Name/Title
MarcasiteEntry/Object ID
2009.62.22Description
Chemical Composition: FeS2
Crystal System: Orthorhombic System
Description: Common Name: Marcasite Dollar
Chemistry: FeS2, Iron Sulfide
Group: Sulfides
Location: Pennsylvania, USA
Description: Similar to pyrite the crystals are in the shape of a sand dollar and is called a marcasiter dollar, the matrix is slate.
PHYSICAL CHARACTERISTICS:
Color is brassy yellow with a greenish tint at times. A multi-colored tarnish may exist that is the result of oxidation.
Luster is metallic.
Transparency: Crystals are opaque.
Crystal System is orthorhombic; 2/m 2/m 2/m
Crystal Habits include the tabular, bladed or prismatic forms. A twinning effect produces spear shaped crystal and repeated twinning produces a "cock's comb" cluster. Also massive, botryoidal, stalactitic and nodular. Sometimes as a replacement mineral of fossils and a pseudomorph of pyrite.
Cleavage is poor in two directions.
Fracture is uneven.
Hardness is 6 - 6.5
Specific Gravity is approximately 4.8+ (average for metallic minerals)
Streak is greenish to brownish black.
Other Characteristics: A sulfur smell is sometimes detectable.
Associated Minerals are calcite, dolomite, quartz, goethite, fluorite, pyrrhotite, bornite, chalcocite, sphalerite, pyrite, galena and other sulfides.
Best Field Indicators are crystal habit, smell and greenish tint
Fracture: irregular- uneven
Hardness: 6 Orthoclase
Luster: Metallic
Occurrence: Marcasite can be formed as both a primary or a secondary mineral.
As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, such as at Dover, Kent, England, where it forms as sharp individual crystals and crystal groups, and nodules (similar to those shown here) in chalk. It can also be found in low-temperature hydrothermal veins.
As a secondary mineral it forms by chemical alteration of a primary mineral such as pyrrhotite or chalcopyrite. On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. 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."
Marcasite may go through a condition known as "pyrite decay", in which a specimen slowly disintegrates into a white powder. Little is known about this detrimental condition. It only affects certain marcasite specimens seemingly at random, while other specimens remain unaffected. When a specimen goes through pyrite decay, the marcasite reacts with moisture in the air, the sulfur combining with water to produce sulfuric acid that attacks other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals to prevent this "disease" from spreading.
Crystal structure of marcasite. Sulfur atoms are red.Some research has suggested bacteria may aid and accelerate this process by literally 'eating' the marcasite. What is known is that samples with a rough surface tend to decay faster than those with bright, shiny faces, probably due to the greater surface area to react with water in the air, and also it's clear that samples kept in a dry environment (low humidity) are less likely to decay.
Rock Type: Sedimentary
Specific Gravity: 4.8
Streak: greenish to brownish
Texture: Microcrystaline
Variety: The mineral marcasite, sometimes called white iron pyrite, is iron sulfide (FeS2). Marcasite is often mistakenly confused with pyrite, but marcasite is lighter and more brittle. Specimens of marcasite often crumble and break up due to the unstable crystal structure, and it is this crystal structure that is the main difference between marcasite and pyrite. Though marcasite has the same chemical formula as pyrite, it crystallizes in a different crystal system, thereby making it a separate mineral. In jewelry, pyrite used as a gem is improperly termed "marcasite". True marcasite is never used as a gem, due to its brittle and chemically unstable structureCollection
RAGM Mineral CollectionAcquisition
Accession
2009.62Source or Donor
Museum Collection of MineralsAcquisition Method
DonationDimensions
Width
9-1/2 inDepth
1-3/4 inLength
12-1/4 inWeight
6.93 ozDimension Notes
Dimension taken at widest pointsLocation
Location
Display Case
FS-3-BRoom
Frieda Smith HallBuilding
Crater Rock MuseumCategory
ExhibitMoved By
Jillian Mather KettleyDate
January 29, 2025Notes
Location/ inventory