Pyrrhotite

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Pyrrhotite

General
Category Mineral
Chemical formula iron sulfide:Fe1-xS (x = 0 to 0.2)
Identification
Color Bronze
Crystal habit Tabular or prismatic in hexagonal prisms; massive to granular
Crystal system hexagonal, 6/m2/m2/m and monoclinic, 2/m
Cleavage Absent
Fracture Uneven
Mohs Scale hardness 3.5 - 4.5
Luster Metallic
Refractive index Opaque
Streak Dark grey - black
Specific gravity 4.6
Fusibility 3
Solubility Soluble in hydrochloric acid
Other Characteristics Weakly magnetic, strongly magnetic on heating

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. Also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic, the magnetism increases as the iron content decreases.

Pyrrhotite is odd also because it has two crystal symmetries. When pyrrhotite is high in iron and the formula is closer to true FeS the structure is hexagonal. But, when it is low in iron, the structure is monoclinic. Both symmetries occur together in the same specimen.

The name is derived from Greek pyrrhos, flame-colored.

Pyrrhotite is a rather common trace constituent of igneous rocks. It occurs as segregation deposits from mafic igneous rocks associated with pentlandite, chalcopyrite and other sulfides. It also occurs in pegmatites and in contact metamorphic zones.

The troilite endmember, though only rarely encountered in the Earth's crust, is found in many meteorites. One iron meteorite, Mundrabilla, contains 25 to 35 volume percent troilite.[1]

[edit] References and external links

  1. ^ Vagn Buchwald, Handbook of Iron Meteorites, 1975.