Hi
Me and my collegues found a strange rock aggregates here Lunda Sul, within Kalahari sand suite.
Similar formations (ferricrite, ferricrete) are found within Lunda Sul, Angola on the surface and in the strata of the Neogene-Paleogene red-brown sand deposits of the Kalahari as different unshaped fragments with different sizes.
I guess it could came from more ancient sedimentary rocks. I have attached some photos and description
So, we have these cylinders splited on 4 sector, 5 sector and 6 sector bofy filled by iron mineralization mixture (mainly composed by goethite, hematite, relics of fayalite, few inclusions of quartz). We have done only X-ray diffraction analysis and single thin sections, but wave microprobe have not yet been done. The spaces between sectors filled by Kalahari sands. Aggregate of goethite botryoidal form, porous filling, brownish dark-gray color, metallic luster, yellowish-brown streak, hardness 5, density about 4 g/cm3, slightly oxidized on chips, in voids and pores. Magnetic susceptibility about 0.16 SI*10^5.
I have one exotic idea about origin but with many questions.
It could be tree trunks, splited into pieces after drying or fire, filled with clay soil during wet season, which was then consolidated after next dry season or new fire. Subsequently the wood (or coal) in sectors were weathered and..... replaced (?) by iron-rich mineralization. Perhaps the processes of psephdomorphic replacement occurred in several stages. As a result, we obtained botryoidal goethite.))
It remains unclear the olivine origin in the botryoidal aggregate?
Why, instead of vertical zoning and the ring structure of the tree, we have, on the contrary, horizontal sinter structures?
What do you think? Please look at the concretes. May be you have seen similar or have idea what is it? Which signs and features I could check without any complex analytics??
Below attached images of aggregates:
https://disk.yandex.ru/d/DXgZNmPkNSQ5UQ
Maybe fulgurite caused by lightning strikes due to the Fe-Si reaction which points to hight-T processes leading to fayalite which is also common to Fe slags or high-T degassing tubes of unknown source
HGD
Thank you Dr Harald.
This is not fulgurite as I believe , because of cylinder shape was rule separated on sectors.
I'm really confused because of this strange morphology cylinders were not ever evidenced for any fayalite crystallization systems. Besides the hydrothermal fluids with low Si/Fe ratio could precipitate fayalite (Dobrica et al., 2022), but my samples are to small and unusual for this kind of formation. May be you are right about unknown tube High-T activity.
Dr Harald,
I want to update that last made thin section and more detail cross cutting show that these examples could be some kind of lava (paralava), rich of fayalite crystals (about 60%). There also much abundance of amorphous Fe-mineralization, and also there are perfect vesicles inside which contain rounded quartz.
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