PYRRHOTITE
Pyrrhotite is an iron monosulfide, unlike pyrite,
which is an iron disulfide (FeS2). So, the chemical formula
for pyrrhotite should be FeS. It isn't, however. The atomic
structure of pyrrhotite often has holes - the mineral has an insufficient
number of iron atoms. The formula for
pyrrhotite ends up being Fe1-xS. In meteorites, iron
monosulfide lacks the atomic-scale “holes” of pyrrhotite, and is called troilite
(FeS).
Pyrrhotite is superficially like pyrite in appearance
and chemistry, but they are definitely different minerals. Pyrrhotite has
a metallic luster, a brownish-brassy or bronzish color, a black streak, no
cleavage, and is magnetic. What’s particularly distinctive about
pyrrhotite is that it is variably magnetic. The holes in the
atomic structure gives pyrrhotite its magnetism. But, there's variation
in the number of missing iron atoms from sample to sample. So, pyrrhotite
ends up having variable magnetism. The more holes, the more magnetic the
sample. The fewer holes, the less magnetic the sample.
Pyrrhotite has economic significance, as it often
occurs with nickel-, copper-, and platinum-bearing minerals. A great
example is the Sudbury Mining District in Ontario, Canada.
Pyrrhotite (all but the peacock iridescent areas - that's tarnished chalcopyrite)
(field of view ~3.2 cm across) from Falconbridge, Ontario, Canada. This
specimen is noticeably magnetic, but there's no magnetite component.