Formation of Weathering Rim of "Laos Tianhuang" under Supergenic Process: Mineralogical Evidence

The identification of Tianhuang and its origin identification are hot topics that needs to be solved urgently at present. The weathering rim of Tianhuang, as a special product during the formation of the stone, recorded rich geological information. In this paper, a detailed comparison of the mineralogical characteristics between the interior and weathering rim of "Laos Tianhuang" was carried out by conventional gemmological testing, X-ray powder diffractometer, Fourier transform infrared spectrosmeter, Raman spectrometer and scanning electron microscope, and the formation process of the weathering rim has been discussed in depth. The results showed that the weathering rim of "Laos Tianhuang" had various colours, including yellow, white and black, or mixed with the colours above as multiple layers. Goethite and hematite are the main colour-causing minerals of the yellow weathering rim; pyrite and amorphous carbon make the stone weathering rim black; the white stone weathering rim is mainly caused by the dissolution and loss of iron oxides. The "Laos Tianhuang" are generally well-polished and rounded with thick weathering rims. Compared with the interior, the weathering rim of "Laos Tianhuang" has a looser structure and larger porosity, which are related to the transportation and water-rock reaction of Tianhuang during the deposition process. The climatic conditions of high temperature, high precipitation, and humic acid-rich hydrological conditions in Laos may have promoted the alteration of "Laos Tianhuang". The different weathering rim colours "Laos Tianhuang" are possibly related to their burial localities and microenvironment. The yellow weathering rim may be formed in an environment with blocked drainage, which makes Fe ions more likely to accumulate in the micropores on the surface of the stone; the white weathering rim is formed in an environment with well drainage condition, which leads to the loss of Fe ions more easily; the black weathering rim may be related to the reducing environment and the role of microorganisms in the shallow burial process. Studying Tianhuang from the perspective of supergenic or surficial genesis can provide ideas for us to understand the similarities and differences between Tianhuang in different origins, and meanwhile to beneficially solve problems such as the formation, identification and origin identification of the stone crust of Hetian Yu pebble and jadeite.