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Troilite (FeS) is by far the most abundant lunar sulphide mineral. Although this minor mineral has never been recorded exceeding 1 vol%, it is ubiquituous. It is commonly associated with spinel, ilmenite, and native Fe.
Some troilite is thought to have formed at a 988oC eutectic point in Fe/S melts, where native Fe and troilite form simultaneously. Some troilite is devoid of native Fe, so this must have formed directly from a sulphur-rich silica melt. As well as late-stage crystallization, troilite may occur in solid rocks, where the partial pressure of sulphur increases rapidly. Some Apollo 16 rocks appear to show this, where native Fe has been "sulphurized" during shock metamorphism from meteoroid impacts. Primary troilite usually has less Ni and P, and more Co than that of troilite of meteoritic origin.
The mineral troilite is actually stoichiometric pyrrhotite. Pyrrhotite ranges in composition from Fe7S8 through to FeS. Most terrestrial pyrrhotite lack some iron. The troilite end-member (FeS) occurs mainly in meteorites and lunar samples.
Fe7S8 is monoclinic (pseudohexagonal), whilst Troilite
(FeS) is hexagonal.
Non-stoichiometric compositions are due to missing iron atoms, and not
the replacement of iron atoms with sulphur.
Ni, Co, Mn, and Cu can substitute for Fe. Such impurities have never been found above 1wt%, in lunar samples.
Many specimens of pyrrhotite are ferromagnetic (i.e., capable of acting as a magnet). This effect decreases as the ideal composition of FeS is reached. Troilite is ideally antiferromagnetic.
Pyrrhotite is decomposed by HCl (releasing H2S) whereas pyrite is not. Troilite is attacked more readily by dilute nitric acid than pyrrhotite.
Both are opaque in thin section.
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