pyrrhotite

Pyrrhotite
General
Category	Mineral
Chemical formula	iron sulfide:Fe1-xS (x = 0 to 0.2)
Identification
Color	Bronze, dark brown
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
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.

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. Troilite can be found as a native mineral on Earth, but is more abundant in meteorites, in particular those originating from the Moon and Mars. Uniform presence of troilite on the Moon and possibly on Mars has been confirmed by the Apollo, Viking and Phobos space probes. The relative intensities of isotopes of sulfur are rather constant in meteorites as compared to the Earth minerals, and therefore troilite from Canyon Diablo meteorite is chosen as the international sulfur isotope ratio standard.

Etymology and history

The name pyrrhotite is derived from Greek pyrrhos, flame-colored.^[1] Troilite is named after Italian abbot Domenico Troili who first noted the mineral in a meteorite that fell in 1766 at Albareto, Modena (Italy). Troili wrote the first description of the fall of a meteorite in a 43-page document published in 1766.^[2] He collected reports from many eyewitnesses, closely examined the stone and detected in it small grains of a brassy mineral he called "marchesita", which was long assumed to be pyrite, FeS₂. However, in 1862, German mineralogist Gustav Rose analyzed the composition and determined it as FeS. Rose named this new mineral troilite after Troili.^[3]

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.^[4][1]

Troilite has hexagonal structure (Pearson symbol hP24, Space group P-62c No 190). Its unit cell is approximately a combination of two vertically stacked basic NiAs-type cells of pyrrhotite, where the top cell is diagonally shifted.^[5] For this reason, troilite is sometimes called pyrrhotite-2C.^[4]

Magnetic properties

The ideal FeS lattice, such is 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

Etched surface of a part of the Mundrabilla meteorite (~3.6 cm width). The darker striated material is troilite

Pyrrhotite is a rather common trace constituent of igneous rocks all over the world. 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. 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]

On the contrary, the troilite is extremely rarely encountered in the Earth's crust. (Even pyrrhotite is relatively rare compared to pyrite and Iron(II) sulfate minerals) Most troilite on Earth is of meteoritic origin. One famous iron meteorite, Mundrabilla, which is the 11th heaviest iron meteorite found on Earth,^[8] contains 25 to 35 volume percent troilite.^[9] The most famous troilite-containing meteorite is Canyon Diablo. Canyon Diablo Troilite (CDT) is used as a standard of relative concentration of different isotopes of sulfur.^[10] Meteoritic standard was chosen because of the constancy of the sulfur isotopic ratio in meteorites, whereas the sulfur isotopic composition in Earth materials varies due to the bacterial activity. In particular, bacteria reduce ³²SO2−4 1.07 times faster than ³⁴SO2−4 that may increase the ³⁴S/³²S ratio by up to 10%.^[11]

Troilite is the most common sulfide mineral at the lunar surface. It forms about 1 % of the lunar crust and is present in any rock or meteorite originating from moon. In particular, all basalts brought by the Apollo 11, 12, 15 and 16 missions contain about 1% of troilite.^{[5][12][13][14]}

Troilite is regularly found in Martian meteorites (i.e. those originating from Mars). Similar to the Moon's surface and meteorites, the fraction of troilite in Martian meteorites is close to 1%.^[15][16] Those observations are supported by the measurements performed by Viking 1, by Viking 2 X-ray fluorescence soil analyses and by Phobos-2 orbital γ-ray data.^[17][18]

Based on observations by the Voyager spacecraft in 1979 and Galileo in 1996, troilite might also be present in the rocks of Jupiter’s satellites Ganymede and Callisto. ^[17] Whereas experimental data for Jupiter's moons are yet very limited, the theoretical modeling assumes large percentage of troilite (~22.5%) in the core of those moons.^[19]

References