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Cause of coloration in green fluorite
  
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protium




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PostPosted: Sep 17, 2017 11:45    Post subject: Cause of coloration in green fluorite  

Hello, my wife and I are very new to mineral collecting. I have been researching this, but have not been able to find an answer. I thought the question was simple. What impurity causes fluorite to appear green. In this case a pale green.

From what I have learned there may not be a simple answer, but I thought I would run the question by you fine folks anyway.

I have learned more about fluorite in the last 24 hours than I thought possible, the fact that both Fluorine and fluorescence are named after the mineral is fascinating! I have spent hours scouring the web, but I am no closer to understanding what causes green color centers.

Thanks in advance for any guidance you can give.
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Pierre Joubert




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PostPosted: Sep 17, 2017 12:02    Post subject: Re: cause of coloration in green fluorite  

Very good question! I will also be following this post and hope all the fluorite experts chip in.
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'The tree of silence bears the fruit of peace. '
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lluis




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PostPosted: Sep 17, 2017 12:43    Post subject: Re: cause of coloration in green fluorite  

Dear Pierre, protium

As chemist I always thought that green fluorite is mainly due to copper traces.... Maybe also nickel traces depending from places....

Not a very hard color to explain.
Other is pink fluorite, that seems to be due to net voids.....

With best wishes

Lluís
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protium




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PostPosted: Sep 17, 2017 13:09    Post subject: Re: cause of coloration in green fluorite  

Thank you for the quick response. I will take your word for it that it is either copper or nickel. This particular specimen was found in Madoc Ontario, but I realize that is not enough to go on to determine which metal may be the culprit.

When I was researching this last night, it seemed that many thought the different colors displayed in fluorite were due to complicated mixes of light and heavy rare earth minerals. However, I am a complete layman. I had to look up what you meant by net voids, and I certainly still do not have a complete understanding.

I find it intriguing that fluorite can be found, in varying intensities, in every color in the visible spectrum.

Thanks again,
Richard
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lluis




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PostPosted: Sep 17, 2017 14:09    Post subject: Re: Cause of coloration in green fluorite  

Hi, Richard

Well, a quick search make me think that I should eat my words...
As you say, in this article, seems that many are due to rare elements....
http://rruff.info/doclib/am/vol37/AM37_910.pdf
(link normalized by FMF)

Pink I always heard that are due to voids in net, and violets, due to radiation...

Anyway, if you wish to read the article, looks very complete...
And, in the other side, the green fluorite from Tsumeb, a mine rich in copper, well, looks to me more closer due to copper traces than to rare elements.... But to think is not to know,,,, And I could not due a flame spectroscopy analysis... So....

With best wishes

Lluís
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Gerhard Brandstetter




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PostPosted: Sep 17, 2017 14:26    Post subject: Re: Cause of coloration in green fluorite  

I found a very helpful table in Mineralienatlas and tried to translate it:
Source: https://www.mineralienatlas.de/lexikon/index.php/Mineralienportrait/Fluorit/Eigenschaften?lang=de

Blue
Fe+3/Fe+2 complex with copper
Colloid Calcium
Y-associated centers
REE
Colloidal Calcium
Przibaum (1953)
Allen (1952), Mackenzie et al. (1971), Braithwaite et al. (1973)
Calas (1972), Bill et al. (1973)
Calas (1995)
Calas (1995)



Brown
Organic inclusions
Mn2+, Mn3+, Thorium
Calas et al. (1976)
Kempe (1994)


Yellow
Eu+2
Fe and REE
OF2 or OF
O3
Przibaum (1938)
Przibaum (1953)
Neuhaus et al. (1967)
Bill et al. (1978)


Yellowgreen
Y/Ce associated centers
Bill et al. (1978)


Green
Colloid Calcium
Fe+2 and Mn, Cr. Ni or Cu
Sm+2
Sm+3
Allen (1952)
Przibaum (1953)
Bill et al. (1967, Neuhaus et al. (1967)
Bill et al. (1978)
Bailey et al. (1974)


Orange
Mn+2
Bailey et al. (1974)


Pink/Red
Fe+3
Cr + Mn+3
YO2
Przibaum (1953)
Przibaum (1953)
Bill et al. (1967, 1978)


Violet
Colloidal Calcium
Allen (1952)
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lluis




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PostPosted: Sep 17, 2017 15:10    Post subject: Re: Cause of coloration in green fluorite  

Dear Gerhard

Many thanks for info

Nevertheless, the causes of pink cause me a big shock...
Till today, I have always heard that was caused by color centers, voids in net, a cause why they are sensible to light....

Apart of that, colloidal calcium should give a blue (dark blue....) colour, as far as I know....

Well, maybe colours in fluorite are just till today bad understood.....
As was the theory of complexes till last time.....

Knowledge increases with time... or so should be... (not always happens, though.... :-( )

With best wishes

Lluís
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protium




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PostPosted: Sep 17, 2017 15:35    Post subject: Re: Cause of coloration in green fluorite  

Thank you all! I will definitely read the article and thank you to Gerhard for the table.

Unfortunately, I have reading to do for school for the rest of the evening. So it may be a bit before I get to it, but I will definitely check it out.

Richard

EDIT:
Lluis, I took a quick look, that is the same paper I read last night. Much of it is over my head, I am currently studying computer programming and systems analysis and have no background in chemistry. However, I did manage to glean some information from it. :)

Thanks again.
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Jesse Fisher




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PostPosted: Sep 18, 2017 13:42    Post subject: Re: Cause of coloration in green fluorite  

There are a number of different mechanisms that may be responsible for color in minerals, and similar colors in various minerals may be the result of different processes. A good introduction can be found at: http://gemologyproject.com/wiki/index.php (link normailized by FMF)

The color of green fluorite from the Weardale region in Northern England (Rogerley Mine, Heights Mine, etc.) is thought to be caused by elevated concentrations of a suite of REEs, which are also likely the cause of their intense fluorescence. The evidence I've seen suggests samarium (2+) substituting for calcium in the fluorite structure. REEs are generally not metallic chromophores, so it is likely that the color is caused by a structural defect due to the ionic substitution. The fact that the green color will fade over time with exposure to direct sunlight suggests annealing of these defects by the UV in the sunlight. Purple fluorite from the region is, for the most part, color-stable, suggesting a different color-inducing mechanism for these.
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lluis




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PostPosted: Sep 18, 2017 13:51    Post subject: Re: Cause of coloration in green fluorite  

Completely agreed with you , Jesse...

I fear that causes of color could be many. And REE would be very numerous, but not the most important....

Depending in what place fluorite is found, others cromophores could be there....

But, yes, chemist, not geologist, and another view on theme

With best wishes

Lluís
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James
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PostPosted: Sep 18, 2017 13:59    Post subject: Re: Cause of coloration in green fluorite  

On the site Jesse mentioned search for 'causes of color'

James
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protium




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PostPosted: Sep 18, 2017 14:21    Post subject: Re: Cause of coloration in green fluorite  

Thanks for the link, I will check it out. However, it is now very clear that the cause of the color in my sample will remain a mystery to me. This is fine. When I started looking into this I thought there might be a simple answer. The fact that there isn't one only caused me to learn new and unexpected things, which is great.

I am very happy to have found such a knowledgeable group of people. You can expect more questions from me in the near future, school work permitting.

Richard
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Roger Warin




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PostPosted: Sep 18, 2017 15:08    Post subject: Re: Cause of coloration in green fluorite  

Hi,
About colors of fluorite, here are some thoughts.
If the questions arise easily, it is not the same for the answers.
Physicochemistry that explains these colorations is a difficult science for amateurs.
A first reflection, of geologist. There is not much likelihood of finding copper in a fluorite crystal. Copper is a chalcophilic element that prefers sulfur, while calcium is a very lithophilic element. All the opposites.
Anhydrous copper cations are colorless. They become blue when coordination water complexes them thus Cu(II) (H2O)4. Other colorations are possible with other ligands, for example green.
On the other hand, the probabilities of finding compatible Rare Earths are great in rocks.

A second reflection of physicochemist, the main substitutions observed in the fluorite are those where Ca is substituted by the Rare Earths, like Yttrium and Cerium. These traces will be at the origin of the phenomena of luminescence (fluorescence). Some yttrofluorites contain up to 10% YF3.
Pure, fluorite is colorless. Different hues are possible as has been said by others.

What is the origin of the colors of fluorite?
As already suggested above by others, structural defects of the crystal do exist. A center F (Frenkel defect) appears in the fluorine as soon as an F- ion is missing. There are several reasons for this absence: growth of the crystal in the presence of an excess of calcium ions, exposure to high energy radiation which can displace an ion from its usual position, etc. To compensate electrically for this positively charged gap, an electron can be trapped and held in place by the crystal field created by the surrounding ions. In this field, the electron can occupy a fundamental state and various excited states, as in the atoms of the transition metals. It is the movements of these electrons between these states that explain both the color and the fluorescence of this mineral.

In spectroscopy, the width of the absorption bands is a signature of the existence of colored centers F.
Green fluorite, California, USA. The bands are located around 580 and 400 nm. They are associated with trapped electrons (F-center) in the presence of yttrium (Y3+). The band at 714 nm is associated with an F-center around a cerium ion (Ce).
Pink Fluorite, Huanzala, Peru. This specimen shows a 485 nm band from an O23- ion stabilized by an adjacent Y3+ ion in addition to the 400 and 580 nm bands associated with the F-center Y.
Purple fluorite, undetermined origin. The band at 560 nm is associated with colloidal calcium.

Moreover, during the growth of a crystal, differentiated colorations can appear giving typical zonations of the crystal in growth. Thus in Seilles Quarry, (Liège - Belgium), one can cleave the crystals to make appear "Mercedes crosses" corresponding to the dodecahedron seen along a ternary axis.

What do you think about this ?
Roger.
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