thermodynamically stable) than marcasite at low pH. Due to the association of marcasite with low pH, the occurrence of marcasite in sedimentary rocks in the geologic record implies the presence of highly acidic conditions during the formation and early diagenesis of those rocks. However, sedimentary pore waters below the modern ocean are typically buffered at near-neutral to slightly alkaline pH by dissolved carbonate species. This raises the question of how sedimentary pore waters became sufficiently acidic to promote marcasite formation in the past. Several theories have been proposed for the formation of early diagenetic marcasite, including: partial oxidation of primary pyrite by molecular oxygen infiltrating from the overlying water column, and rapid anoxic organic matter decomposition and organic acid generation by fermentation and methanogenesis. Varieties and blends Blueite (S.H.Emmons): Nickel variety of marcasite, found in Denison Drury and Townships, Sudbury Dist., Ontario, Canada. Lonchidite (August Breithaupt): Arsenic variety of marcasite, found at Churprinz Friedrich August Erbstolln Mine (Kurprinz Mine), Großschirma Freiberg, Erzgebirge, Saxony, Germany; ideal formula Fe(S, As)2. Synonyms for this variety: kausimkies, kyrosite, lonchandite, metalonchidite (Sandberger) described at Bernhard Mine near Hausach (Baden), Germany. Sperkise: designates a marcasite having twin spearhead crystal on {101}. Sperkise derives from the German Speerkies (Speer meaning spear and Kies gravel or stone). This twin is very common in the marcasite of a chalky origin, particularly those from the Cap Blanc-Nez. Decay Marcasite reacts more readily than pyrite under conditions of high humidity. The product of this disintegration is iron(II) sulfate and sulfuric acid. The hydrous iron sulfate forms a white powder consisting of the mineral melanterite, FeSO4·7H2O. This disintegration of marcasite in mineral collections is known as "pyrite decay". When a specimen goes through pyrite decay, the marcasite reacts with moisture and oxygen in the air, the sulfur oxidizing and combining with water to produce sulfuric acid that attacks other sulfide minerals and mineral labels. Low humidity (less than 60%) storage conditions prevents or slows the reaction. References External links Sulfide minerals Iron(II) minerals Alchemical substances Orthorhombic minerals Minerals in space group 58 Pyrite group Disulfides