Numerous, thin plates of pyrrhotite to about 1.6 cm which have been overgrown with/replaced by marcasite (or maybe pyrite), accented with colorless gypsum. Sphalerite (as cleavages in the matrix) and quartz (as sparse, small points) are also present. Marcasite has a (well deserved) reputation as a mineral with fickle stability, often a victim of 'pyrite rot' resulting in crumbling and foul odors. However, I have possessed this specimen since 2009 with no evidence of rot. Based on Mindat photos, pyrrhotite is rare and marcasite not reported at Naica. These minerals (including pyrrhotite replacement specimens) are more common from the Santa Eulalia Mining District, so the locality listed here (and on a previous dealer label) may in fact be Santa Eulalia and not Naica.