PYRRHOTITE OLD STOCK

PYRRHOTITE OLD STOCK

An amazing mineral Pyrrhotite Old Stock!  A collection-worthy piece.  This specimen displays a crystal of Pyrrhotite and is from old stock.  Pyrrhotite crystals are rare and hard to come by.  A single partial crystal with laminated structure, minor surface Pyrite and some Calcite.  An interesting difference between Pyrite and Pyrrhotite is a magnet will stick to Pyrrhotite but not to Pyrite.  Please read the interesting details below about weakly magnetic and two minerals in one.

Location:

Dimensions:
5cm x 4.8cm x 5.5cm, 299g.

Pyrrhotite: Fe₇S_8,
Also given as Fe_1-x (x = 0 to 0.17).

Named in 1847 by Ours Pierre Armand Petit Dufrenoy from Greek “pyrrhos”, flame-coloured.  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, 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 coming from a very unusual mineral.