FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Cristobalite in Libyan Desert Glass.

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

[marco campos-venuti]

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.

[Roger Warin]

Many thanks Marco.

[marco campos-venuti]

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

[Roger Warin]

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

[marco campos-venuti]

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.

[alfredo]

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.

[marco campos-venuti]

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.