Reidite -- a high pressure polymorph of mineral Zircon (ZrSiO₄)
 

NOT the proposed indicator for an impact event ?

Norbert Brügge, Germany
Dipl.-Geol.

mailto: [email protected]

The rare mineral reidite has only been found in a few structures classified as impact craters. This includes this eight locations:

Apart from the doubt that these mostly imaginary structures were created by an impact, this topic has now actually become a "smoking gun" through the detection of fingerprints of the mineral reidite in the
 

 The evidence for the impact origin of the structures mentioned is weak, because pressure-formed minerals or rocks can also arise by endogenous events. On the other hand, the Libyan Desert Glass is now without a doubt in the broader sense originated volcanically. This also includes the Tektites. It is also doubted that reidites result from short shock effects on zircons. The pressure was exerted on the crystal structure should be more permanent and, is like the Reidite itself, probably from a very old period of the Earth history. The finds in Ordovician sandstone in the Rock Elm structure certainly had something to do with a rearrangement.

The best example of the uncertainty as to whether a structure was created by an impact or not is the Nördlinger Ries. The deep underground is almost unknown. A bore hole in the structure ended at a depth of 1206 m in broken gneiss interspersed with rubble. Prior to this, 270 m of "Suevite" (melt + breccias) had been detected. The volcanic formation of the structure is still not excluded. The melt has a dacitic-andesitic composition.

What are reidites and how do they come about?

Reidite is a high-pressure polymorph of zircon.The pressure makes minerals tightly repack their molecules into denser crystal structures. Reidite has the same composition as regular zircon but is about 10 percent denser.
Reidite is not stable when hot, and reverts back to zircon above 1,200℃. This results in the formation of a network of new, tiny interlocking grains. Above 1,700℃ zircon ultimately breaks down to zirconia.
 
The key to finding evidence of former reidite lies in analysing the crystal orientations of the tiny interlocking grains in reverted zircon.The initial transformation to reidite occurs along specific directions in a zircon crystal. When reidite changes back to zircon, it leaves a fingerprint of its existence that can be detected through orientation analysis.

My Conclusion

Finally, the question arises whether finds of reidite are actually suitable to prove an impact event. Recently, reidites were even found in Thai tektites and the Libyan Desert Glass. Tektites and the LDG are clearly volcanic glasses. It is a matter of time before further discoveries of zircon-reidites are made, which have their origin in high-pressure changed crustal basement-rocks and can be transported in recrystallized form to the surface of the earth by volcanic activity.
 

References

1. Zirkon-reidite relations in Breccias from the Chesapeake impact structure
   L. Malone e al. -- 41st Lunar and Planetary Science Conference (2010), 2286.pdf
      
2. Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater
   A. J. Cavosie  et al. -- GeoScienceWorld, Geology, vol.43, no.4, p.315-318 (2015)
    
3. Shock-metamorphosed Zircon in terrestrial impact craters
   A. Wittmann et al.-- Meteoritics & Planetary Science, vol.41, no.3, p.433�454 (2006)
    
4. Shocked Zircon from the Woodleigh-1 core: The largest known section of coherent redeite-bearing bedrock ?
   M. A. Cox et al. -- 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132), 1505.pdf
    
5. Precambrian Reidite discovered in shocked Zircon from the Stac Fada impactite, Scotland
   S. M. Reddy et al. -- GeoScienceWorld, Geology, vol.43, no.10, p.899-902 (2015)
    
6. Natural occurrence of Reidite in the Xiuyan crater of China
   Ming Chen et al. -- Meteoritics & Planetary Science, vol.48, no.5, p.796�805 (2013)
    
7. Effects of shock metamorphism on zircons from the Haughton impact structure
   A. C. Singleton et al. -- 74th Annual Meteoritical Society Meeting (2011),  5142.pdf
    
8. Anatomy of impactites and shocked zircon grains from Dhala reveals Paleoproterozoic meteorite impact in the Archean basement rocks of Central India
   Shan-Shan Li et al. -- Science Direct, Gondwana Research, vol.54, p.81-101 (2018)