Fluorite

• Calcium fluoride

Fluorite, also known under the name of fluorspar mining or its chemical name calcium fluoride, is the calcium salt of hydrofluoric acid and a very common mineral from the mineral class of simple halides. Fluorite is crystallized in a cubic crystal system having the chemical composition CaF2 and developed cubic crystals with predominantly würfeliger or octahedral crystalline form, as well as rare penetration twins, but also granular, massive aggregates.

Pure Fluorite is colorless and transparent, impurities also gray. He may, however, by foreign admixtures almost all colors, mostly in low intensity accept. Spreads are green, violet to black violet and yellow crystals ( " Honigspat "), but also blue, red and brown Fluorite be found. Likewise, it can be observed frequently zonarer color change. The stroke color, however, is always white.

Fluorite is the Leitmineral ( mineral scale ) the Mohs scale of hardness 4

• 7.1 The raw material
• 7.2 As a gemstone

Special Features

By incorporation of lanthanides, for example, Eu2 , fluorite can show under excitation with UV light strong fluorescence and phosphorescence by heating, as well as high mechanical stress Triboluminescence.

Contact with strong acids such as sulfuric acid is highly toxic Fluorite free hydrogen fluoride. However, a direct toxicity of the fluorite is not known.

Fluorite is an electrical insulator.

Color

Although pure CaF2 is colorless, belongs to the fluorite minerals with the most color variation at all. The dark color of many Fluorite created by embedded rare earth or radioactive irradiation of fluorspar ( Stinkspat ), where ingrown uranium minerals can enhance the color.

The color causes are varied and not always fully understood. Coloring act mostly trace amounts of rare earth elements, which are often ionized only by radioactive irradiation coloring ions. Which rare earth elements are ionized this may depend on the type of radiation. This allows other colors develop as in the area of uranium-bearing minerals Fluorite with equal levels of trace elements in the environment of thorium-containing minerals. Furthermore, the temperature history can affect the color and the incorporation of oxygen ions and OH - ions or other coloring. Not trace amounts of coloring ions such as Na or Fe3 stabilize coloring lattice defects and affect so also the color.

• Yellow: The color yellow fluorites based on the incorporation of O3 and O2, instead of two adjacent F- .Ionen. The charge compensation is done via the replacement of Ca2 by Na .

• Light green: The bright green color of many Fluorite are due to trace levels of Sm2 . Samarium (Sm ) is incorporated as Sm3 instead of Ca2 . Reduction to Sm2 is carried out by inclusion of an electron, which is released by radioactive radiation in the oxidation of other cations. The stability of the green color depends on which cations are oxidized. Fluorite formed under reducing conditions contain traces of Fe2 , which can be oxidized to Fe3 and produced together with samarium a temperature-stable green color. In under more oxidizing conditions fluorites formed the production and stabilization of green coloring Sm2 ions takes place by oxidation of Ce (Pr, Tb) 3 to Ce (Pr, Tb ) 4 . Fluorite, which are colored by this mechanism, fade in daylight or heating.

• Green: Fluorite some localities ( Redruth in England, Tübingen in Germany ) show a green to yellow-green color, which to the common occurrence of traces of yttrium (Y3 ) and cerium ( Ce 3 ), each with a color center (space on a F - position, in which there are two electrons) due in the neighborhood.

• Light Blue: Fluorite containing only Y3 , along with a blank space on a F - position in which there are two electrons are colored light blue.

• Dark blue: Synthetic Fluorite may be ( metallic) intense blue to violet by the formation of colloidal calcium. The blue-violet color of the naturally occurring 'Blue John Fluorite " from Castleton in Derbyshire in England is attributed to the fact.

• Violet: The cause of the widespread violet staining of natural fluorite is not fully understood. Currently electrons are defects in the crystal lattice as a likely cause of the violet color of fluorite.

• Pink, Red: The pink to red color of fluorite is by installing O23 - causing molecules that are stabilized by neighboring Y3 ions.

• Color variations

Blue Violet

Light Blue

Blue Green

Light Green

Yellow

Orange

Red

Pink

Colorless

Stinkspat ( Antozonit )

As Stinkspat is called a dark purple to black variety of fluorite, which develops a pungent odor during crushing. Stinkspat is often (but not always) before, together with uranium minerals that can be incorporated as fine particles in Stinkspat partially. The type locality and most famous German reference is Wölsendorf in the Upper Palatinate.

By rubbing or hitting the crystal gaseous toxic fluorine ( F2) is released, which causes the odor.

The dark purple to black color has several causes. Plays a major role colloidal metallic calcium, which leads to a dark blue to black coloration. There are also free electrons on an empty positions fluorine (F- centers), which are typical of purple fluorite.

All these properties of the Stinkspates are caused by radioactive irradiation of fluorite. Stinkspat typically occurs along with uranium-bearing minerals. The uranium and thorium contained in it falls apart, stating gamma radiation. This radiation electrons from the F - ions and forms an H center, a neutral fluorine atom on an otherwise empty grid position, an atom bond to a neighboring F - ion is formed. The released electrons are trapped by lattice defects, empty fluorine positions and form F- centers, single electrons at a F - position, surrounded by four Ca2 ions. These F- centers are not spatially stable. They diffuse through the crystal lattice and connect with other F- centers to fluorine-free Ca nanoparticles with 5-30 nm in diameter. These clusters are referred to as " colloidal Ca " and contribute to the blue-black color of the Stinkspates at.

Etymology and history

Fluorspar was known in ancient Greece. The German name goes back to use as a flux in the metal processing. 1824 German mineralogist Friedrich Mohs, discovered in the ultraviolet light becomes visible fluorescence. The Irish mathematician and physicist George Gabriel Stokes named the phenomenon of fluorescence after fluorite, in analogy to the opal opalescence.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the fluorite belonged to the general ward of a "Simple halides ," where he along with Coccinit, Frankdicksonit, Gagarinit - (Y), Laurelit, Tveitit - (Y) and Gagarinit - (Ce) (formerly Zajacit - (Ce) ) formed a separate group.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz 's Mineral classification assigns the fluorite in the new and more precise division of a "Simple halides, without H2O". This division is, however, still further subdivided by the molar ratio between metal (M) and halide ( X), so that the mineral according to its composition in the subsection "M: X = 1: 2" to find where it only together with the hypothetical mineral Cerfluorit and the Frankdicksonit forms the unnamed group 3.AB.25.

The commonly used in English-speaking classification of minerals according to Dana assigns the fluorite in the class of " halides ( and relatives ) " and there in the department of " halides ". Here he is the eponymous mineral " Fluoritgruppe " with the system no. 09:02:01 and the other members Frankdicksonit, Tveitit - (Y) and Strontiofluorit within the sub-division of " water clear and hydrated halides of the formula AX2 ".

Education and Locations

Fluorite is usually massive, sometimes in crystalline form and occurs primarily in corridors and is often associated with barite, quartz, topaz, calcite, galena and sphalerite. He is occasionally Nebengemengteil in granites, carbonatites and other igneous rocks such as pegmatites.

The world's largest fluorspar mining is found in Mexico, in the Las Cuevas deposit, which is of volcanic origin. More rich fluorspar deposits are located in China, India Amba Dongar, in South Africa ( Zwartkloof and Witkop in Transvaal ), Namibia ( Okorusu ), in the Kenyan Kario Valley and in the U.S. states of Illinois and Kentucky. A well-known archaeological site of fluorspar in Europe are the mountains and caves around Castleton, in the English Peak District, where it is known under the name " Blue John " and is mined for jewelry production. The name is a corruption of the French "bleu et jaune ", that means " blue - yellow".

German deposits are, for example, the Clara mine in Oberwolfach in the Black Forest and the Pit Bug increase in Pforzheim; next to it is fluorite in the Upper Palatinate, between Nabburg and black box, in Schortetal in Ilmenau in the Thuringian Forest (see also: Mine Full Rose), in the Ore Mountains near Freiberg and bears stone, in the Vogtland at Schönbrunn ( Vogtländische Flußspatwerke or Patriot) and in eastern Harz at Strassberg. In small amounts, he found himself in many mineral deposits in Germany.

In Austria, for example, the - by now exhausted - occurrence Vorderkrimml known today Schaustollen.

Morphology

Fluorite is often well-educated, cubic and octahedral crystals. Combined with these main forms fluorite often show areas of other shapes. Are widespread areas of the rhombic dodecahedron { 110}, { 210 } Tetrakishexaeders ( additional surfaces parallel to the cube edges ), and also the Ikositetraeders {211 } or { 311 } and { 421 } Hexakisoktaeders.

The crystal habit of fluorite is temperature dependent. Thus, at high forming temperatures occur mainly octahedron {111 }, at intermediate temperatures rather rhombic dodecahedron { 110} and at low temperatures cube {100 } as the dominant forms costume.

The cube faces are usually smooth and shiny. Octahedron and Rhombendodekaederflächen contrast, often appear rough and dull and are then usually composed of tiny cube faces. The commercially popular loose, octahedral Fluorite with smooth shiny surfaces, it almost never is in this form grown crystals, but Spaltoktaeder.

Furthermore, fluorite forms spherical and botryoidal aggregates, crusts or stalactites.

• Crystal forms and aggregates

Cube { 100}

Octahedron {111 }

Octahedron {111 }

Cube and rhombic dodecahedron {100 }, { 110}

Cube and octahedron ( cuboctahedron ) {100 }, { 111}

Octahedron, cube and rhombic dodecahedron {111 }, { 100 }, { 110}

Rhombic dodecahedron, octahedron, cube {110 }, { 111 }, { 100}

Spherical Fluorite

Grape -shaped fluorite

Crystal structure

Fluorite crystallizes in the cubic crystal system in the highly symmetric crystal class m3m ( cubic hexakisoktaedrisch ) or the space group Fm3m. In the crystal structure of the Ca2 ions form a cubic close packing of spheres tested that a face-centered cubic lattice (fcc, face centered cubic ) corresponds. The Flächenzentrierung the unit cell (see picture left) can also be read from the space group symbol (" F"). The fluoride ion (F- ) occupy all the tetrahedral holes of the close packing of calcium ions. Since in a closest packing as many tetrahedral holes as packing particles are always included twice, resulting in the structure of a calcium - fluoride ratio of 1:2, which is also reflected in the chemical formula of fluorite, CaF2, reflects. A coordination polyhedron is obtained for the fluoride ions by a tetrahedron of four calcium ions, the calcium ions are in turn surrounded by eight fluorine atoms in the form of a cube. The cation and anion sublattice are not commutative, ie interchangeable. The so-called fluorite structure can be found in a number of other salts, such as in the fluorides SrF2, BaF2, CdF2, HgF2 and PbF2. The fluorite structure is found for example in Li2O, Li2S, Na2O, Na2S, K2O, K2S, Rb2O and Rb2S. Its crystal structure is isotypic with uraninite.

Use

The raw material

Industrial Fluorite is mainly used

• As Hüttenspat in the metal industry as a flux for slag in metallurgical process, particularly as aggregate for open-hearth furnace and electric furnace, and the manufacture of artificial cryolite for aluminum extraction,
• As for the preparation of acid grade fluorspar fluoride and hydrofluoric acid, and various fluorides and derivatives such as hydrofluorocarbons and polymeric fluorine compounds (such as polytetrafluoroethylene),
• As a ceramic Spat in the glass industry as a flux and opacifier for example, Milk glass, frosted glass and opalescent glasses, for ceramic materials and as a raw material for optical lenses (CaF2 single crystals, fluoride glasses based on beryllium, fluorite, and sodium fluoride). Due to the ability to refract the light spectrum evenly, can the chromatic aberration of lenses compensate. The problem here is that for high -performance lenses particularly large crystals are required, they are artificially bred. Crystals of this size have the property is already becoming so warped by heat ( solar radiation) that they significantly alter the bill's appearance.

As a gemstone

Due to its rather low hardness fluorite is rather uninteresting as a gemstone for the commercial jewelry industry. Occasionally, it is processed by Glyptikern and hobby grinders to small, decorative arts objects or faceted gemstones.

Since it can be but confused by its variety of colors with many gem minerals, he often serves as the basis for imitations. To change the color, fluorite is either blown or irradiated. Be to cover to prevent damage or damage caused by splitting cracks Gemstones Fluorite often stabilized with resin (see also gemstone ).