Chemical composition аnd morphology of pyrite and marcasite in blue clay from the Maritsa East lignite basin, Bulgaria

Pyrite and marcasite aggregates were found in a sample of blue silty clay from an exploration drill core at the northern edge of the Troyanovo North mine, Maritsa East lignite basin, Bulgaria. The aggregates have various sizes – from 50 µm (single crystal twins) to 0.5 cm (large twinned aggregates). Pyrite occurs as single cubic and cuboctahedral crystals, penetrative twins forming oval-shaped aggregates with visible cubic to truncated octahedral habits. Marcasite occurs as single tabular, pyramidal crystals, but often as twinned (penetrative, parallel, contact) composite oval-shaped aggregates. Marcasite has higher trace element contents than pyrite. In both minerals are detected Mn, Co, Ni, Cu, Zn, Mo, As, Sb, Hg, Tl and Pb with varying contents. The crystal aggregate growth is slow in varied supersaturation and pH due to the occurrence of both octahedral and cubic faces in pyrite and marcasite presence.