Marcasite, also called a Pyrite of white iron has a crystal structure relating to the orthorhombic form, and is a sulfide of iron (FeS2). You have to remember that this mineral is different from Pyrite in both its crystallography and its physical characteristics. The similarity between the two lies in the iron of disulfide S22 that brings about the sulfur atoms to retain a short bonding length. However, the difference lies in how the Fe2+ cations are surrounded by the di-anions. Another difference is that Pyrite is less brittle and heavier than Marcasite. The unstable crystal formation of Marcasite causes it to break up and crumble.
Derivation of Name
Pyrite’s Arabic name is from where Marcasite’s name has been derived. Similar characteristics between Pyrite and Marcasite often cause people to confuse between the two. In other words, Marcasite, being Pyrite’s polymorph, shares the chemistry, but is differentiated by their structure leading to different crystal shapes and symmetry.
Marcasite may be formed as a primary or a secondary mineral. Generally, it gets formed under low-temperature and highly acidic environment. It is found in sedimentary rocks (shales, limestones and low grade coals) and also in hydrothermal veins of low temperatures. It is usually associated with minerals like Pyrite, Galena, Sphalerite, Pyrrhotite, Fluorite, Calcite and Dolomite.
When it serves a gemstone’s role in Marcasite jewellery it’s termed as "Marcasite", implying that Marcasite jewellery uses Pyrite, not Marcasite. From the late medieval period of early modern times, the term "Marcasite" was used to describe Pyrite plus Marcasite (and iron sulfides generally). Scientifically speaking Marcasite is not used as a gem due its brittleness. Current scientific definition of Marcasite’s structure as an iron sulfide in the orthorhombic form dates back to 1845.
As a primary mineral it forms concretions, nodules and crystals in a range of sedimentary rocks. For instance, at Dover, Kent, England it forms sharp individual crystals plus crystal groups and nodules.
It is realized that Marcasite pseudomorphs other minerals. A pseudomorph is an atom by atom substitution of one mineral's chemistry to another. When subtly done, the substitution can keep the old mineral's shape unharmed resulting to one mineral in the shape of another, therefore the term pseudomorph (Latin for false shape). Marcasite has pseudomorphed Gypsum, Pyrite, Fluorite plus others. At times Marcasite is itself pseudomorphed into the iron oxide mineral Goethite. Usually, it’s only a peripheral replacement, leaving a thin coat of iron oxides on the crystals. These iron oxides are noticed as an iridescent sheen and can impart an attractive and vibrant appearance to Marcasite specimens.
Tin-white on fresh surfaces, darkening on exposure, pale bronze-yellow, dull iridescent.
Mohs scale hardness