三水铝石至一水软铝石转化的机制:对铝土矿中一水软铝石成因的启示

Transformation mechanism of gibbsite to boehmite:Implication for the genesis of boehmite in bauxite

铝土矿的矿石矿物主要为三水铝石、一水软铝石和一水硬铝石,是化学风化作用的产物,主要形成于炎热而潮湿气候条件下的风化壳中,但不同矿床中的主要矿物组合存在显著差异.其中,一水软铝石既是铝土矿中重要的矿石矿物,也是一种重要的无机新材料,查明该矿物的成因机制对理解铝土矿的成因及其资源的高效利用均有重要的意义.研究表明,细粒三水铝石(D_(50)=0.5μm)作为前驱体分别在175℃的水蒸气和热水中恒温12 h后,三水软铝石消耗殆尽,完全转化为具有完好自形晶体的一水软铝石.粗粒三水铝石(D_(50)=120μm)在165℃的水蒸气中恒温12 h后,在三水铝石的(001)晶面上生长出呈平行四边形板片状形貌的一水软铝石微晶;在175℃的热水中恒温12 h后,同样生成了大量平行四边形板片状一水软铝石微晶.一水软铝石晶体的大小和形貌特征表明,三水铝石向一水软铝石的转变受溶解-沉淀机制控制.由此可见,铝土矿中一水软铝石的形成可能与三水铝石的溶解有关,即受控于溶解-沉淀机制.

The main ore minerals of bauxite are gibbsite,boehmite and diaspore,which are the products of chemical weathering under hot and humid climate conditions.The mineral constituents and mineral assemblages can be significantly different from one deposit to the other.Among them,boehmite is not only an important ore mineral in bauxite,but also an important inorganic material.Unraveling the genetic mechanism of this mineral is of significance to understand the genesis of bauxite and the efficient utilization of resources.In this study,the fine-grained gibbsite(D_(50)=0.5μm)was used as the precursor for the transformation experiments that were conducted in water vapor and hot water at 175℃for 12 h,respectively.Our characterizations of the obtained products show that gibbsite was completely transformed into boehmite with intact idiomorphic morphology.In the case of the coarse-grained gibbsite(D_(50)=120μm)that was hydrothermally treated in water vapor at 165℃for 12 h,boehmite microcrystals with parallelogram plate morphology formed on the(001)crystal plane of gibbsite.Similarly,after treated in hot water at 175℃for 12 h,a large number of parallelogram plate-like boehmite microcrystals formed.The size and morphology of the resulting boehmite crystals suggests that the transformation from gibbsite to boehmite was controlled by the dissolution-precipitation mechanism.These observations imply that the formation of boehmite in bauxite might be related to the dissolution of gibbsite,i.e.,a dissolution-precipitation mechanism is involved in the transformation.