Cronstedtite
Cronstedtite is a complex iron silicate mineral belonging to the serpentine group of minerals. It has a formula of Fe2Fe(Si,FeO5)(OH)4.
It was discovered in 1821 and named in honor of Swedish mineralogist Axel Fredrik Cronstedt (1722–1765). It has been found in Bohemia in the Czech Republic and in Cornwall, England.