THE MINERAL PYRRHOTITE
- Chemistry: Fe1-xS (x=0 to x=0.2), Iron Sulfide
- Class: Sulfides and Sulfosalts
- Group: Nickeline
- Uses: mineral specimens and as an ore of sulfur and iron.
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-x
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.
- Color is bronze.
- Luster is metallic.
- Transparency crystals are opaque.
- Crystal System hexagonal, 6/m2/m2/m and monoclinic; 2/m
- Crystal Habits: tabular or prismatic in hexagonal prisms with a pinacoidal termination, but mostly massive rock forming beds with other sulfides.
- Cleavage none but there is a strong parting tendency.
- Fracture is uneven
- Hardness is 3.5-4.5
- Specific Gravity is 4.6
- Streak gray-black
- Associated Minerals pentlandite, quartz, ankerite, pyrite and other sulfides.
- Other Characteristics: weakly magnetic, prism faces striated parallel to pinacoid faces and color will darken with exposure to light.
- Notable Occurances Sudbury, Ontario; Ducktown, Tennessee; Chihuahua, Mexico; Russia; Germany and Brazil.
- Best Field Indicators magnetism, crystal habit, hardness and color.