Mobility of Rare Earth Elements in Basalt‐Derived Laterite at the Bolaven Plateau,Southern Laos

Geochemical and geochronological studies were conducted on basalts and laterites from the Bolaven Plateau in southern Laos in order to evaluate the mobility and mineralization of REE, Y and Sc during laterization. The basalts are classified into three categories: (i) small volumetric alkali basalt (eruption age: 15.7 Ma), large volumetric olivine tholeiite (1.2 Ma) and quartz tholeiite with olivine tholeiite (younger than 0.5 ± 0.2 Ma). Formation of REE minerals during laterization result in mobilization and fractionation of REE and Y in laterite profiles. Occurrence of florencite‐(Ce) in a laterite profile derived from alkali basalt immobilizes REE (particularly LREE) and this leads the laterites to be enriched in LREE relative to the parent basalt. Few positive Ce anomalies in this profile suggest that florencite‐(Ce) [(Ce)Al₃(PO₄)₂(OH)₂] formation was followed by CeO₂ precipitation due to the change of redox condition. In tholeiite‐derived laterite profiles, florencite is not recognized and REE and Y tend to be depleted relative to the parent basalts with positive Ce anomalies. This is interpreted as scavenging REE³⁺ except for Ce⁴⁺ from the laterite profile in oxidizing conditions. Sc behaves similarly to Fe during laterization and it is more abundant in the tholeiitic laterite than that in the alkali basaltic laterite. Results of sequential extraction indicate that REE of the alkali basaltic laterite are contained in residual phase, which is dominantly florencite‐(Ce), but they are rarely present in ion‐adsorption phase. It is concluded that basaltic laterites have a low potential of REE resource in terms of low REE contents and a difficulty in REE extraction.     

纤磷钙铝石类化合物:来自地球具工艺用途的材料(英文)

纤磷钙铝石类化合物系由自纤磷钙铝石CaAl3 (OH) 6(HPO4) (PO4)本身、经磷钙铝石CaAl3 (OH) 6(SO4) (PO4)到钾明矾石KAl3 (OH ) 6(SO4) 2 等的天然矿物族所组成的。它们表现出具有很大范围的阳离子置换系列 ,其中以Sr2 +和Ba2 +置换Ca2 +,REEs3 +置换Ca2 +和H+,As5 +置换P5 +,S6+/Se6+置换H+P5 +最为重要。随着置换的进行 ,其热动力学稳定性也逐渐增加。由此 ,纤磷钙铝石类化合物成为很能抗风化的矿物 ,在红土中尤其如此。厚的纤磷钙铝石层形成在闪长岩 (富Sr) ,碳酸盐岩 (富REEs)和含金石英脉 (富As)之上 ,从而保护整个磷酸盐红土 ,使之免受风化。以此方式 ,形成了桌状山脉或岛屿 ,他们具有未受覆盖的表面。这些“纤磷钙铝”石化合物为上述及其它元素形成一有效的地球化学障。“纤磷钙铝石”的这种天然地球化学固定作用可以用适当的母体或人工合成晶体来进行模拟 ,即通过金属阳离子交换或就地反应来模拟。这种固定作用还可用于处理天然和人造的放射性裂变产物、有毒的重金属元素、砷酸盐和硒酸盐、硫酸盐等。