Two-dimensional pyrite crystals (40–80 μm wide and 2–3 μm thick) and large thin crusts are reported from the mudstones from the Carboniferous coal basin in Poland. Crystals occur on a flat surface, originally probably a crack in the rock, and are composed of uniform particles (150–200 nm wide). A hypothetical pathway of the formation of 2D pyrite crystals is presented: (1) formation of pyrite particles (or monosulphide precursors) in the suspension introduced onto the surface of the crack, and forming a film with a smooth meniscus at the air/suspension interface on the rock substrate; (2) thinning of the suspension film due to the water loss, increase of particle concentration, and formation of the first monolayers; (3) growth leading to the formation of thin crystals complying with pyrite crystallography.

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