Marcasite: The Brother of Pyrite

Marcasite and pyrite have a great deal in common. In fact they are each composed of the same chemical! Read on and learn more about this fascinating mineral--the brother of pyrite.
Marcasite is a polymorph of the chemical iron sulfide, just like pyrite. (You can read more about polymorphs in my post Polymorphism: One Chemical Many Minerals published on this blog May 6, 2011) Marcasite is of a similar pale-yellow color and it shines with a metallic luster as does pyrite. It gets its name from the Arabic word for pyrite and jewelers still call pyrite “marcasite.”
If there are these many similarities between the two minerals then how on earth can anyone tell them apart? Well, it has been difficult. It wasn’t until the 1800s that anyone could differentiate the two. There are some characteristics that are different, though. For starters marcasite has a different crystal shape than pyrite. Pyrite crystals are cubic whereas marcasite’s are orthorhombic. Now, when its crystals twin they can look like spears. Marcasite can also form in bladed, tabular, prismatic, massive, botryoidal, nodular, and stalactitic habits as well as “cockscombs” which look like the combs on chickens’ heads. Unless a marcasite specimen has very well-formed crystals it can still be a challenge to differentiate it from pyrite. Fortunately marcasite’s streak is a little different (read more about the streak test in my Nov. 1, 2010 post Pyrite the Imposter) Marcasite’s streak is greenish brown whereas pyrite’s is greenish black. Marcasite also exhibit’s a sulfur smell, which, as far as I know, pyrite does not.
There is one property that marcasite exhibits that pyrite does not, which I find rather unfortunate. Marcasite is unstable and will decay in the air. How fast it decays depends a lot upon how pure the sample is and how it is stored and handled. I keep mine in an air-tight container with a moisture-absorber, but so far no one has found a way to keep them from decaying indefinitely.
Marcasite is found in such places as: the Czech Republic, France, Greece, Belgium, Russia, Peru, Mexico, Peru, Japan, and the U.S. states of Michigan, Oklahoma, Wisconsin, Pennsylvania, Arkansas, and Illinois.
I hope that you enjoyed today’s post about pyrite’s brother marcasite. Next time I plan to write about marcasite’s unfortunate decaying problem.