Phase transformation processes in karst-type bauxite deposit from Yunnan area, China

Bauxite is the only resource of aluminum metal and forms through weathering, leaching, and deposition (i.e., bauxitization processes). Here we characterize the bauxite deposit in Yunnan area, China, through mineralogical and textural characteristics using XRD, SEM-EDX, and micro-Raman analyses. The Al-oxyhydroxide polymorphs (AlO(OH)) such as diaspore and boehmite are the dominant phases in most of the samples. Gibbsite (Al(OH)₃) is the main mineral in two samples from Quaternary bauxite, and these three Al-rich minerals mixed with goethite, hematite, kaolinite, and small amounts of TiO₂ polymorphs (anatase and rutile). Considering the textural characteristics and the stratigraphy of the bauxite deposit, some of the Al-oxyhydroxides and Ti-oxides are inferred to be products of the bauxitization in the lateritic soil during late Permian weathering. In the subsequent transgression stage, the earlier formed (Al-rich minerals and anatase) phases were transformed to diaspore or boehmite as spherulitic form and to rutile, respectively, with the processes of diagenesis, burial metamorphism, tectonic movements and magmatic activity. Following crustal uplift in the Yunnan Province after the late Triassic, diaspore was transformed into boehmite and the bauxite deposit formed earlier was exposed again to weathering resulting in the formation of gibbsite, hematite, goethite, kaolinite, and anatase, with diaspore, boehmite, and rutile remaining as relics. Our study provides insights into phase transformation during bauxitization and subsequent processes.