FLUORITE INFORMATION
|
Quick |
Facts |
| Group: |
Halides |
| Crystal system: |
Cubic |
| Chemical formula: |
CaF2 |
| Hardness: |
4 |
| Density: |
3.18 |
| Cleavage: |
Perfect |
| Fracture: |
Conchoidal |
| Colour: |
Colourless (allochromatic) |
| Streak: |
White |
| Lustre: |
Vitreous |
| Fluorescence: |
Pink or
violet |
Fluorite, sometimes still known by its
old name, fluorspar, is variable in colour. It is most often
blue, green, purple or yellow, but can sometimes be black, pink, red
or altogether colourless. In addition, individual specimens
may be banded or "colour-zoned". The name comes from the Latin fluere, meaning "to flow": this is principally a reference to the
mineral's low melting point. Fluorite has a wide range of
industrial applications - it is used to make opalescent glass and is
a component of iron and steel enamelware; it is also a flux in the
manufacture of some forms of steel. Most importantly, fluorite
is the main source of fluorine gas and hydrofluoric acid. Fine
specimens of blue fluorite, known as blue john, came from
Derbyshire, England, and were widely used in the 19th century to
make stunning vases.
Fluorite is a member of the
halide group of minerals. Each molecule of the mineral is made
up of one atom of metallic calcium joined to two atoms of fluorine.
The basic structure of fluorite crystals is cubic, too, although
eight-sided and 12-sided shapes may also be found. Crystals
often run into each other in the phenomenon known as twinning.
Fluorite is a fairly soft mineral, scoring only 4 on the Mohs Scale
of hardness - it can e easily scratched with the blade of an
ordinary knife. It is extremely sensitive to heat and melts
(fuses) easily in an open flame. Fluorite is usually
fluorescent, showing pink or violet under an ultra-violet light.
However, the strength of this effect can vary enormously from
specimen to specimen.
Although Fluorite
is primarily an industrial mineral, some crystals are gem-quality.
However, even the finest samples have limited use because they are
not hard enough to withstand the wear and tear of everyday use.
Yet, although most fluorite is fashioned for display in
mineralogists' collections, some pieces are used in jewellery.
These are typically cut in cabachons and then capped with a
protective outer layer of a form of quartz called rock crystal.
In addition to being more resilient than fluorite, rock crystal is
translucent and consequently the colours of the fluorite can easily
be seen through it. Fluorite is also used to make carvings and
brooches with carved designs called intaglios. When fluorite
is faceted, the usual cutting angles are 40-50 degrees along the
crown surfaces and 43 degrees on the pavilions. Fluorite is
usually quite pure, but in some specimens as much as 20% of the
calcium in the mineral may be replaced by metallic yttrium.
This variant form is usually violet, grey or reddish brown in colour
and is known as yttrian fluorite. Less frequently, some of the
calcium in fluorite may be replaced by another metallic element,
cerium.
As its name
implies, fluorite may show a strong fluorescence under ultra-violet
light. Specimens are often covered with layers of calcite.
These can be removed with dilute hydrochloric acid. Iron
stains are also common and can be removed by wiping the surface with
oxalic acid. Importantly, all acids should be treated with
great care and never used without adult supervision. No
attempt should be made to clean specimens of yttrian fluorite.
Fluorite is a
widespread mineral found in a large number of mineral veins which
have been altered by hot water (hydrothermals) and around hot
springs. It may form on its won or in association with various
metallic ores, especially those of lead and silver. In
deposits of the latter type it tends to form in small quantities and
is known as a gangue mineral. Among the many minerals with
which fluorite may be associated in this way are barite, calcite,
cassiterite, celestine, dolomite, galena, quartz, sphalerite and
topaz.
The following are some of the main locations in which
substantial deposits of fluorite have been found: Freiberg
(Germany); Bolzano, Lazio and Tuscany (Italy); and St Gotthard
(Switzerland). In the United States, fluorite is found at
Crystal Peak, Colorado, and in Harden County, Illinois.
Yttrian fluorite is found in Sussex County, New York State; the
other main source of this form of the mineral is Falun (Sweden).
Fluorite is reasonably common all over the British Isles, and is
principally mined in the following places: Cleator Moor, Cumbria;
Derbyshire; Devon; Weardale, Durham and the lead veins of
Northumberland. The beautiful blue john can still be found,
but the main veins are now worked out and it is no longer mined on
an industrial scale. |
