pyrrhotite

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A mineral with composition Fe_1−xS (x = 0 to 0.2). Eskebornite, Fe_1−xSe, is the selenium analog. The iron-deficient pyrrhotites are ferrimagnetic at room temperature, but at some higher temperature they become paramagnetic, presumably because of vacancy disorder.

The mineral occurs as rounded grains to large masses, more rarely as tabular pseudohexagonal crystals and rosettes. Color is brownish bronze-yellow with dark grayish-black streaks. Hardness is 4 on Mohs scale and specific gravity 4.6 (for the composition Fe₇S₈).

Pyrrhotite occurs in basic igneous rocks as a late-stage fractional differentiate, particularly in norites and gabbros, and sufficient quantities may constitute an ore of iron. Pyrrhotite also occurs with magnetite and chondrodite in contact metamorphic marbles, and in low-temperature veins with calcite and other sulfides and sulfosalts.

Environment

Widespread in many types of occurrences, mostly those formed at higher temperatures.

Crystals usually tabular, their most common form being piles of hexagonal plates with sides that have deep horizontal striations. Its major occurrences, however, are massive and granular.

Bronze. Luster metallic; hardness 4; specific gravity 4.6-4.7; fracture subconchoidal; cleavage none, but crystals commonly show a basal parting. Brittle; magnetism varies from strong to negligible.

Ferrous sulfide (approximately 60.4% Fe, 39.6% S). There is a slight deficiency of Fe in this mineral, which makes it somewhat unstable and easily decomposed. The x in its formula ranges from 0.0 to 0.2.

Fuses easily to black magnetic mass, dissolves readily in hydrochloric acid, producing hydrogen sulfide (rotten-egg smell).

The magnetic nature of its powder is usually sufficient to distinguish it from anything similar in color (fresh bornite and niccolite) and from pyrite and chalcopyrite.

Pyrrhotite is a common mineral of magmatic sulfide segregations and high-temperature ore veins. It also occurs in pegmatites and in contact-metamorphic deposits. Good crystals have been found in Romania, exhibiting the laminated vertical development (the deep, discontinuous, horizontal grooving mentioned under Crystal description) and somewhat concave basal faces. The largest crystals of good form have come from the San Antonio mine at Aquiles Serdán (formerly Santa Eulalia), Chihuahua, Mexico. Well-formed platy crystals were found in a pegmatite at Standish, Maine. Morro Velho, Brazil's deep gold mine in Minas Gerais, is the source of small sharp hexagonal plates that are perched on calcite rhombohedrons. The main ore body at Sudbury, Ontario, is pyrrhotite, and the ore minerals pentlandite (an iron-nickel sulfide), sperrylite (a hard, white, cubic crystallized platinum arsenide), and chalcopyrite are embedded in it. A bit unstable, some pyrhotites tend to crumble in collections.

Meteorites contain the closely related nonmagnetic mineral troilite (FeS), which is regarded as a charge-balanced ferrous sulfide. Pyrrhotite's structure has been repeatedly studied, and most examples prove to be mixtures of hexagonal and monoclinic lattices. Those with less sulfur are likely to be hexagonal; those with great sulfur excesses, monoclinic. Heating to 350°C rearranges the structure to full hexagonal symmetry.

• Mineral compounds

Pyrrhotite
Pyrrhotite – Santa Eulalia Mine (Chihuahua) Mexico (7,5x7cm)
General
Category	Mineral
Chemical formula	Fe1-xS (x = 0 to 0.2)
Strunz classification	02.CC.10
Crystal symmetry	Monoclinic prismatic H-M symbol: (2/m) Space group: A2/a
Unit cell	a = 11.88 Å, b = 6.87 Å, c = 22.79 Å; β = 90.47° Z = 26
Identification
Color	Bronze, dark brown
Crystal habit	Tabular or prismatic in hexagonal prisms; massive to granular
Crystal system	Monoclinic, with hexagonal polytypes
Cleavage	Absent
Fracture	Uneven
Mohs scale hardness	3.5 – 4.5
Luster	Metallic
Streak	Dark grey – black
Specific gravity	4.58 – 4.65, average = 4.61
Refractive index	Opaque
Fusibility	3
Solubility	Soluble in hydrochloric acid
Other characteristics	Weakly magnetic, strongly magnetic on heating; non-luminescent, non-radioactive
References	[1][2][3]

Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: Fe_(1-x)S (x = 0 to 0.2). The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic. The magnetism increases as the iron content decreases, and the troilite is non-magnetic.

Contents 1 Etymology and history 2 Crystal structure 3 Magnetic properties 4 Occurrence 5 References

Etymology and history

The name pyrrhotite is derived from Greek pyrrhos, flame-colored.^[1]

Crystal structure

NiAs structure of basic pyrrhotite-1C

Pyrrhotite has a number of polytypes of hexagonal or monoclinic crystal symmetry; several polytypes often occur within the same specimen. Their crystalline structure is based on the NiAs unit cell, where metal occurs in octahedral coordination and anions in trigonal prismatic arrangement. An important feature of this structure is an ability to omit metal atoms with the total fraction up to 1/8, thereby creating iron vacancies. One of such structures is pyrrhotite-4C (Fe₇S₈). Here "4" indicates that iron vacancies form a superlattice which is 4 times larger than the unit cell in the "C" direction. The C direction is conventionally chosen parallel to the main symmetry axis of the crystal; this direction usually corresponds to the largest lattice spacing. Other polytypes include: pyrrhotite-5C (Fe₉S₁₀), 6C (Fec₁₁S₁₂), 7C (Fe₉S₁₀) and 11C (Fe₁₀S₁₁). Every polytype can have monoclinic (M) or hexagonal (H) symmetry, and therefore some sources label them, for example, not as 6C, but 6H or 6M depending on the symmetry.^[1][4] The monoclinic forms are stable at temperatures below 254 °C, whereas the hexagonal forms are stable above that temperature. The exception is for those with high iron content, close to the troilite composition (47 to 50 % atomic percent iron) which exhibit hexagonal symmetry.^[5]

Magnetic properties

The ideal FeS lattice, such as that of troilite, is non-magnetic. The ferromagnetism which is widely observed in pyrrhotite is therefore attributed to the presence of relatively large concentrations of iron vacancies (up to 20%) in the crystal structure. Vacancies lower the crystal symmetry. Therefore, monoclinic forms of pyrrhotite are in general more defect-rich than the more symmetrical hexagonal forms, and thus are more magnetic.^[6] Upon heating to 320 °C, pyrrhotite loses its magnetism, but also starts decomposing to magnetite. The saturation magnetization of pyrrhotite is 0.12 tesla.^[7]

Occurrence

Pyrrhotite is a rather common trace constituent of mafic igneous rocks especially norites. It occurs as segregation deposits in layered intrusions associated with pentlandite, chalcopyrite and other sulfides. It is an important constituent of the Sudbury intrusion where it occurs in masses associated with copper and nickel mineralisation.^[5] It also occurs in pegmatites and in contact metamorphic zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite. Pyrrhotite does not have specific applications. It is mined primarily because it is associated with pentlandite, sulfide mineral that can contain significant amounts of nickel and cobalt.^[1]

References

1. ^ ^{a b c d} "Pyrrhotite". Mindat.org. http://www.mindat.org/min-3328.html. Retrieved 2009-07-07. 
2. ^ Handbook of Mineralogy
3. ^ Webmineral data
4. ^ Hubert Lloyd Barnes (1997). Geochemistry of hydrothermal ore deposits. John Wiley and Sons. pp. 382–390. ISBN 0-471-57144-X. http://books.google.com/?id=vy2_QnyojPYC&pg=PA383. 
5. ^ ^{a b} Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 20th ed, 1985, pp. 278-9 ISBN 0-471-80580-7
6. ^ Suna Atak, Güven Önal, Mehmet Sabri Çelik (1998). Innovations in Mineral and Coal Processing. Taylor & Francis. p. 131. ISBN 90-5809-013-2. http://books.google.com/?id=fI8Yo0bX7BwC&pg=PA131. 
7. ^ Jan Svoboda (2004). Magnetic techniques for the treatment of materials. Springer. p. 33. ISBN 1-4020-2038-4. http://books.google.com/?id=WFBpOXSe8kQC&pg=PA33. 

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