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PhotosThis is an incorrect ID

23rd Sep 2019 03:49 UTCChris Emproto

09861550016028682551856.jpg
Calcite, Amethyst
Rio Grande do Sul, Brazil
Copyright © Kuno Stoeckli
The white mineral is most likely calcite. Cristobalite is geologically implausible for this deposit given the formation conditions. 

23rd Sep 2019 08:45 UTCAlfredo Petrov Manager

Should be easy to check with a drop of acid where that upper ball is exposed at the surface.

16th Oct 2019 16:41 UTCUwe Kolitsch Manager

Message sent.

18th Oct 2019 17:12 UTCUwe Kolitsch Manager

07203050016028682567714.jpg
Cristobalite, Amethyst
Ametista do Sul, Rio Grande do Sul, Brazil
Copyright © Marilha / Brazil Collection Minerals
Kuno Stöckli replied (in German, so I translate here) as follows:

The occurrence of cristobalite in amethyst is well known, see e.g. Gübelin/J.Koivula "BILDATLAS der Einschlüsse in Edelsteinen", p. 157.
The crystal is now in the Natural History Museum in Budapest. 

Another crystal with cristobalite is shown in the above photo.

EDIT: Considering the size of these cristobalite aggregates, they will have been analysed at some time by either powder XRD or Raman spectroscopy (at least one hopes so).


19th Oct 2019 22:07 UTCRalph S Bottrill 🌟 Manager

Rock Currier uploaded a photo of one of these and I queried it. He said it hadn’t been tested to his knowledge so he sent me quite a nice specimen to test. He died very shortly after, but given the circumstances and that I would have had to destroy a nice sample for testing I kept it as a momento, feeling sure somebody else would test one. It does seem an odd paragenesis.
 
Mineral and/or Locality  
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