Abstract
We studied pyrite spherules built of radiating crystals to establish the relationships among microstructure, composition, and macroscopic appearance. Results of high-resolution transmission electron microscopy (HRTEM) imaging and selected-area electron diffraction (SAED) show that many pyrite crystals contain planar faults perpendicular to one of the [001] axes. A comparison between HRTEM micrographs and images simulated for defect model structures indicates that the faults can be interpreted as single (101) layers of marcasite that disrupt the regular sequence of (002) layers in pyrite. Such a marcasite layer can be described as a boundary between two pyrite crystals that are related by a 21 screw axis parallel to [100]. Energy-dispersive X-ray spectrometry (EDS) indicates that such disordered pyrite crystals contain about 3 at% As. However, the distribution of As is uniform over heavily faulted and fault-free regions, indicating the As content is not related to the occurrence of marcasite lamellae in pyrite.
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