Occurrence and environmental constraints of gray monazite in red soils from the Campo de Montiel area (SW Ciudad Real province, south central Spain)

Abstract

Monazite ((Ce, La, Nd, Th) PO 4 ) is a rare and strategic mineral that occurs naturally as an accessory and minor mineral in diverse igneous and metamorphic rocks. This mineral does not frequently form mineable ore deposits and it has different typologies, including those formed by endogenous processes (generally "yellow monazite" mineralizations) and those formed by exogenous processes ("gray monazite" mineralizations). The mineral is an important ore of Rare Earth Elements (REEs), which have been identified by the European Union as critical raw materials. Monazite can be considered a weathering-resistant mineral, and the mobility of the REE and associated elements is low. The study reported here concerns a mineralogical and geochemical assessment of the occurrence and risks associated with the presence of concentrations of monazite in a typical, well-developed, and representative red Mediterranean soil, in order to establish the associated risk with their future mining. The results confirmed that monazite ore is particularly poor in radioactive elements, and it is concentrated in the most surficial soil horizons. The chemical mobility of REEs present in the soil, as assessed by selective extraction with ammonium acetate in acidic media, follows the order Y > Dy > U > Tb > Gd > Eu > Sm > La > Th > Ce. The mobility of REEs contained in monazite proved to be higher than that of the REE compounds in the upper horizons of the soil profile suggesting the immobilization in other REE-containing minerals, while light REEs show lower mobility rates than heavy REEs, due to an immobilization of LREE by sorption with iron oxy-hydroxides. Further studies are required in order to obtain better speciation data for REEs in soils aimed to identify soluble and insoluble compounds.

Exploring the Presence of Five Rare Earth Elements in Vineyard Soils on Different Lithologies: Campo de Calatrava, Spain

“…REEs are mobile elements during soil formation [45] and the typical weathering profiles of dolomite and limestone were analyzed [33], albeit under different conditions than those reported here. Recently, it was revealed that the greater or lesser mobility of these elements depends on whether they are heavy or light elements and also on whether they are in the surface or subsurface horizons [46]. These findings are consistent with the values for surface and subsurface horizons in the study area (both horizons are very similar values with slight differences between them).…”

Occurrence of some rare earth elements in vineyard soils under semiarid Mediterranean environment

“…Analysis of all these data leads to the conclusion that the concentrations vary by several orders of magnitude, with their mean values within the normal ranges reported worldwide (Table 3, Laul et al, 1979;Wedepohl, 1995;Diatloff et al, 1996;Lijun et al, 2004;Salminen et al, 2005;Li et al, 2008;Zhang et al, 2008;El-Ramady, 2010;Kabata Pendias, 2010;Oliveira et al, 2012;Cheng et al, 2012;Li et al, 2013;Dolegowska et al, 2013). The values are also similar to, or slightly higher than, the reported regional data in this region (Jiménez-Balleta et al, 2010;Bravo et al, 2019;Higueras et al, 2020). Diaz et al (2013) found that in Ck horizons, the highest contents of REEs corresponded to the stable interfluvial planes of a lower basin (La 25.2 mg kg −1 , Ce 41 mg kg −1 , Nd 28 mg kg −1 ).…”