Pyrite varieties in Estonian Tremadocian argillite (Dictyonema shale)

Estonian Tremadocian organic-rich argillite (Dictyonema shale) contains up to 20% organic matter, and 4-6% pyrite. Most of the pyrite occurs in fine-dispersed form invisible with the naked eye. About 60% of the thin sandstone interlayers in argillite are cemented with pyrite (type 1), half of them are covered with a thin finegrained pyrite seam (type 2). The constitution of the latter prompted to consider its composition and genesis similar to the fine-dispersed pyrite that cannot be separated from argillite. Besides these two types, very thin ( < 1 mm) light-grey siltstone interlayers, pyrite knobs, and some pyrite findings other than the described varieties have been studied by X-ray diffractometry and emission spectroscopy methods.