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Why this fluorapatite is purple?
  
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Luis Domínguez




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PostPosted: Mar 24, 2014 21:00    Post subject: Why this fluorapatite is purple?  

Hello
I have this question: Why is this fluorapatite purple?
Which metal is present in this specimen?

I have never seen anything like this before, talking about apatites.

I took this photo from Jordi's Tucson show post ( https://www.mineral-forum.com/message-board/viewtopic.php?p=37446#37446 )

Greetings from Mexico.

If my English is not good, excuse me ;-)

Luis.



fluorapatite morada.jpg
 Description:
Fluorapatite on Siderite
Minas da Panasqueira, Aldeia de São Francisco de Assis, Covilhã, Castelo Branco, Cova da Beira, Centro, Portugal
Specimens size: 10x7 cm
Main crystal: 4.3 x 3 cm
Specimen: Gail & Jim Spann collection
Photo: Tom Spann
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fluorapatite morada.jpg



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Reyhane




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PostPosted: Mar 25, 2014 03:11    Post subject: Re: Why this fluorapatite is purple?  

Hi Luis.
I know radioactive elements could turn colors to blue and purple...and if I remember correctly, Mn in quartz make it purple (Amethyst)

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Jordi Fabre
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PostPosted: Mar 25, 2014 03:49    Post subject: Re: Why this fluorapatite is purple?  

Luis Domínguez wrote:
...I have this question: Why this fluorapatite is purple?
Which metal is present in this specimen?...

It is more complex than just "metals" or natural radioactivity, it is due to the color centers. If you use the search Search for a text , and you search by "color centers" you will find information about these color centers in different threads, although there is not one specific thread for this topic, so maybe some of our native English speakers could help you writing a short informative abstract...
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PostPosted: Mar 25, 2014 05:38    Post subject: Re: Why this fluorapatite is purple?  

There are lots of mechanisms that impart color to minerals. A reasonably concise explanation is at:
https://www.enmu.edu/services/museums/miles-mineral/colors.shtml

A more detailed explanation:
https://www.minsocam.org/msa/collectors_corner/arc/color.htm
(links normalized by FMF)

As to why a Panasqueira may be purple, Lithographie Monograph No. 17, titled, "Apatite the Great Pretender," on p. 13 and p. 76 states that some think the purple in apatites from Panasqueira may be due to the presence of Mn3+ substituting for Ca2+ the structure, but they don't say what mechanism causes Mn3+ to impart the color. I went to the original source cited in Apatite the Great Pretender (Lagerwey, 1977), and it said that the mechanism of lilac/purple coloration in the Panasqueira apatites is just like those from Ehrenfriedersdorf, but again didn't specify how the Mn3+ imparted the color. I could find no info on why Ehrenfriedersdorf apatites are purple.

On the other hand, on mindat, Andrew G. Christy posted ( https://www.mindat.org/forum.php?read,9,47163,47369,quote=1 ):

(i) Mn2+ tends to cause pink colour (i.e. manganOan), while Mn3+ causes intense red-purple colours (as in piemontite, namansilite...).

(ii) Mn3+ is a Jahn-Teller distorted cation that has a very strong preference for distorted octahedral sites when coordinated by oxygen. No such sites are available in the apatite structure, and I doubt that there is much scope for incorporating Mn3+. On the other hand, Mn2+, as a moderately large divalent cation that can handle >6 coordination, should be able to replace Ca up to minor-element levels.

(iii) The only Mn that is believed to replace phosphorus in the tetrahedral sites of apatite, as far as I know, is a trace of Mn5+ (unusual oxidation state, hence TRACE) to produce the colour in blue (not pink) "manganapatites". One of the defects of the -ian/-oan nomenclature was that it did not allow more than two oxidation states to be distinguished, although manganate (V) could perhaps have been indicated as "manganATian" if it was high enough inconcentration.

This leaves me confused because Lagerwey states that it is Mn3+ causing the purple color and Christy stating that Mn3+ can’t substitute for Ca2+ in the M sites. I give up for now.

BTW, I’d like to congratulate the Spanns on the wonderful specimen!
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Jordi Fabre
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PostPosted: Mar 25, 2014 06:13    Post subject: Re: Why this fluorapatite is purple?  

Jordi Fabre wrote:
...so maybe some of our native English speakers could help you writting a short informative abstract...

Matt_Zukowski wrote:
There are lots of mechanisms that impart color to minerals. A reasonably concise explanation is at:
https://www.enmu.edu/services/museums/miles-mineral/colors.shtml

A more detailed explanation:
https://www.minsocam.org/msa/collectors_corner/arc/color.htm

Thank you so much Matt!
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John S. White
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PostPosted: Mar 25, 2014 09:22    Post subject: Re: Why this fluorapatite is purple?  

I might add that Mn is not the cause of the color of amethyst, it is iron 3+.
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John Rakovan




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PostPosted: Mar 25, 2014 11:28    Post subject: Re: Why this fluorapatite is purple?  

Dear Luis,

The rich purple of this apatite and others from locations like the Pulsifer Quarry in Maine, https://www.mindat.org/gallery.php?loc=6199&min=1572 , and Greifenstein Rocks, Ehrenfriedersdorf, Germany, https://www.mindat.org/gallery.php?loc=1804&min=1572 has inspired the same question for many people. After a thorough review of the literature on causes of color in apatite (for Rakovan, J. and Waychunas, G. (2013) Color and Fluorescence in Apatite. In, Apatite - The Great Pretender. Mineral monographs V. 17. Lithographie, LLC.) I found that this question has yet to be answered convincingly. In other words we don’t know. I suspect that it does have to do with Mn but the chemistry and structure of apatite lead to many possibilities for an exact mechanism. Manganese can exist in multiple oxidation states in the structure including +2, +3, +5 and possibly +4 (although the latter has not been shown experimentally). Depending on the oxidation state, and subsequent ionic radius, Mn can reside in one or more of three cation sites, the Ca1, Ca2 or tetrahedral P site; each with its own unique crystal field. If the mechanism is a simple crystal field transition, e.g. a d-d electron transition on a Mn ion, then we have quite a few possible scenarios. Add to this the possibility of coupled transitions and defect sites and the conceivable mechanisms grow significantly larger. The blue color in many apatites has also been attributed to Mn and the same probably applies to this as well.

Cheers,

John Rakovan
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PostPosted: Mar 25, 2014 11:44    Post subject: Re: Why this fluorapatite is purple?  

Magisterial John. Thanks!

john Rakovan wrote:
...After a thorough review of the literature on causes of color in apatite (for Rakovan, J. and Waychunas, G. (2013) Color and Fluorescence in Apatite. In, Apatite - The Great Pretender. Mineral monographs V. 17. Lithographie, LLC.)...

To see the mentioned monograph about the Apatite "The Great Pretender", you can check: Lithographie Monographs



Apatite The Great Pretender - Lithographie.jpg
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Apatite The Great Pretender - Lithographie.jpg


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Don Lum




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PostPosted: Mar 25, 2014 17:03    Post subject: Re: Why this fluorapatite is purple?  

Matt,

Thank you for sharing these two interesting and informative articles.

Don

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Luis Domínguez




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PostPosted: Mar 25, 2014 22:28    Post subject: Re: Why this fluorapatite is purple?  

Wow!, very nice explanation, I was suspecting a replacement of calcium sites or probably in tetrahedral phosphorus sites, as you said, but I was expecting cobalt.

And what about geochemical conditions?

I think this could be a thesis project haha.

Thank you for your nice links and excellent explanation.

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Kevin Conroy




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PostPosted: Dec 10, 2020 13:20    Post subject: Re: Why this fluorapatite is purple?  

An interesting link in a similar discussion was just posted on Mindat:
https://www.mindat.org/mesg-544963.html
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Peter Lemkin




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PostPosted: Dec 11, 2020 02:04    Post subject: Re: Why this fluorapatite is purple?  

Jordi Fabre wrote:
...so maybe some of our native English speakers could help you writting a short informative abstract...

Matt_Zukowski wrote:
There are lots of mechanisms that impart color to minerals. A reasonably concise explanation is at:
https://www.enmu.edu/services/museums/miles-mineral/colors.shtml

A more detailed explanation:
https://www.minsocam.org/msa/collectors_corner/arc/color.htm

I would add:
http://minerals.gps.caltech.edu/files/visible/
(link normalized by FMF)

for general possibilities for color in different minerals
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