Subtropical weathering profiles developed on granites in Southern China are the worlds dominant source for heavy rare earth elements (HREE); metals that are crucial in many user electronics and green technologies. In these weathering profiles, feldspars and other magmatic minerals that contain REE break down to form secondary clay minerals. In the process of soil formation, REE are released from the minerals and are generally inferred to adsorb loosely to the surfaces of secondary minerals, dominantly clays and Fe-Mn oxides.
Although these so-called ‘ion adsorption deposits’ are typically very low grade compared to magmatic REE deposits (carbonatites/alkaline rocks), exploitation of these resources is economically viable because the REE can be extracted at low cost via surface or heap leaching. The key requirement for this to work is that the REE are indeed adsorbed to the surfaces of clay minerals, instead of being hosted in insoluble secondary or relict mineral phases.
In our new paper published in Nature Communications (Open Access: https://lnkd.in/dJf46UH) we compare the mineralogy and distribution of #REE in prospective weathering profiles from the peralkaline #Ambohimirahavavy Complex in #Madagascar to economically exploited weathering profiles from the #Zhaibei Granite in #China.
Specifically, we use X-ray Absorption Spectroscopy to find out exactly where the #REE were hiding in lateritic soil profiles from Madagascar and China.
Although primary mineralogy plays a key role in the development of easily-leachable REE deposits formed by weathering, we found that the adsorption mechanisms of REE to clay minerals, at the atomic level, are identical at both sites.
Our data demonstrates that Yttrium, as a proxy for the heavy REE, is dominantly adsorbed as 8 to 9-fold hydrated outer sphere complexes to clay minerals, dominantly kaolinite and minor halloysite. This proves the general assumption that the REE are adsorbed to clay minerals, and justifies the name ‘Ion Adsorption Clay Deposits’ for these ore types.