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Marcasite
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<p>The <a href="/facts/Mineral/LkUFbNmv">mineral</a> marcasite, sometimes called "white iron pyrite", is <a href="/facts/Iron_sulfide/EJQPNLFT">iron sulfide</a> (FeS2) with <a href="/facts/Orthorhombic_crystal_structure/3zo4F8P0">orthorhombic crystal structure</a>. It is physically and crystallographically distinct from <a href="/facts/Pyrite/41CYREyr">pyrite</a>, which is iron sulfide with <a href="/facts/Cubic_crystal_system/92Co7BKM">cubic crystal structure</a>. Both structures contain the disulfide S22− <a href="/facts/Ion/3bePpDna">ion</a>, having a short bonding distance between the <a href="/facts/Sulfur/IK0amTfM">sulfur</a> atoms. The structures differ in how these <a href="/facts/Dianion/wrbwhF3z">di-anions</a> are arranged around the <a href="/facts/Fe2%2B/AGImjWqq">Fe2+</a> <a href="/facts/Cations/wrbwhF3z">cations</a>. Marcasite is lighter and more <a href="/facts/Brittleness/cn27H2vg">brittle</a> than pyrite. Specimens of marcasite often crumble and break up due to the unstable <a href="/facts/Crystal_structure/bJ4bCeHd">crystal structure</a>. </p><p>On fresh surfaces, it is pale yellow to almost white and has a bright metallic <a href="/facts/Lustre_(mineralogy)/TCobwMGC">luster</a>. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs". </p><p>In the late medieval and early modern eras, the word "marcasite" meant all iron sulfides in general, including both pyrite and the mineral marcasite. The narrower, modern scientific definition for marcasite as specifically orthorhombic iron sulfide dates from 1845. Jewellery where <a href="/facts/Pyrite/41CYREyr">pyrite</a> is used as the gemstone is called <a href="/facts/Marcasite_jewellery/tVpRoNAj">marcasite jewellery</a>; a term which pre-dates the scientific definition, using the original sense of the word. Marcasite in the scientific sense is not used as a gem due to its brittleness. </p>