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Cristobalite in Lybian Desert Glass
  
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Roger Warin




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PostPosted: Dec 23, 2016 00:57    Post subject: Cristobalite in Lybian Desert Glass  

Cristobalite in Libyan Desert Glass.

A terrestrial mineral in a terrestrial environment. Isn’t it?



Cristobalite#1_R.jpg
 Mineral: Cristobalite
 Locality:
Egypt
 Description:
Alignment of the cristobalite crystals within the lamellae results in birefringence producing a variety of patterns, including Maltese cross, when spherulites are viewed between crossed polarizers in an optical microscope.It is not possible to see the fibro-radiated cristobalite crystals but the Maltese cross proves their texture. The size of the spherule is a bit large to have a well-defined Maltese cross in this slide.It would be interesting to correlate this texture of cristobalite with the temperature. I do not know the origin of the Libyan glass, but the thermal shock should not have been produced by a meteorite impact.
 Viewed:  1389 Time(s)

Cristobalite#1_R.jpg



Verre_Libyque_3637_R.jpg
 Mineral: Cristobalite
 Locality:
Egypt
 Description:
Cristobalite spherulites in Lybian Desert Glass
 Viewed:  1394 Time(s)

Verre_Libyque_3637_R.jpg


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marco campos-venuti




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PostPosted: Dec 23, 2016 04:12    Post subject: Re: Cristobalite in Lybian Desert Glass  

Exact!
Hi, here few slides I made for a presentation, about the origin of Libyan Glass. Spherulites are the result of secondary devitrification as happen in all glass, natural as obsidians or man made.



Imagen4.jpg
 Mineral: General info about Libyan Glass
 Description:
 Viewed:  1344 Time(s)

Imagen4.jpg



Imagen1.jpg
 Mineral: Diagram showing origin of tectites
 Description:
 Viewed:  1343 Time(s)

Imagen1.jpg



Imagen3.jpg
 Mineral: Main Tecite Fields
 Description:
 Viewed:  1344 Time(s)

Imagen3.jpg



Imagen2.jpg
 Mineral: From where comets are from
 Description:
 Viewed:  1342 Time(s)

Imagen2.jpg


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Roger Warin




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PostPosted: Dec 23, 2016 05:10    Post subject: Re: Cristobalite in Lybian Desert Glass  

Many thanks Marco.
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marco campos-venuti




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PostPosted: Dec 23, 2016 05:32    Post subject: Re: Cristobalite in Lybian Desert Glass  

Here is an interesting piece of Libyan Glass overgrown by limonite pseudo after pyrite from the Farafra Oasis, Egypt.


IMG_4130.JPG
 Mineral: Libyan Glass overgrown by limonite pseudo after pyrite
 Locality:
White Desert, Farafra Oasis, Matruh Governorate, Egypt
 Dimensions: 7 cm
 Description:
 Viewed:  1306 Time(s)

IMG_4130.JPG


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Roger Warin




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PostPosted: Dec 24, 2016 08:30    Post subject: Re: Cristobalite in Lybian Desert Glass  

Hi Marco,
There are also Philippinites and the Anda type.
Finally, what is the best explanation for this texture?
Roger.



204-Tectite_Anda_3_R.jpg
 Mineral: Tectite
 Locality:
Anda Island-Municipality, Pangasinán Province, Ilocos Region, Filipinas
 Description:
 Viewed:  1179 Time(s)

204-Tectite_Anda_3_R.jpg



205-Tectite_Anda3_R.jpg
 Mineral: Tectite
 Locality:
Cabarruyan Island, Anda Island-Municipality, Pangasinán Province, Ilocos Region, Filipinas
 Description:
 Viewed:  1177 Time(s)

205-Tectite_Anda3_R.jpg



206-Tectite_Anda_R.jpg
 Locality:
Cabarruyan Island, Anda Island-Municipality, Pangasinán Province, Ilocos Region, Filipinas
 Description:
detail
 Viewed:  1205 Time(s)

206-Tectite_Anda_R.jpg


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marco campos-venuti




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PostPosted: Dec 24, 2016 12:31    Post subject: Re: Cristobalite in Lybian Desert Glass  

Hi,
all grooves on the surface of tectites are re-entry structures. They are formed when they fall down. Due to the lower fusion temperature compared with meteorites, grooves are deeper in tectites. Consider that they travelled for thousands of kilometers.
Philippinites and Anda are local type of the big australasian group.
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alfredo
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PostPosted: Dec 24, 2016 18:55    Post subject: Re: Cristobalite in Lybian Desert Glass  

Depends in part on the climate. The grooves on moldavites are of post-flight origin, by long-term etching in acid soils. Some of that might happen with other tektites too, especially in humid countries like the Philippines. In a desert, like Australia, they are more likely to be of aerodynamic origin.
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marco campos-venuti




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PostPosted: Dec 25, 2016 08:45    Post subject: Re: Cristobalite in Lybian Desert Glass  

Yes you are right Alfredo. Almost part of the sculpture is secondary, but follows pre-defined lines of weakness created at the time of formation of the tektite. This explains why tektite sculpture varies geographically in different areas of its strewn field. Moldavites found in ironstone nodules are much less sculptured.
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