Despite the natural abundance of pyrite and marcasite and their intergrowth, and a wealth of information they can provide on the physical–chemical conditions of mineral deposits, a complete mechanistic and kinetic study on the phase transformation from the thermodynamically metastable polymorph marcasite to the stable polymorph pyrite is yet to be made. This limits the application of marcasite as an indicator mineral for low-temperature geological environments. Here, we report results from in situ synchrotron powder X-ray diffraction and ex situ anneal/quench experiments at 400–540 °C, demonstrating that the mechanism and kinetics of this transformation depend not only on temperature, but also on particle size, the presence of water vapor, and the presence of pyrite inclusions in marcasite. Under dry conditions, the transformation is limited by surface nucleation and occurs via epitaxial nucleation of pyrite on marcasite, with {100} pyrite //{101} marcasite and {001} pyrite //{010} marcasite . In contrast, in the presence of water vapor, there is little crystallographic orientation relationship between the two phases; the transformation is still limited by surface nucleation, but modification of the surface properties by water vapor results in a different nucleation mechanism, and consequently different kinetics. Kinetic analysis estimates a half-life of 1.5 Ma at 300 °C for the transformation under dry conditions with small and pyrite-free marcasite grains, but this estimation should be used with extreme caution due to the complexity of the transformation. From synchrotron X-ray fluorescence elemental mapping, trace elements (As and Pb) play an insignificant role in the transformation. However, the presence of a fluid phase changes the behavior of Pb. Under dry conditions randomly oriented particles of galena formed in pyrite, while under water vapor conditions arrays of nano- to microparticles of galena precipitated in pores. This study highlights that although the natural occurrence of marcasite can indicate low-temperature environments, precise estimation of temperature should not be made without considering the influences from various reaction parameters.

中文翻译：

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白铁矿向黄铁矿转变的机制和动力学：原位和非原位实验及地质意义

尽管黄铁矿和白铁矿的天然丰度及其共生，以及它们可以提供关于矿床物理化学条件的大量信息，但对从热力学亚稳态多晶型白铁矿到稳定多晶型的相转变进行了完整的机械和动力学研究黄铁矿尚未制成。这限制了白铁矿作为低温地质环境指示矿物的应用。在这里，我们报告了原位同步加速器粉末 X 射线衍射和 400–540 °C 下的非原位退火/淬火实验的结果，表明这种转变的机制和动力学不仅取决于温度，还取决于粒径、水蒸气的存在，以及白铁矿中黄铁矿夹杂物的存在。在干燥条件下，转变受到表面成核的限制，并通过黄铁矿在白铁矿上的外延成核发生，{100} 黄铁矿 //{101} 白铁矿和 {001} 黄铁矿 //{010} 白铁矿。相比之下，在有水蒸气存在的情况下，两相之间几乎没有晶体取向关系；转变仍然受到表面成核的限制，但水蒸气对表面性质的改变会导致不同的成核机制，从而导致不同的动力学。动力学分析估计干燥条件下在 300°C 下转化的半衰期为 1.5 Ma，具有小且不含黄铁矿的白铁矿晶粒，但由于转化的复杂性，应极其谨慎地使用这种估算。从同步加速器 X 射线荧光元素映射，微量元素（As 和 Pb）在转化中的作用微不足道。然而，流体相的存在会改变 Pb 的行为。在干燥条件下，方铅矿颗粒在黄铁矿中形成随机取向，而在水蒸气条件下，方铅矿纳米到微米颗粒阵列沉淀在孔隙中。这项研究强调，虽然白铁矿的自然存在可以表明低温环境，但不应该不考虑各种反应参数的影响就对温度进行精确估计。