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Pyrrhotite

Pyrrhotite

General
Category	Mineral
Chemical formula	iron sulfide:Fe_1-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.

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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]