Marcasite in Blue Agate

Marcasite is sometimes called white iron pyrite and is an iron sulfide with an orthorhombic crystal structure. It is physically and crystallographically different from Pyrite, which has a cubic crystal structure. Marcasite is also lighter and more brittle than Pyrite. Somestimes, Marcasite can crumble and break up due to its unstable crystal structure.

It is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish to brownish color and gives a black streak. It can be scratched with a knife.

In jewelry, when Pyrite is used as a gemstone, it is called “marcasite" but it is not made from the actual Marcasite mineral. In the late medieval and early modern eras, the word “marcasite” meant all iron sulfides in general, which meant both Pyrite and the mineral Marcasite.

Marcasite can form as both a primary or a secondary mineral. It typically forms under low-temperature and highly acidic conditions. Commonly associated minerals include Pyrite, Pyrrhotite, Galena, Sphalerite, Fluorite, Dolomite and Calcite. As a primary mineral, it forms nodules, concretions and crystals in a variety of sedimentary rock where it forms as sharp individual crystals and crystal groups. As a secondary mineral, it forms by chemical alteration of a primary mineral, like Pyrrhotite and Chalcopyrite.

Blueite is a nickel variety of Marcasite found in Denison Drury and Townships, Sudbury District, Ontario Canada.

Lonchidite is an arsenic variety found at Kurprinz Mine, Saxony, Germany.

Sperkise is a twin-spearhead crystal Marcasite commonly found in a Marcasite found in a chalky origin.