Playing with Polymorphs

FeS₂
The Pyrite-Marcasite Polymorph
by
Dina Wingfield

A Geology 336 Mineralogy Web Page Project

Pyrite in Pyritohedron Form

Image taken from:
http://mineral.galleries.com/
scripts/item.exe?ENLARGEC+
Minerals+Sulfides+Pyrite+PYR-85

Marcasite in Cockscomb Habit

Image taken from:
http://mineral.galleries.com/
scripts/item.exe?ENLARGEC+
Minerals+Sulfides+Marcasite+MAR-15

Specific Information Regarding the Polymorphs,
Pyrite and Marcasite
What is a polymorph?	Chemical Composition and Crystallography	Physical Properties
Associated Minerals	Origin of Formation Field Indicators	Habits and Forms
Mineral Use	Other Polymorph Examples	References

What is a polymorph?

Polymorphs occur when a specific chemical compound exists in nature in more than one structural or atomic arrangement. In other words, same chemical make-up, different arrangement of atoms. To give you a simplified example, I have included a pair of polymorphs made from pattern blocks. Note that they are composed of exactly the same materials (1 large yellow hexagonal figure and 6 red rhombus shaped figures). However, they are clearly arranged differently, resulting in 2 very distinct, shapes (aka crystal shape).

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Chemical Composition and Crystallography
of Pyrite and Marcasite

Mineral Species Chemical Composition Crystal System and Space Group
Pyrite

The small black spheres represent
the Fe ion, whereas the larger gray
spheres represent the sulfur anions. FeS₂, based on the NaCl structure, where the Na ions are replaced with iron, and the Cl anions are replaced with the sulfur anion pair. Pyrite crystallizes in the Isometric crystal system, bar32/m. Space Group = Pa3.
Marcasite

The small black spheres represent
the Fe ion, whereas the larger gray
spheres represent the sulfur anion. FeS₂, and not based upon any familiar crystal structure. Marcasite crystallizes in the Orthorhombic crystal system, 2/m2/m2/m. Space Group = Pnnm.

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Physical Properties
of Pyrite and Marcasite

Property Pyrite Marcasite

Luster Metallic Metallic

Hardness 6-6.6 6-6.5

Specific Gravity 5.02 4.89

Cleavage Indistinct Poor in two directions

Fracture Conchoidal, brittle Uneven, brittle

Streak Greenish or brownish black Greenish to brownish black

Color Pale brassy yellow to black. May show an iridescence when tarnished. Brassy yellow with a greenish tint at times. Tarnish may form as a result of oxidation.

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Associated Minerals

Mineral Species Associated Minerals

Pyrite Pyrite is often associated with quartz, calcite, gold, copper, sphalerite, galena, fluorite and marcasite among others.

Marcasite Marcasite is often associated with calcite, dolomite, quartz, pyrite, galena, sphalerite, pyrrhotite, and goethite among others.

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Origin of Formation and Field Indicators
for Pyrite and Marcasite

Mineral Species Origin of Formation Field Indicators

Pyrite Pyrite forms most commonly at high temperatures (though it has been found to occur at lower temperatures as well) as a result of an iron-sulfur reaction in dry conditions. It can be found in hydrothermal deposits or as accessory minerals in igneous and metamorphic rocks. Has a deeper color than marcasite (paler than chalcopyrite). Cubic and pyritohedron forms are distinctive. Often termed "fools gold", pyrite is more brittle and much harder than actual gold.

Marcasite Marcasite forms at lower temperatures and must be in solution. It can often be found in association with lead and zinc ores, and often occurs in replacement deposits in limestones, or in concretions embedded in clays or shales. Color is often a paler yellow in comparison to pyrite. The cockscomb habit is distinctive as is the presence of a sulfur smell (due to oxidation breakdown).

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Habits and Forms of
Pyrite and Marcasite

Pyrite is found in a many different forms. Perhaps the most easily identified is the cube. It can also be found in the pyritohedron and octahedron form. Common habits include massive, reniform and penetration twins (called iron crosses). See photos below for examples of pyrite forms and habits.

   Pyrite cubes with quartz.
Image courtesy of Bob's Rock Shop,
http://www.rockhounds.com/rockshop/keller2.html.
Image taken from Mineral Museum at University of Delaware,
http://www.museums.udel.edu/mineral/collection/ .html.
Image taken from:
http://www.museums.udel.edu/mineral/collection/Regional/D4738.html.
Image courtesy of:
http://www.mineral.galleries.com/minerals/sulfides/pyrite/pyrite.html.

Marcasite can be found in tabular, bladed or prismatic forms. Habits include massive, botryoidal, stalactitic and nodular. Twinning can produce spear shaped crystals (sometimes called spear pyrite) and most notably, the cockscomb cluster.   See photos below for examples of marcasite forms and habits.

Image taken from:
http://mineral.galleries.com/minerals/sulfides/marcasit/marcasit.htm.
Image taken from:
http://www.museums.udel.edu/mineral/collection/periodictable/periodic.html.
Image taken from:
http://www.museums.udel.edu/mineral/displaycollection/Sulfides/100049.html.

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Uses for Pyrite and Marcasite

Mineral Species Common Uses

Pyrite Pyrite is most widely used as a source of sulfur and sulfuric acid.   Since it commonly occurs with other metals (gold and copper), it is also used as an ore. Sometimes pyrite is polished and cut and used as a gemstone, however, it is marketed under the name "marcasite."

Marcasite Marcasite is also used as a source of sulfur, but not nearly so often as its polymorph, pyrite. It is also cut and polished and sold as a gemstone.

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Other Examples of Polymorphs

A few other minerals known for their polymorphic abilities include diamond and graphite, and calcite and aragonite.

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References

Books

Dennen, W.H., (1960). Principles of mineralogy. NY: The Ronald Press Company. pp.206-207 and 243-245.

Klein, C. (2002). Mineral science. (22nd Ed.) NY: John Wiley and Sons, Inc. pp. 364-367.

Electronic References

The Mineral Gallery, The Sulfides Class, World Wide Web URL: http://mineral.galleries.com/minerals/sulfides/class.htm. Retrieved on November 17, 2002.

Mineralogical Database, World Wide Web URL: http://webmineral.com. Retrieved on November 17, 2002.

http://cobra.aml.arizona.edu/

http://www.theimage.com/mineral/index.htm

Mineral Species	Chemical Composition	Crystal System and Space Group
Pyrite The small black spheres represent the Fe ion, whereas the larger gray spheres represent the sulfur anions.	FeS₂, based on the NaCl structure, where the Na ions are replaced with iron, and the Cl anions are replaced with the sulfur anion pair.	Pyrite crystallizes in the Isometric crystal system, bar32/m. Space Group = Pa3.
Marcasite The small black spheres represent the Fe ion, whereas the larger gray spheres represent the sulfur anion.	FeS₂, and not based upon any familiar crystal structure.	Marcasite crystallizes in the Orthorhombic crystal system, 2/m2/m2/m. Space Group = Pnnm.

Return to student web pages.

	Pyrite cubes with quartz. Image courtesy of Bob's Rock Shop, http://www.rockhounds.com/rockshop/keller2.html.
	Image taken from Mineral Museum at University of Delaware, http://www.museums.udel.edu/mineral/collection/ .html.
	Image taken from: http://www.museums.udel.edu/mineral/collection/Regional/D4738.html.
	Image courtesy of: http://www.mineral.galleries.com/minerals/sulfides/pyrite/pyrite.html.

Property	Pyrite	Marcasite
Luster	Metallic	Metallic
Hardness	6-6.6	6-6.5
Specific Gravity	5.02	4.89
Cleavage	Indistinct	Poor in two directions
Fracture	Conchoidal, brittle	Uneven, brittle
Streak	Greenish or brownish black	Greenish to brownish black
Color	Pale brassy yellow to black. May show an iridescence when tarnished.	Brassy yellow with a greenish tint at times. Tarnish may form as a result of oxidation.

Mineral Species	Associated Minerals
Pyrite	Pyrite is often associated with quartz, calcite, gold, copper, sphalerite, galena, fluorite and marcasite among others.
Marcasite	Marcasite is often associated with calcite, dolomite, quartz, pyrite, galena, sphalerite, pyrrhotite, and goethite among others.

Mineral Species	Origin of Formation	Field Indicators
Pyrite	Pyrite forms most commonly at high temperatures (though it has been found to occur at lower temperatures as well) as a result of an iron-sulfur reaction in dry conditions. It can be found in hydrothermal deposits or as accessory minerals in igneous and metamorphic rocks.	Has a deeper color than marcasite (paler than chalcopyrite). Cubic and pyritohedron forms are distinctive. Often termed "fools gold", pyrite is more brittle and much harder than actual gold.
Marcasite	Marcasite forms at lower temperatures and must be in solution. It can often be found in association with lead and zinc ores, and often occurs in replacement deposits in limestones, or in concretions embedded in clays or shales.	Color is often a paler yellow in comparison to pyrite. The cockscomb habit is distinctive as is the presence of a sulfur smell (due to oxidation breakdown).

Mineral Species	Common Uses
Pyrite	Pyrite is most widely used as a source of sulfur and sulfuric acid. Since it commonly occurs with other metals (gold and copper), it is also used as an ore. Sometimes pyrite is polished and cut and used as a gemstone, however, it is marketed under the name "marcasite."
Marcasite	Marcasite is also used as a source of sulfur, but not nearly so often as its polymorph, pyrite. It is also cut and polished and sold as a gemstone.