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本工作首次在实验室条件下对浅生区紫硫镍矿(Ni,Fe)3S4交代镍黄铁矿(Ni,Fe)9S8水热反应的机理及动力学进行了研究。起始反应矿物采用高纯自然镍黄铁矿,合成纯镍黄铁矿或合成镍黄铁矿-磁黄铁矿集合体。反应pH值采用0.2M醋酸-醋酸纳缓冲溶液控制在3～5的范围内。反应进程由X-射线衍射物相定量分析及扫描电镜观察进行跟踪。结果表明,当反应温度恒定在80℃时,交代20(4)%的镍黄铁矿需792h。相同条件下加入少量H2S可将反应速率提高一倍。当反应在125℃饱和蒸汽压水热环境下进行时,完全交代纯镍黄铁矿需约168h。此过程由于磁黄铁矿的存在而被催化,交代集合体中的镍黄铁矿仅需68h,进一步反应磁黄铁矿被交代成白铁矿。磁黄铁矿的催化作用可能源于溶解产生的微裂纹加速了流体的传质过程。当反应温度升高至145℃时,速率反而下降,不遵循Arrhenius经验规律。动力学分析得80℃速率常数介于5.8×10-8～3.0×10-7/s之间,125℃及145℃速率常数分别介于2.8×10-6～2.08×10-5/s及1×10-6～5.1×10-6/s之间,远高于同温度下固相扩散反应的速率常数,表明该反应在地质时标上为一快速反应。此外,用背散射电子显微技术对矿物表面形貌进行了分析,发现交代产物紫硫镍矿具有颗粒细小及存在微裂纹等特征,与自然界浅生矿床中的紫硫镍矿非常相似;电镜实验还表明该交代作用是一个典型的耦合溶解-再沉淀反应。其耦合机制的驱动力可能与反应界面处微空隙对流体饱和度的控制有关。
In this work, the mechanism and kinetics of hydrothermal reaction of nickel (Fe, Ni) Fe (subscript x) (subscript 9S8) of purple sulfur nickel ore (Ni, Fe) 3S4 in supergene are studied for the first time under laboratory conditions. The starting reaction mineral is purified high-purity natural pentlandite, synthetic pure pentlandite or synthetic pentlandite-pyrrhotite assemblages. The reaction pH is controlled within a range of 3 to 5 using a 0.2 M acetic acid-sodium acetate buffer solution. The progress of the reaction was tracked by quantitative analysis of X-ray diffraction and scanning electron microscopy. The results show that when the reaction temperature is constant at 80 ℃, account for 20 (4)% of pentlandite need 792h. The same conditions, adding a small amount of H2S can double the reaction rate. When the reaction was carried out at a saturated vapor pressure of 125 ° C under hydrothermal conditions, it took about 168 hours for the pure account of pure pentlandite. This process was catalyzed by the presence of pyrrhotite, accounting for only 68 h of the pentlandite in the assemblage, further reacting the pyrrhotite to be interpreted as a white iron ore. Pyrrhotic catalysis may result from dissolution of microcracks that accelerate the mass transfer of fluids. When the reaction temperature was raised to 145 ℃, the rate decreased instead of following the Arrhenius empirical rule. The kinetic analysis shows that the rate constants at 80 ℃ are between 5.8 × 10-8 ~ 3.0 × 10-7 / s, the rate constants at 125 ℃ and 145 ℃ are 2.8 × 10-6 ~ 2.08 × 10-5 / s and 1 × 10-6 ~ 5.1 × 10-6 / s, which is much higher than that of the solid state diffusion reaction at the same temperature, indicating that the reaction is a fast reaction on the geologic time scale. In addition, backscattered electron microscopy was used to analyze the surface morphology of the minerals. It was found that the purple-nickel-nickel ore, which is a product of the metathesis, is characterized by fine grains and micro-cracks and is similar to the purple-sulfur nickel ore in shallow deposits in nature. It is also shown that this interaction is a typical coupled lysis-reprecipitation reaction. The driving force of the coupling mechanism may be related to the control of fluid saturation at the micro-voids at the reaction interface.