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Gismondine-Ca : CaAl2Si2O8·4H2O

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Copyright © Christof Schäfer
 
 
 
 
minID: 3L5-226

Gismondine-Ca : CaAl2Si2O8·4H2O

Copyright © Christof Schäfer  - This image is copyrighted. Unauthorized reproduction prohibited.

Two crystals terminated by (110) and (011) and elongated in direction of [100] are forming a cross-like twin. Twinning plane is (101). Associated minerals are Phillipsite and Calcite. Old specimen from A.A. Damour´s collection. Field of view 6 mm, collection C.&H. Schäfer

This photo has been shown 1504 times
Photo added:21st Oct 2008
Dimensions:640x529px (0.34 megapixels)

Data Identifiers

Mindat Photo ID:191664 📋 (quote this with any query about this photo)
Long-form Identifier:mindat:1:4:191664:2 📋
GUID:46585c06-7c5d-4ce8-9b46-513f59621032 📋
Specimen MinID3L5-226 (note: this is not unique to this photo, it is unique to the specimen)

Discuss this Photo

PhotosCross-twinned Phillipsite, not Gismondine

4th Dec 2020 10:43 UTCGianfranco Ciccolini 🌟

They are not gismondine crystals, but rather a typical phillipsite cross twinned. See https://www.mindat.org/photo-859768.html. This specimen was confirmed as phillipsite with Raman examination at the Sapienza University of Rome. Gismondine has very different twins: "On the normal to {100}, giving pseudotetragonal or pseudo-orthorhombic individuals "(Handbook of Mineralogy).

5th Dec 2020 13:28 UTCVolker Betz 🌟 Expert

For me this looks also like phillipsite.

5th Dec 2020 15:07 UTCChristof Schäfer Manager

Such gismondines have been typical for Capo di Bove. Several drawings of precisely such twins from Capo die Bove are given in Goldschmidt´s Atlas der Krystallformen.
In addition this type of twins is also common for the Eifel area. The picture https://www.mindat.org/photo-345763.html shows such a twin. In this case the two individuals are less prismatically developed.
My specimen from Capo i Bove was never examined in Rome. Why you state that these crystals were determined at Sapienza to be phillipsite? This absolutely false!

6th Dec 2020 12:10 UTCVolker Betz 🌟 Expert

04405310016072534008928.jpg
The drawings of credner (1847)  illustrated in Goldschmidt show  at Fig. 6 very clear a phillipsite twin and at Fig. 8 an epitaxy of gismondine and phillipsite.


We  have to take in consideration that at the most occurrences of gismondines are epitaxies with Phillipsite.  This is sometimes easy visible but often very hidden and can only seen as unusual face on gismondine crystals. Tschernich (1992) shows examples for capo di Bove.

So the specimen 3L5-226  hows most likely an epitaxy, as phillipsite is also present on the specimen.


6th Dec 2020 08:28 UTCGianfranco Ciccolini 🌟

05159670016072428505927.png
Your photo of hypothetical gismondine is very similar to this one of a different but very similar location in Lazio and which I had analyzed, because it was given to me as gismondine.

Now, if you look carefully at the ends of the cross arms, both in your photo and in mine, you will notice the presence of small prisms typical of phillipsite and nonexistent in gismondine. In fact, in Volker's gismondine photo, the ends are straight, smooth. 


 
 
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