## 4 Discussion  
### Supergene Enrichment of REE  
#### 4.1.1 Enrichment of REE in Terra Rossa in Northern Vietnam  
REE supergene enrichment under the weathering process had been widely reported on laterite overlaying granite, basalt, and alkaline rocks [41; 42; 43; 44]. Some studies focused on the carbonate rock weathering crusts in subtropical areas [32; 45], but few had been conducted on tropical terra rossa [13; 14; 24; 46].  
To identify the main factors that affect the enrichment of REE in Northern Vietnam, we analyzed the correlation between REE and major elements. The results are shown in Table 4. REE, LREE, and HREE are extremely significantly and positively correlated with Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and Zr, and especially with Fe. The correlation with CaO and MgO was negative, especially with SiO\({}_{2}\). This indicates that the content of REE in the study area is controlled by SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and Zr, etc. The laterite profile of SC and TG profiles experienced the aluminum-rich and desilication stage, during which the Si was lost, and Fe and Al were enriched. At this stage, rare earth elements were co-deposited with Al and Fe.  
Figure 5: C1 Chondrite-normalized REE patterns of samples of the SC and TG profiles. The concentration was normalized to Chondrite after [PERSON] and [PERSON] [22].  
Figure 6: Bedrock-normalized REE distribution patterns for samples from the SC and the TG profiles.  
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c} \hline  & **RE** & **LERE** & **HERE** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **MgO** & **CaO** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **MnO** & **TiO\({}_{2}\)** & **P\({}_{3}\)O\({}_{5}\)** & **FeO** & **Zr** \\ \hline RERE & 1 & 1 & & & & & & & & & & & \\ LLERE & 0.999 \({}^{\ast}\) & 1 & & & & & & & & & & & \\ HREE & 0.974 \({}^{\ast}\) & 0.966 \({}^{\ast}\) & 1 & & & & & & & & & & \\ SO2 & 0.818 \({}^{\ast}\) & \(-\)0.814 \({}^{\ast}\) & \(-\)0.827 \({}^{\ast}\) & 1 & & & & & & & & & \\ Al2O3 & 0.782 \({}^{\ast}\) & 0.779 \({}^{\ast}\) & 0.783 \({}^{\ast}\) & \(-\)0.947 \({}^{\ast}\) & 1 & 1 & & & & & & & \\ Fe2O3 & 0.810 \({}^{\ast}\) & 0.977 \({}^{\ast}\) & 0.824 \({}^{\ast}\) & \(-\)0.886 \({}^{\ast}\) & 1 & 1 & & & & & & \\ MgO & -0.137 \({}^{\ast}\) & 0.184 \({}^{\ast}\) & -0.285 & 0.237 \({}^{\ast}\) & \(-\)0.247 & \(-\)0.503 & 1 & & & & & & \\ CaO & -0.003 \({}^{\ast}\) & -0.011 \({}^{\ast}\) & 0.186 \({}^{\ast}\) & \(-\)0.225 \({}^{\ast}\) & \(-\)0.342 \({}^{\ast}\) & 0.108 \({}^{\ast}\) & 1 & & & & & \\ Na2O & -0.326 \({}^{\ast}\) & -0.317 \({}^{\ast}\) & -0.377 \({}^{\ast}\) & 0.411 \({}^{\ast}\) & \(-\)0.456 \({}^{\ast}\) & \(-\)0.229 \({}^{\ast}\) & 0.040 \({}^{\ast}\) & 1 & & & & \\ K2O & -0.530 \({}^{\ast}\) & -0.518 \({}^{\ast}\) & -0.595 \({}^{\ast}\) & \(-\)0.799 \({}^{\ast}\) & \(-\)0.700 \({}^{\ast}\) & \(-\)0.71 Rare earth minerals are generally thought to mainly exist in three forms: in mineral lattice in the form of ionic compounds, such as monazite, fluorocarbon, REE-phosphates, and REE-carbonates; dispersed in impure minerals in the form of isomorphs, such as apatite and fluorite; and adsorbed on the surface of some minerals in an ionic state, such as clay minerals and mica minerals [33].  
The main clay minerals in the red soil of Northern Vietnam include illite, kaolinite, chlorite, vermiculite, montmorillonite, plagioclase, potassium feldspar and goethite (discussed separately). According to the analysis of the correlation between REEs and major elements, REEs are adsorbed in clay minerals in an ionic state at the end of weathering. REEs are thought to be mainly adsorbed in kaolinite, chlorite, montmorillonite, and goethite. REEs have a strong correlation with TiO\({}_{2}\) and Zr, indicating that REEs may occur in minerals containing Ti and Zr in the form of cerium niobium perovskite, titanium-uranium, ilmenite, zircon, rutile, spinel, etc.