Pyrrhotite Inclusion — Iron Sulphide Trapped in Diamond
Pyrrhotite Inclusion — Iron Sulphide Trapped in Diamond
A common sulphide inclusion type used in mantle research
Pyrrhotite is a non-stoichiometric iron sulphide with the formula Fe1−xS, occurring as inclusions in diamond and, less commonly, in other gemstones. Pyrrhotite is among the most common sulphide inclusions found in natural diamonds and is used in research on diamond formation as one of the principal indicator phases for the chemistry and oxidation state of the deep mantle environment in which diamond crystallises. The mineral appears in inclusion suites as dark, opaque, often irregularly shaped grains, sometimes accompanied by graphite, magnetite, or other sulphide phases.
Mineralogy and significance
Pyrrhotite has a crystal structure derived from the nickel arsenide type, with iron vacancies producing the non-stoichiometric composition that distinguishes it from stoichiometric troilite (FeS). The vacancy concentration controls magnetic behaviour: monoclinic 4C pyrrhotite is ferrimagnetic and noticeably magnetic, while the hexagonal high-temperature polymorphs are antiferromagnetic. The mineral commonly occurs in association with pentlandite (a nickel-iron sulphide) in mantle assemblages, and the pyrrhotite-pentlandite ratio in diamond inclusions provides information on the temperature and chemistry of diamond crystallisation.
Pyrrhotite inclusions in diamond are typically of mantle origin and have been radiometrically dated by the Re-Os isotopic system to constrain the age of the host diamond. Such studies have demonstrated that many lithospheric diamonds formed billions of years ago, with mantle pyrrhotite providing the geochronometric evidence. Sulphide inclusions, including pyrrhotite, are therefore among the most scientifically valuable inclusions in diamond despite — or because of — their visual unattractiveness in cut stones.
In the trade and gemmological documentation
Pyrrhotite inclusions are documented in the standard inclusion photoatlases of Gübelin and Koivula and in numerous research papers in Gems & Gemology and other journals. Visually, the inclusions appear as small dark spots or irregular grains, sometimes surrounded by stress haloes from the differential thermal expansion of inclusion and host during cooling from mantle temperatures. The presence of sulphide inclusions does not significantly affect diamond clarity grading except where the inclusions are large or numerous enough to impact the overall clarity assessment under standard 10× examination.
Pyrrhotite has been documented less commonly as an inclusion in other gemstones, principally in some sapphires from sediment-hosted deposits and in certain garnet specimens. In these contexts the mineralogical significance is reduced, and the inclusions are essentially incidental rather than diagnostic. See also sulphide inclusion in diamond.