Blue Johns

A small but artistically interesting use of fluorspar is ia the productioa of vases, cups, and other ornamental objects popularly known as Blue John, after the Blue John Mine, Derbyshire, U.K. Optical quaUty fluorite, sometimes from natural crystals, but more often artificially grown, is important ia use as iafrared transmission wiadows and leases (70) and optical components of high energy laser systems (see Infrared and RAMAN spectroscopy Lasers) (71). 

The name Blue John is a corruption of the French term bleu-jeune, which was used to describe the blue... 

The fluorite structure is named after the mineral form of calcium fluoride, CaF2, which is found in the U.K. in the famous Derbyshire Blue John mines. The structure is illustrated in Figure 1.39. It can be described as related to a ccp array of calcium ions with fluorides occupying all of the tetrahedral holes. There is a problem with this as a description because calcium ions are rather smaller than fluoride ions, and so, physically, fluoride ions would not be able to fit into the tetrahedral holes of a calcium ion array. Nevertheless, it gives an exact description of the relative positions of the ions. The diagram in Figure 1.39(a) depicts the fourfold tetrahedral coordination... 

Fluorite is a ubiquitous mineral and is so widespread in ils occurrence that only the most noteworthy can be mentioned. The English localities at Cumberland. Durham, and Weardale are world famous. Exceptionally heauiiful handed material of hlue fibrous character fruin Derbyshire, known as Blue-John, has been much-used for decorative curved pieces, such as vases and other ornamental objects. Norway has produced exceptional specimens from the famous Kongshcrg silver veins, as well as yttrium-rich fluorite from northern Norway associated with rare-earth minerals. Fine material has been obtained from the Transvaal in the Republic of South Africa, Tasmania and Australia. Large quantities of fluorite are mined in Mexico at Guadalcazar and Guanajuato. 

Fortunately, these crystals eventually ended up in the hands of artists who converted them into vases, cups, holders and other ornamental objects which magnify our show rooms, living rooms or conference tables. These objects are popularly known as Blue Johns , named after the Blue John mine in Derbyshire, England which operated in the 18th Century. 

The electron produced in oxidation step 27.1 remains trapped in the crystal lattice and occupies a lattice site, leaving a CD site vacant. Excitation and subsequent relaxation of the electron results in the emission of radiation in the visible region. The electron centre is known as an F-centre (from the German Farbe for colour). The origins of F-centres are varied, but their presence has some dramatic consequences. For example, some variants of transparent minerals are coloured owing to the presence of F-centres, e.g. Blue John is a rare blue-purple form of fluorspar and is much prized in jewellery and decorative ornaments. 

In Napoleonic times Derbyshire fluorspar was exported to France where it was termed the bleu jaune or blue-yellow stone it was shaped into fancy articles which were subsequently re-imported into Britain as the anglicised Blue John. 

The mineral CaF2 is known as fluorite, fluorspar, and Blue John. The ionic radii are rca + = 100 pm and fp- = 130pm, so Tca /rp- is -0.8. By Pauling s rules the ions should have CN = 8 and the F" ions should have CN = 4. Since the fluoride ions are larger, we should think of the structure as a simple cubic stacking of the F ions with the Ca ions filling every other cube interstice. However, you may remember the structure better by arranging the Ca ions on an fee... 

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