Publication: Formation of hollow concretions in Northeastern Thailand
Issued Date
2009-01-01
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2-s2.0-84969640958
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Mahidol University
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SCOPUS
Bibliographic Citation
Advances in Geosciences: Volume 13: Solid Earth (SE). (2009), 13-22
Suggested Citation
Prinya Putthapiban, Sutatcha Hongsresawat Formation of hollow concretions in Northeastern Thailand. Advances in Geosciences: Volume 13: Solid Earth (SE). (2009), 13-22. doi:10.1142/9789812836182_0002 Retrieved from: https://repository.li.mahidol.ac.th/handle/20.500.14594/27523
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Formation of hollow concretions in Northeastern Thailand
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Abstract
© World Scientific Publishing Company. The mysterious rocks “Naka’s eggs” commonly found in Northeastern Thailand are hollow concretions derived from clastic rocks of the Khorat Group. The concretions appear in different shapes, such as spheroidal, ellipsoidal, and irregular with sizes varying from a few cm up to 60 cm. Their dark brown outer shells are much harder than the hosted rocks, and the inner surfaces of the hollows are rugged and occasionally contain remnants of pyrite (FeS2) minerals indicating incomplete oxidation processes. The result of extensive examinations of these hollow concretions suggests that their formation involves subsurface water that penetrates through fractures of rocks and the boundaries of sand grains forming several species of iron solutions. Due to their exothermic nature, these solutions sieve outward to the region with lower temperature and pressure where chemical reactions can continue. When equilibrium is reached, reddish brown iron oxide sediments remained as hard shells of the concretions. The hollow is then created in situ as a result of these chemical processes. The size and shape of these hollow concretions clearly depend on the quantity of pyrite crystals and the morphology of the pyrite nodules. As an external erosion process subsequently takes place, the outer shells which are more resistant and have a smaller porosity due to the secondary cemented iron oxides survive with shapes of sphere, ellipsoid and others, whereas other sandy parts of the host were eroded away. Because it is evidently clear that the reddish-brown color of the clastic rocks in our study areas is secondary in origin, parts of chemical reactions discussed here are promising candidates for actual chemical alterations responsible for the reddish color of the Khorat Group red beds in Thailand.