MARCASITE

Marcasite, often referred to as "white iron pyrite," is an iron sulfide (FeS2) characterized by its orthorhombic crystal structure, distinguishing it from pyrite, which has a cubic structure. Both minerals contain the disulfide ion S22−, but they differ in the arrangement of these di-anions around the Fe2+ cations. Marcasite is generally lighter and more brittle than pyrite, leading to its tendency to crumble due to its unstable crystal formation. 

Freshly exposed surfaces of marcasite appear pale yellow to nearly white with a bright metallic sheen, but they tarnish to yellowish or brownish hues and produce a black streak. This brittle mineral cannot be scratched with a knife, and its thin, flat, tabular crystals, when clustered, are known as "cockscombs." Historically, the term "marcasite" encompassed all iron sulfides, including pyrite, but since 1845, it has been specifically defined as orthorhombic iron sulfide. The term "marcasite jewellery" refers to pieces featuring pyrite, reflecting the original broader meaning of the word, while true marcasite is not utilized as a gemstone due to its fragility.

Specimen originates from Wissant (Calais) France, 1976 (collected by me)