JFD50AN8: Marcasite is a mineral with a bad reputation since it is less stable than other iron sulphides, but the vast majority of Marcasites from Panasqueira are very stable since they are not usually pure marcasites but epitactic intergrowths of Arsenopyrite-Marcasite and that gives them great stability. This particularly elegant specimen is very well preserved despite the number of years I've had it and despite the fact that I live in a city with very high humidity, which does not help the conservation of sensitive species.
Minas da Panasqueira, level 0, Aldeia de São Francisco de Assis, Covilhã, Castelo Branco, Cova da Beira, Centro Region  Portugal (12/2014)

Specimen size: 7.2 × 5.4 × 3.1 cm = 2.83” × 2.13” × 1.22”

Main crystal size: 1.2 × 1 cm = 0.47” × 0.39”