Talk:Fluorite
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Article Grading: The article is currently given low importance and concentrates on gemology / jewellry Fluorite / fluorspar is an important (strategic?), internationally traded industrial mineral commodity. Importance of article should therefore be up graded The world market price is currently set by china, and with reduced supply from china, global prices are currently rising as users seek secure supplies. It is currently the major source of fluorine and fluorides. Brief (and incomplete)list of uses: production of synthetic cryolite for aluminium smelting Flux in metal smelting and (occasionally)in cement manufacture hydrofluoric acid, used for etching, cleaning metals, as reagent in analytical chemistry Fluorocarbon plastics and rubbers e.g. Viathon & PTFE Enrichment of uranium (UF6) Refrigerant and propellant gasses Aneasthetics SSRI Anti depressants Fluorides used in anti refflective coating of lenses optical grade fluorite is used in optics for UV and in apochromatic correction of lenses. |
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The photograph of "Fluorite crystals" image:Fluorite_crystals_270x444.jpg actually shows cleaved fluorite octahedra. Dlloyd 21:36, 17 Aug 2004 (UTC)
I got to this page from a redirect via Blue John and whilst Blue John is Fluorite not all fluorite is Blue John. Given that Blue John can only be found in Castleton,Derbyshire - surely a distinction should be made? CustardJack 09:56, 21 Apr 2005 (UTC)
[edit] Alternative images
 Pig carved in fluorite, 5 cm (2 inches) long |
[edit] Colour in Fluorites
You're correct, saying that "blue john only comes from Derbyshire" is a bit like saying "only Nestle make Nescafe" I don't think anyone has patented "blue John" as a trade mark, but that is what it effectively is.
Equivelant very dark blue banded fluorite has been known to come from china and the Romans may have sourced some from the arabian peninsula.
Colour in fluorites appears to have been the subject of much debate. Early theories that the dark blue colour of some Derbyshire fluorites was due to hydrocarbon inclusions now seems unlikely (although hydrocarbon traces are present in that area). More likely theories are for colour centres caused by excess calcium in the lattice.
This is in contrast to the traces of transition metal ions, thought to colour most other minerals.
Dark blue colour in fluorite can be produced by x-ray or Gamma ray bombardment. (FW Smith, PHD Thesis, University of Durham (date?))this causes lattice deffects.
This may explain why some natural crystals only have a thin skin of dark blue; The mines and show caves around Castleton in Derbyshire have carefully monitored ventilation systems to control radon gas levels, which in some of the wild caves in the area can give a years recommended max exposure for a human in a few hours. this may over a long period give some lattice damage and colouring.
The delicate greens yellows and blues in English North Pennine fluorite may be due to rare earth metal traces causing lattice deffects, Prizibram, some time in the 1960's or 70's reported rare earth spectra from a specimen from Ramshaw Mine in Co Durham. FW (Rick) Smith found that Yttrium concentration increased towards vein intersections which were suspected feeder channels to the mineralisation (fluid inclusion hommogenization temperatures also followed this trend). Ixer appears to have done considerable research into trace quantities of rare earth minerals present in the north pennine mineralization.
Annealing of the crystal will destroy the colour in fluorites.
Some of the colours in North Pennine fluorites are meta-stable, for example, some (?most) green fluorites from this area will slowly change to mauve/ purple colour with exposure to sunlight.
[edit] Optics of Fluorite
My understanding of the reason for using fluorite in visible light optics is that it is:
1)cubic and therfore optically isotropic (only single refractive index)
2)it offered a third refractive index/dispersion combination, so that when combined in an "Apochromatic Triplet" with the conventional "flint" and "crown" type optical glasses, it allowed Red, Green and Blue light to be brought into focus on the same plane("apo-chromatic correction"), where using the conventional Achromatic double only two colours could be brought into focus on the same plane, the others focusing either infront or behind it.
I gather that the same correction effect can be achieved by leaving an air gap in a "doublet" so effectively creating an "air element".
3)it was available in clear, relatively colourless and relatively insoluble (in water)crystals.
The refractive index of fluorite is surprisingly low. In microscopic examination this causes fluorite grains in thin sections to stand out with high relief boundary contacts against the cementing medium used to mount the slide, and most other minerals, due to the fluorite having a significantly lower refractive index.
This low refractive index can be used to "look into" a scratched or worn specimen, as submerging it in water, especially salty or sugary water will surpress many of the surface reflections.
Apparently, the mineral cryolite has approximately the same refractive index as water, and gives the illusion of disappearing when submerged.
