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The mineral marcasite, sometimes called "white iron pyrite", is iron sulfide (FeS 2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures contain the disulfide S 2 2− ion, having a short bonding distance between the sulfur atoms.
Marcasite jewellery is jewellery made using cut and polished pieces of pyrite (fool's gold) as gemstone, and not, as the name suggests, from marcasite. [1] Both pyrite and marcasite are chemically iron sulfide, but differ in their crystal structures, giving them different physical properties. Pyrite is more stable and less brittle than marcasite.
Martensite is formed in carbon steels by the rapid cooling of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe 3 C). Austenite is gamma-phase iron (γ-Fe), a solid solution of iron and alloying elements.
Marcasite FeS 2; Molybdenite MoS 2; Sulfarsenides: Cobaltite (Co,Fe)AsS; Arsenopyrite FeAsS; Gersdorffite NiAsS; Sulfosalts: Pyrargyrite Ag 3 SbS 3; Proustite Ag 3 AsS 3; Tetrahedrite Cu 12 Sb 4 S 13; Tennantite Cu 12 As 4 S 13; Enargite Cu 3 AsS 4; Bournonite PbCuSbS 3; Jamesonite Pb 4 FeSb 6 S 14; Cylindrite Pb 3 Sn 4 FeSb 2 S 14
A mineral's hardness is a function of its structure. Hardness is not necessarily constant for all crystallographic directions; crystallographic weakness renders some directions softer than others. [70] An example of this hardness variability exists in kyanite, which has a Mohs hardness of 5 1 ⁄ 2 parallel to but 7 parallel to . [71]
Extended Wulff constructions refers to a number of different ways to model the structure of nanoparticles as well as larger mineral crystals, and as such can be used to understand both the shape of certain gemstones or crystals with twins.as well as in other areas such as how nanoparticles play a role in the commercial production of chemicals using heterogeneous catalysts.
Arsenopyrite has a related structure with heteroatomic As–S pairs rather than S-S pairs. Marcasite also possesses homoatomic anion pairs, but the arrangement of the metal and diatomic anions differs from that of pyrite. Despite its name, chalcopyrite (CuFeS 2) does not contain dianion pairs, but single S 2− sulfide anions.
Drawings are available showing their presence in marcasite, gold, silver, copper and diamond. New mineral forms with a fiveling structure continue to be found, for instance pentagonite, whose structure was first decoded in 1973, is named because it is often found with the five-fold twinning. [2] [15]