FMF - Friends of Minerals Forum, discussion and message board

[Luis Domínguez]

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.

[Reyhane]

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)

[Jordi Fabre]

[Matt_Zukowski]

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!

[Jordi Fabre]

[John S. White]

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

[John Rakovan]

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

[Jordi Fabre]

Magisterial John. Thanks!

[Don Lum]

Matt,

Thank you for sharing these two interesting and informative articles.

Don

[Luis Domínguez]

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.

[Kevin Conroy]

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

[Peter Lemkin]