Berthierite

Berthierite is a mineral, a sulfide of iron and antimony with formula FeSb2S4 (FeS·Sb2S3). It is steel grey in colour with a metallic lustre, which an iridescent tarnish can cover. Because of its appearance, it is often mistaken for stibnite (Sb2S3, an antimony trisulfide mineral, free of Fe2+ sulfide). It was discovered in France in 1827 and named for the French chemist, Pierre Berthier (1782–1861). Berthierine, an iron(II)-rich phyllosilicate from the serpentine subgroup, with as chemical formula (Mg,Fe)3(Si2O5)(OH)4, was also named in honor of Pierre Berthier. The great similarity between the two names is a frequent source of confusion between these two very different mineral species.