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Amir Akhavan
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Mar 27, 2022 04:44 Post subject: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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
Mineral: | Quartz |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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Dimensions: | 29 mm |
Description: |
Quartz-filled perimorphs of quartz after an unknown mineral, probably fluorite. Width 29 mm, diameter of the hemisphere 9 mm. |
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_________________ Amir C. Akhavan, Hamburg, Germany |
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Pete Richards
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Joined: 29 Dec 2008
Posts: 842
Location: Northeast Ohio



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Posted: Mar 27, 2022 20:35 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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alfredo
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Joined: 30 Jan 2008
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Posted: Mar 28, 2022 03:55 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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"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
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Mar 28, 2022 04:16 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
_________________ Amir C. Akhavan, Hamburg, Germany |
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Amir Akhavan
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Mar 28, 2022 04:23 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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 ;-)
_________________ Amir C. Akhavan, Hamburg, Germany |
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Amir Akhavan
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Mar 28, 2022 04:31 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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Demonstration of its "prettiness".
The rest will be in the small article I'm working on.
Mineral: | Quartz, Goethite |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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Dimensions: | Diamter of hemisphere 9 mm |
Description: |
Perimorph of quartz after unknown mineral, filled by transparent quartz. |
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_________________ Amir C. Akhavan, Hamburg, Germany |
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John S. White
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Joined: 04 Sep 2006
Posts: 1298
Location: Stewartstown, Pennsylvania, USA



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Posted: Mar 28, 2022 10:15 Post subject: Re: Spherical inclusions in amethyst - cristobalite? |
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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.
Mineral: | Quartz (variety amethyst) with inclusions |
Locality: | Rio Grande do Sul, Brazil |  |
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Description: |
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_________________ John S. White
aka Rondinaire |
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silvio steinhaus

Joined: 15 Jan 2011
Posts: 51
Location: São Paulo



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Posted: Mar 28, 2022 15:32 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
Mineral: | Quartz |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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silvio steinhaus

Joined: 15 Jan 2011
Posts: 51
Location: São Paulo



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Posted: Mar 28, 2022 15:36 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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Another Amethyst sample from the same region, different mining with Calcite association.
Mineral: | Quartz |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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Description: |
Amethyst with calcite and bubbles maybe Mordenite inside. |
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silvio steinhaus

Joined: 15 Jan 2011
Posts: 51
Location: São Paulo



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Posted: Mar 28, 2022 15:46 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
Mineral: | Quartz |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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Description: |
Amethyst with inclusions. |
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14928 Time(s) |

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Amir Akhavan
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Mar 28, 2022 16:03 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
_________________ Amir C. Akhavan, Hamburg, Germany |
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silvio steinhaus

Joined: 15 Jan 2011
Posts: 51
Location: São Paulo



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Posted: Mar 29, 2022 07:28 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
Mineral: | Quartz |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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Description: |
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Mineral: | Quartz |
Locality: | Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, Brazil |  |
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Dimensions: | 7 cm |
Description: |
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Amir Akhavan
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Mar 29, 2022 18:49 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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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Posted: Mar 30, 2022 10:23 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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One from Spain. I think the form of the original crystals is very clear
Mineral: | Quartz |
Locality: | Vértice Quarry, Cerro Vértice, Villaviciosa de Córdoba, Comarca Valle del Guadiato, Córdoba, Andalusia, Spain |  |
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Dimensions: | 7cm x 6cm x 4cm |
Description: |
Epimorphs after Aragonite or Calcite, you can see a broken one at the top, right |
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Amir Akhavan
Joined: 01 Dec 2009
Posts: 95
Location: Hamburg


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Posted: Jun 30, 2022 14:06 Post subject: Re: Spherical inclusions in amethyst - cristobalite? vs fluorite |
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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.
_________________ Amir C. Akhavan, Hamburg, Germany |
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