Pyrite and marcasite are two minerals that share many similarities. They represent a mineralogical phenomenon known as a polymorph. Both have the same chemical composition but they crystallize into different crystal systems. Examples of other mineral polymorphs are calcite and aragonite and diamonds and graphite.
Crystal Formation
Pyrite and marcasite are both iron sulfide minerals with the chemical formula FeS2. Pyrite crystallizes in the isometric system and commonly forms cubic crystals. It may also form an octahedron or pyritohedron. Marcasite crystallizes in the orthorhombic system and forms needle, cockscomb or spear-shaped crystals. When tested using the streak test, both pyrite and marcasite form dark gray to black streaks. Both have a 6 to 6.5 Mohs hardness.
Appearance
Both pyrite and marcasite have a metallic luster and are often mistaken for one another. However, pyrite has a distinct brassy-to-gold color and is often mistaken for gold. Marcasite is a paler yellow or silver color. It oxidizes readily to red, brown or black. As they oxidize, both pyrite and marcasite may exhibit iridescence.
Where to Find
Pyrite is found in a variety of geological settings, including sedimentary, metamorphic, and igneous rocks. It is often associated with other minerals such as quartz, calcite and fluorite. Pyrite can also be found in hydrothermal veins and as a replacement mineral in fossils. Pyrite is a common mineral and is found all over the world, with significant deposits in Spain, Peru and the United States.
Marcasite is typically found in sedimentary rocks. It is often associated with organic matter such as coal. It is also found in hydrothermal veins. Unlike pyrite, marcasite is not as common and is found in limited locations around the world including France, Germany and the United States.
Uses
Pyrite is used as a source of sulfur dioxide in the production of sulfuric acid and paper production. It can also be used as a replacement mineral in the production of lithium batteries called “fool’s gold batteries.” Marcasite is used in jewelry and ornamental objects because of its metallic luster and unique crystal shapes. Both can be prone to decay over time, especially when exposed to air and moisture.
Alternate Uses
For those who believe that minerals and crystals possess energetic properties, pyrite is thought to enhance willpower, motivation and assertiveness. Marcasite is said to promote a sense of stability and balance and to promote emotional healing.
This story about pyrite and marcasite previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Richard Gross and Pam Freeman.
