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Structure of framboidal pyrite: a single crystal X-ray diffraction study

¹ School of Earth, Ocean and Planetary Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK
² EPSRC National X-Ray Crystallography Service, Department of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK

^* Corresponding author, present address: Geodynamics Research Center, Ehime University, Matsuyama, 790-8577, Ehime, Japan; e-mail: ohfuji{at}sci.ehime-u.ac.jp

The crystallographic nature of pyrite framboids, self-assembled multicrystalline pyrite, was investigated by single crystal X-ray diffraction. The results show that even morphologically well-organised framboids are not extreme examples of single crystals but are microcrystalline aggregates. Two dimensional diffraction patterns obtained from well-organised framboids composed of relatively few (1000) microcrystals, are single-crystal like but detailed analysis shows that they are actually produced by the superimposition of XRD reflections from similarly, but not identically, oriented microcrystals. The number of coherent crystallographic orientations of pyrite crystals estimated from a complete Ewald sphere data set obtained from a larger single framboid, were found to be approximately four to five times fewer than the number of the microcrystals that are actually contained in the framboid. The result shows the presence of multiple single crystal-like domains that are arranged in similar crystallographic orientations. The results show that framboids are not formed by a process in which crystallographic control is a primary factor. Framboid self-assembly is probably caused by the aggregation of a large number of similarly sized and shaped pyrite microcrystals that are nucleated in a limited volume. Self-organisation occurs as these microcrystals adopt a minimum energy configuration.

Key-words: framboidal pyrite, framboid, microcrystal, single crystal XRD, self-organisation.

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