Berthierite

Berthierite is a mineral, a sulfide of iron and antimony with formula. 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.
It was discovered in France in 1827 and named for the French chemist, Pierre Berthier.
Berthierine, an iron-rich phyllosilicate from the serpentine subgroup, with as chemical formula, 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.