南岭热带—亚热带风化壳中稀土元素赋存形式的初步研究

A PRELIMINARY STUDY ON MODES OF OCCURRENCE OF RARE EARTH ELEMENTS IN THE TROPICALSUBTROPICAL WEATHERING CRUST OF NANLING REGION

通过对南岭及海南岛热带-亚热带地区几十个风化壳剖面中稀土元素分布特征的研究表明,REE有如下五种主要赋存形式:(1)呈可交换状态赋存于粘土中;(2)富集于次生铁锰氧化物中;(3)呈次生稀土矿物或类似的富稀土微质点;(4)风化残余稀土(或含稀土)副矿物;(5)赋存于残留造岩矿物碎屑中。控制化学风化条件下REE淋滤、沉淀的主要因素是原岩中稀土赋存矿物的稳定性,介质环境的pH和Eh条件、REE的电子层结构及其化学键性质以及风化粘土矿物的种类和阳离子交换能力。

From a preliminary investigation into REE characteristics along tens of weathering profiles developed on the basis of different rock types in a tropical-sutropical climate of Nanling region and Hainan Island, it is known that rare earth elements have at least five modes of occurrence in the weathering crust:(1)REE in an exchangeable state in clay, accounting for 30%～90% of the total REE amount;(2)those enriched in ferromanganese oxides, which could make up 1.3% REE with enrichment coefficients ranging from 5 to 80 relative to the surrounding clays; (3)those in secondary REE minerals or, likewise, in REE-rich colloidal particles; (4)those in residual REE (or REEbearing) accessory minerals such as monazite and xenotime; and ( 5 ) those accomodated in residual rock-forming minerals. Factors controlling the dissolution, transportation and precipitation of REE under the chemical weathering condition include stability of REE-hosting minerals in the parent rock, pH and Eh values of environments, bond types of REE and anions, and species of clay minerals as well as their cation exchange capacities (CEC).