JFD154AO7: A classic from Panasqueira: large sharp crystals of Marcasite, in 'cock's comb' habit, with some Arsenopyrite, and associated with carbonates (Calcite-Dolomite in this case) which very possibly contributes to making them more stable than those from other deposits.
Minas da Panasqueira, level 2, Aldeia de São Francisco de Assis, Covilhã, Castelo Branco, Cova da Beira, Centro Region  Portugal (1993)

Specimen size: 10.4 × 7.1 × 5.1 cm = 4.09” × 2.80” × 2.01”

Main crystal size: 1.9 × 1.8 cm = 0.75” × 0.71”