Description
The mineral marcasite, or spear stone, is a pale bronze-yellow to tin-white iron sulfide with the chemical formula FeS2. The name is derived from the Arabic word Makraschit, the original name for pyrite and similar minerals. Marcasite is rarely used as a gemstone, but was especially popular in France during the Middle Ages, where it was used as a folk ornament. Today, small cut marcasites are set in silver, especially in Thailand. In prehistoric times, it was used as a fire starter in combination with flint. It is found in hydrothermal, postvolcanic, hypergenic, and sedimentary rocks. Marcasite has the same chemical formula as cubic pyrite but a different crystal structure (orthorombic). This is called isomorphism. Marcasite darkens rapidly in air, and specimens of this mineral can even crumble or disintegrate over time. It is of little importance as an iron or sulfur ore mineral. In the gemstone trade, the name marcasite is incorrectly used for cut pyrite. This "marcasite," with a brass-yellow color and a high metallic luster, is primarily set in silver jewelry as a decorative element. The name marcasite is of Arabic or Moorish origin. It was used for all pyrite-like minerals such as marcasite, chalcopyrite, sphalerite, and others. Marcasite is typically formed hydrothermally and is found in sedimentary rocks (such as limestone, shale, or lignite) or in veins (with zinc and lead sulfide). Marcasite forms at low temperatures and under reducing conditions. It is found in chalk deposits, such as those at Cap Blanc-Nez near Calais, France. It is sensitive to heat and sweat; sulfur dioxide is released in fire. The mineral corrodes at the surface.
