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Spherical inclusions in amethyst - cristobalite? vs fluorite
  
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Amir Akhavan




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PostPosted: Mar 27, 2022 04:44    Post subject: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Hi,

Alfredo has recently sent me a specimen of Rio Grande do Sul amethyst with these infamous hemispheric inclusions for study. So far I can tell these are simply quartz pseudo- and perimorphs after some dissolved radially grown mineral, probably fluorite.
Specimens of that kind are often sold as "cristobalite". However, I am not aware of any published analytical evidence. Niedermayr and Hyrsl report cristobalite intergrown with fluorite based on personal communication with Koivula in their book on quartz inclusions. Analytical methods are not given. John S. White has uploaded images of spherical inclusions deep inside the crystals and mentions cristobalite has been found by XRD, but points to their tubular shape, and not to the spheres:
https://www.mineral-forum.com/message-board/viewtopic.php?p=5491#5491

Has anything been published about this?

Attached is a photo of the specimen I got.

Regards
Amir



am_br_ametista_do_sul_Q776_1_25.jpg
 Mineral: Quartz
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Dimensions: 29 mm
 Description:
Quartz-filled perimorphs of quartz after an unknown mineral, probably fluorite. Width 29 mm, diameter of the hemisphere 9 mm.
 Viewed:  2204 Time(s)

am_br_ametista_do_sul_Q776_1_25.jpg



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Pete Richards
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PostPosted: Mar 27, 2022 20:35    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Amir, you probably know all of this better than I do....

I know of no publications.

But I wonder - do the spheroids penetrate into the amethyst or are they strictly on the surface of the amethyst?

I also wonder whether the "cristobalite" precursor is thought to be high-cristobalite or low-cristobalite?

When I think of cristobalite, I think of high-cristobalite, and it could only have formed on amethyst in a much hotter environment after the amethyst had grown, in which case, it would be strictly on the amethyst, not partly in it, right? At high-cristobalite temperatures, wouldn't the amethyst bake into citrine?

Low-cristobalite as a primary mineral seems unlikely to form spheres of radial structure of the size that is shown here.

If these thoughts are correct, they would seem to rule out the inclusions being cristobalite. If the inclusions are now pseudomorphed to quartz, it will probably be impossible to establish the precursor with certainty, unless some specimens are found in which the replacement is incomplete.

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alfredo
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PostPosted: Mar 28, 2022 03:55    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

"But I wonder - do the spheroids penetrate into the amethyst or are they strictly on the surface of the amethyst?"

Usually they are completely enclosed inside the amethyst crystal. I had selected this one for Amir to study because it was one of very few that had the ball protruding outside the crystal, making it easier to analyze.

Amethyst crystals from this Brazilian locality have been on the market for years labelled as "cristobalite inclusions", which over the years became a self-perpetuating ID, but one which I doubted. So this one was sacrificed for science by Jewel Tunnel (not me personally) and Amir can cut it up for a thin section if he likes.
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Amir Akhavan




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PostPosted: Mar 28, 2022 04:16    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Hi Pete,

maybe people just took the cristobalite devitrification spherulites in obsidian as an example for radial growth of cristobalite, but of course this is a completely different environment.

I guess most cristobalite on earth forms at low temperatures, as an intermediate during the diagenesis of chert from opaline sediments. But as a late formation in geodes already filled out by quartz - that's rather exceptional.
The example shown by John is a better candidate because the balls are embedded deep inside the crystals and could have formed during an early phase and a more unrest environment with large fluctuations in silica content in the fluids.

I have some super tiny (pseudo)-octahedral transparent crystals of (supposed) cristobalite in my collection that sit in a celadonite matrix between calcite of prismatic habit and no quartz anywhere. The specimen is from Rio Grande do Sul. It has only been EDX analyzed to be silica, but nothing more has been done, and doing an SXRD would certainly be a challenge. It is very likely still cristobalite, because cristobalite has so far been found to typically turn into quartz by dissolution and recrystallization of the silica, not by reconstructive transformation (breaking and forming chemical bonds).
(I wonder how many of the supposed paramorphs of quartz after tridymite or cristobalite have actually been XRD-analyzed)

Anyway, I don't doubt that cristobalite can form in such environments, but I'd rather like to see some more experimental data ;-)

The hemisphere on my specimen sits only about 1-2 mm inside the amethyst, but I've seen photos of hemispheres that are fully included and also have this mottled surface caused by goethite bundles. I think fluorite balls from that locality are well established, although I still need to get a copy of this article:
Pöllmann, H. (2010). Kugelfluorit auf Amethyst aus Rio Grande do Sul, Brasilien. Aufschluss, Jg.61, 6, 321-323.

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Amir Akhavan




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PostPosted: Mar 28, 2022 04:23    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Hi Alfredo,

thanks for confirming that most from this batch were fully enclosed. I already had forgotten that.
Unfortunately, the one I got now is my only specimen and is quite pretty and interesting, so I don't plan to sacrifize it and turn it into a thin section, as I should.
I will have to look for a cheap ugly one, and if possible, one that is more enclosed.
These specimens are very well known, but difficult to find on mineral shows.
So Alfredo, if there are still some left, I'll take one more, if I can afford it ;-)

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Amir Akhavan




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PostPosted: Mar 28, 2022 04:31    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Demonstration of its "prettiness".
The rest will be in the small article I'm working on.



am_br_ametista_do_sul_Q776_2_50.jpg
 Mineral: Quartz, Goethite
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Dimensions: Diamter of hemisphere 9 mm
 Description:
Perimorph of quartz after unknown mineral, filled by transparent quartz.
 Viewed:  2045 Time(s)

am_br_ametista_do_sul_Q776_2_50.jpg



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John S. White
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PostPosted: Mar 28, 2022 10:15    Post subject: Re: Spherical inclusions in amethyst - cristobalite?  

I no longer have these crystals but I do have a photo of the pair. Attached here is a photo of two amethysts from Brazil, one with many spheres totally included and another with a sphere on the surface and many very small spherical inclusions deep within.. I have labelled these spheres cristobalite?, but I was never able to test their true identity. I am sharing this photo only to display more examples. Please note that the spherical inclusions on the left have very busy and beautiful surfaces similar to Amir's while those on the right appear to have a thin brown surface.


Quartz - Brazil 23-2-14 (with Cristobalite incl.).JPG
 Mineral: Quartz (variety amethyst) with inclusions
 Locality:
Rio Grande do Sul, Brazil
 Description:
 Viewed:  1991 Time(s)

Quartz - Brazil 23-2-14 (with Cristobalite incl.).JPG



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silvio steinhaus




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PostPosted: Mar 28, 2022 15:32    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Jaroslav Hyrsl had analyzed some samples that the late Luiz Menezes sent him, and the result was Cristobalite in one of the samples and Fluorite in another sample. I must have about 40 samples in the collection of amethysts with inclusions from Rio Grande do Sul, in his mostly Ametista do Sul, some associated with Calcites and others with pseudo-cubic shapes, some with Goethites and others with Hematites and probably Mordenite or Pseudo Mordenite Quartz.


Ametista (32).JPG
 Mineral: Quartz
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Description:
Amethyst with bubbles
 Viewed:  1943 Time(s)

Ametista (32).JPG


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silvio steinhaus




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PostPosted: Mar 28, 2022 15:36    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Another Amethyst sample from the same region, different mining with Calcite association.


Ametista (49).JPG
 Mineral: Quartz
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Description:
Amethyst with calcite and bubbles maybe Mordenite inside.
 Viewed:  1944 Time(s)

Ametista (49).JPG


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silvio steinhaus




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PostPosted: Mar 28, 2022 15:46    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Another Amethyst sample from the same region, different mining. In this sample, the white inclusion is not homogeneous as it normally is, it is associated with Goethite and the calcite grows from above this inclusion towards the outside of Amethyst.


Ametista com Calcita, Cristobalita e Goetita1 - 5,3 X 6,3 X 3,3 cm_1.jpg
 Mineral: Quartz
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Description:
Amethyst with inclusions.
 Viewed:  1925 Time(s)

Ametista com Calcita, Cristobalita e Goetita1 - 5,3 X 6,3 X 3,3 cm_1.jpg


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Amir Akhavan




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PostPosted: Mar 28, 2022 16:03    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Thanks, John and Silvio for the photos of the nice specimens and infos!

Silvio, I have already sent an email to Jaroslav, but so far got no response.

I'm pretty sure mine and most of those on sale are nothing but quartz, and I think only by studying a fully embedded inclusion one will have a chance of determining the original minerals.

I've seen a few offers on the Internet, including specimens from John's collection.

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silvio steinhaus




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PostPosted: Mar 29, 2022 07:28    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Amir, good morning.
I'm out of São Paulo this week, when I get back I can separate a sample and send it to you to analyze the inclusions.



IMG_6235.JPG
 Mineral: Quartz
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Description:
Amethyst with inclusions
 Viewed:  1804 Time(s)

IMG_6235.JPG



Ametista (34).JPG
 Mineral: Quartz
 Locality:
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil
 Dimensions: 7 cm
 Description:
 Viewed:  1807 Time(s)

Ametista (34).JPG


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Amir Akhavan




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PostPosted: Mar 29, 2022 18:49    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Silvio, thank you very much for the kind offer!
I've sent you a P.M. :-)

Amir

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James Catmur
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PostPosted: Mar 30, 2022 10:23    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

One from Spain. I think the form of the original crystals is very clear


IMG_20220330_161115516.jpg
 Mineral: Quartz
 Locality:
Vértice Quarry, Cerro Vértice, Villaviciosa de Córdoba, Comarca Valle del Guadiato, Córdoba, Andalusia, Spain
 Dimensions: 7cm x 6cm x 4cm
 Description:
Epimorphs after Aragonite or Calcite, you can see a broken one at the top, right
 Viewed:  1699 Time(s)

IMG_20220330_161115516.jpg


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Amir Akhavan




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PostPosted: Jun 30, 2022 14:06    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

One of the tasks at Saint Marie aux Mines ( apart from catching covid in the badly ventilated crowded halls ) was finding more samples.
All I had to do was to look for unpolished amethyst tips at *all* Brazilian amethyst dealers, patiently go through dozens of boxes and hundreds of crystals, pick every crystal with these inclusions (only 8 at two dealers), and pay a total of 43 Euros for it - they came in the boxes with the other crystals which were sold by 50 Euro/kg.

However, I could not find amethysts with fluorite spheres on them, so the hunt is not over yet.

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PostPosted: Jun 30, 2022 14:20    Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite  

Faced with such relevant opinions, dare I formulate another hypothesis?
Chalcedony.
It seems to me that the Temperature & pressure parameters would be more compatible.
Thank you for contradicting me because I still want to learn about quartz, which in its pure state (not twinned) seems to me to be a rather exceptional mineral.
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