shops and museum gift shops at a small cost. Fluorite mine workers are reported to sit down at lunch breaks and cleave the octahedrons for the extra cash. The octahedrons are very popular due to their attractive colours, clarity, “diamond-shaped” and low costs, but to a serious collector they are nothing more than “cleavage fragments”.
Fluorite not only is attractive in its own right but is often associated with other attractive minerals. Fluorite crystals will frequently accompany specimens of silver gray galena, brassy yellow pyrite, chalcopyrite or marcasite, golden barite, black sparkling sphalerite, intricately crystallized calcite and crystal clear quartz, even amethyst.
The origin of the word fluorite comes from the use of fluorite as a flux in steel and aluminium processing. It was originally referred to as fluorospar by miners and is still called that today. Fluorite is also used as a source of fluorine for hydrofluoric acid and fluorinated water. The element fluorine also gets its name from fluorite, fluorines only common mineral. Other uses of fluorite include an uncommon use as a gemstone (low hardness and good cleavage reduce its desirability as a gemstone), ornamental carvings (sometimes misleadingly called Green quartz) and special optical uses.
Fluorite is the most popular mineral for mineral collectors in the world, second only to quartz. Every mineral collection owned by even the newest and youngest of mineral collectors must have a specimen of fluorite. Fluorite is by far one of the most beautiful and interesting minerals available on the mineral markets.
Colour: extremely variable and many times can be an intense purple, blue, green or yellow; also colourless, reddish orange, pink, white and brown. A single crystal can be multi-coloured
Transparency: Crystals are transparent to translucent
Crystal System: Isometric; 4/m bar 3 2/m
Crystal Habits: include the typical cube and to a lesser extent, the octahedron as well as combinations of these two and other rarer isometric habits. Always with equant crystals; less common are crusts and botryoidal forms. Twinning also produces penetration twins that look like two cubes grown together
Cleavage: perfect in 4 directions forming octahedrons
Fracture: irregular and brittle
Specific Gravity: 3.1+ (average)
Other: Often fluorescent blue or more rarely green, white, red or violet and may be thermoluminescent, phosphorescent and triboluminescent
Associated Minerals: include calcite, quartz, willemite, barite, witherite, apatite, chalcopyrite, galena, sphalerite, pyrite and other sulfides
Major Occurrences: in addition to those mentioned above Cumberland, England; Spain; China; Brazil; Morocco; Bancroft, Ontario, Canada; Naica, Chihuahua, Mexico; Germany; Elmwood, Tennessee; Rosiclare, Illinois; Fort Wayne, Indiana; Pugh Quarry and Wood County, Ohio; Nancy Hanks Mine, Colorado and many other USA localities as well as many other localities from around the world
Best Indicators: crystal habit, colour zoning, hardness (harder than calcite, but softer than quartz or apatite), fluorescence and especially the octahedral cleavage