Description:
Pyrrhotite has some unusual characteristics. First,
it has an unusual formula. The amount of sulfur does
vary by roughly 20% or 50 to 55 atoms of sulfur per
50 atoms of iron. Or is it the iron that varies? Really
the same difference. Thus the unusual formula of Fe1-xS.
Secondly, it has two symmetries. While this should indicate
that there are two minerals and not one, in the case
of pyrrhotite, mineralogists have made an exception.
When pyrrhotite is low in sulfur and the formula is
closer to true FeS, then the structure is hexagonal.
But when it is high in sulfur, the structure is monoclinic.
Clearly two different symmetries, two different formulae;
therefore, two different minerals . . . except, that
in natural pyrrhotite crystals both phases are present
in the same crystal. If you are a purist, you can think
of a pyrrhotite crystal as an assemblage of two minerals,
but most minerologists treat it as one. Thirdly, pyrrhotite
is magnetic or at least weakly so. It is the next most
common magnetic mineral to magnetite. Although not all
specimens will show great evidence of magnetism if any,
some will attract a paperclip or needle suspended from
a string or move the needle of a compass. Massive pyrrhotite
is common and magnetism is sometimes the only way to
distinguish it from other brassy colored sulfides such
as chalcopyrite, pyrite, pentlandite or marcasite Good
crystals are rare and should rightly be treasured as
comming from a very unusual mineral. |
