,
Mecklenburg-West Pomerania, Germany Marcasite can be formed as both a primary or a secondary mineral. It typically forms under low-temperature, highly
acidic conditions. It occurs in
sedimentary rocks (
shales,
limestones and low grade
coals) as well as in low temperature
hydrothermal veins. Commonly associated minerals include
pyrite,
pyrrhotite,
galena,
sphalerite,
fluorite,
dolomite, and
calcite. As a secondary mineral, it forms by chemical alteration of a primary mineral, such as pyrrhotite or
chalcopyrite.
Sedimentary marcasite and low pH In laboratory experiments, marcasite forms preferentially to pyrite at a
pH of less than about 5.
Ab initio calculations suggest that this is due to pyrite having a higher surface energy (thus being less 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==