Mechanism and kinetics of pseudomorphic mineral replacement reactions: A case study of the replacement of pentlandite by violarite

作者：Xia Fang; Brugger Joel; Chen Guorong; Ngothai Yung; O'Neill Brian; Putnis Andrew; Pring Allan^*

来源：Geochimica et Cosmochimica Acta, 2009, 73(7): 1945-1969.

DOI：10.1016/j.gca.2009.01.007

Although pseudomorphic mineral replacement reactions are common in all geological environments, and have long been considered important to many geological processes such as metamorphism, metasomatism, diagenesis, and chemical weathering, their mechanisms are still not well known. We present a combined textural and kinetic study of the replacement of pentlandite, (Fe,Ni)(9)S(8), by violarite (NiFe)(3)S(4), and describe the mechanisms and kinetic behavior of this reaction by considering the role of the fluid phase, the causes of coupling between pentlandite dissolution and violarite precipitation, the rate-limiting steps controlling the kinetic behavior, and the origin of the length scale of the features preserved during pseudomorphism.
The experiments were conducted under mild hydrothermal conditions (80-210 degrees C, vapor saturated pressures). Reaction kinetics shows a complex behavior depending on various physical and chemical parameters including temperature, pH, concentrations of various reaction species, solid-weight-to-fluid-volume-ratio and specific surface area. The rate of replacement (i) increases with temperature from 80 to 125 degrees C, then decreases as temperature further increases to 210 degrees C, (ii) first increases then decreases with decreasing pH from pH 6 to 1, (iii) increases with increasing concentration of oxidants such as O(2(aq)), H(2)O(2), and KMnO(4), but decreases with increasing concentration of Ni(2 ) and Fe(3 ), and with increasing solid-weight-to-fluid-volume ratio, (iv) increases linearly with the specific surface area. This kinetic behavior as well as the resulting textures revealed a coupled dissolution-reprecipitation reaction mechanism.
Nanometer-scale pseudomorphic replacement, through which the crystallographic orientation of pentlandite is inherited by violarite, occurs only between 1This case study illustrates some general principles of replacement reactions proceeding via the coupled dissolution-reprecipitation mechanism: (i) primary mineral dissolution needs to be rate-limiting compared to the secondary mineral precipitation in order to achieve a high degree pseudomorphic replacement; (ii) the effects of solution composition on reaction kinetics can be qualitatively rationalized by considering the rate-limiting step reaction.

• 出版日期2009-4-1
• 单位东华大学