重晶石的水热原位混合合成实验

Hydrothermal synthesis of barite using the in-situ high temperature mixing method

重晶石(BaSO_(4))是自然界常见的硫酸盐矿物,它既可以作为矿石矿物供开采利用,同时也是热液矿床常见的脉石矿物。开展重晶石的水热合成实验对于揭示其形成条件具有重要意义。目前重晶石的水热合成主要采用室温混合钡盐和硫酸盐溶液成核、后续加温生长的实验方法(室温混合-高温熟化生长法)。为了更好地模拟重晶石的热液形成环境,本文开展了高温原位混合钡盐和硫酸盐溶液(高温原位混合法)而合成重晶石的探索性实验,并同室温混合实验进行对比研究。结果显示,2种不同方法合成的重晶石晶体大小不一样,形貌截然不同。室温混合-高温熟化生长法合成的重晶石主要为近等轴的粒状晶体,而高温原位混合法合成的重晶石为长短轴比很大的树枝状。室温混合-高温熟化生长形成的重晶石,在室温成核阶段,成核温度较低,产生的晶格缺陷或空位少,有利于沿晶体构型生长,且在后续的高温熟化生长阶段,由于已有大量晶核形成,生长环境一直处于相对较低的过饱和度条件,晶体生长较慢,主要沿原有晶面生长,不发生定向生长,从而形成晶面发育良好的近等轴的粒状晶体;而高温原位混合形成的重晶石,高温下成核与生长近乎同时进行,没有足够的时间产生完美晶核,且晶核周围处于过饱和度相对于室温混合较高的环境,晶体择优生长,从而形成长短轴比很大的树枝状,并且与海底热液喷口因流体混合作用而形成的树枝状重晶石的形貌特征很相似。实验结果表明树枝状可能是重晶石在高温流体混合条件下形成的重要形貌标型特征。

Barite(BaSO_(4))is a common sulfate mineral in nature.It can be as an ore mineral for exploitation and utilization.It is also a common gangue mineral in hydrothermal deposits.Hydrothermal synthesis of barite is of great significance to reveal its forming conditions.At present,the hydrothermal synthesis of barite mainly adopts the experimental method of the first nucleation of barite in mixed barium salt and sulfate solution at room temperature and the subsequent growth of barite by heating(the method of nucleation at room temperature and ripening growth at high temperature).In order to make better simulation for the growth condition of barite at hydrothermal system,a novel barite synthesis method(the in-situ mixing at high temperature method)has been introduced in this paper to undertake exploratory experiment of the barite synthesis by using the in-situ mixed barium salt and sulfate solution at high temperature and to compare with experimental results of the barite synthesis by using the mixed barium salt and sulfate solution at room temperature.The experimental results show that the size and morphologies of barite crystals synthesized by using the two different methods were completely different.The barite synthesized by using the method of nucleation at room temperature and ripening growth at high temperature is mainly near-equiaxed granular crystal.In contrast,the barite synthesized by using the method of in-situ growth at high temperature is dendritic crystal with large axial ratio.The barite synthesized by using the nucleation at room temperature and ripening growth at high temperature method,due to the low nucleation temperature,has a few lattice defects or vacancies.It is conducive to the crystal growing along the crystal configuration.In the subsequent ripening growth stage at high temperature,due to the existence of a large number of formed crystal nuclei,,the barite crystal is grown slowly and mainly along the original crystal growth direction rather than the preferred growth direction in the relatively less supersaturated solution to have formed the near-equiaxed granular crystal with well-developed faces.However,the barite formed by using the method of in-situ mixing of barium salt and sulfate solution at high temperature,due to its almost simultaneous nucleation and growth in a very short time at high temperature,has no produced perfect crystal nuclei.In addition,due to the crystal nuclei were surrounded by relatively high degree of supersaturated solution,the barite crystals had grown preferentially to have formed dendritic crystals with large axial ratios.These dendritic crystals are morphologically similar to the dendritic barite formed in the submarine hydrothermal vents due to the mixing of hydrothermal fluid and seawater.The experimental results show that the dendritic shape of barite crystal may be the important morphological typomorphic characteristics of barite grown under the high-temperature fluid-mixing condition.