Commonly know as iron
pyrite or "fools gold", and often found in nature as golden cubes.
Pyrite is the most
common of the sulfide minerals. The name pyrite is from a
Greek word meaning fire. When struck by steel, brilliant
sparks are emitted.
- Chemistry: FeS2, Iron Sulfide
- Class: Sulfides
- Group: Pyrite
- Uses: A very minor ore of sulfur
for sulfuric acid, used in jewelry under the trade name
"marcasite" and valued as mineral specimens and
collection pieces.
Pyrite is the classic
"Fool's Gold". There are other shiny brassy yellow minerals,
but pyrite is by far the most common and the most often
mistaken for gold. Whether it is the golden look or
something else, pyrite is a favorite among rock collectors.
It can have a beautiful luster and interesting crystals. It
is so common in the earth's crust that it is found in almost
every possible environment, hence it has a vast number of
forms and varieties.
Bravoite is the
name given to a nickel-rich iron sulfide. It is closely
related to pyrite but contains up to 20% nickel. Some
mineral books treat it as a variety of pyrite.
Pyrite is a polymorph
of marcasite, which means that it has the same
chemistry, FeS2, as marcasite; but a different structure and
therefore different symmetry and crystal shapes. Pyrite is
difficult to distinguish from marcasite when a lack of clear
indicators exists.
Pyrite's structure is
analogous to galena's structure with a formula of PbS.
Galena though has a higher symmetry. The difference between
the two structures is that the single sulfur of galena is
replaced by a pair of sulfurs in pyrite. The sulfur pair are
covalently bonded together in essentially an elemental bond.
This pair disrupts the four fold symmetry that a single atom
of sulfur would have preserved and thus gives pyrite a lower
symmetry than galena.
Although pyrite is
common and contains a high percentage of iron, it has never
been used as a significant source of iron. Iron oxides such
as hematite and magnetite, are the primary
iron ores. Pyrite is not as economical as these ores
possibly due to their tendency to form larger concentrations
of more easily mined material. Pyrite would be a potential
source of iron if these ores should become scarce.
Pyrite has been mined
for its sulfur content though. During WWII, sulfur was in
demand as a strategic chemical and North American native
sulfur mines were drying up. A sulfide deposit near
Ducktown Tenn. was found to be able to mine pyrite and other
sulfides such as
pyrrhotite and
pentlandite and produce the needed sulfur as well as
iron and other metals. The sulfur was used in the production
of sulfuric acid, an important chemical for industrial
purposes. Now most sulfur production comes from H2S gas
recovered from natural gas wells.
PHYSICAL CHARACTERISTICS:
- Color is brassy yellow.
- Luster is metallic.
- Transparency: Crystals are
opaque.
- Crystal System is isometric;
bar 3 2/m
- Crystal Habits include the
cube, octahedron and pyritohedron (a dodecahedron with
pentagonal faces) and crystals with combinations of these
forms. Good interpenetration twins called iron
crosses are rare. Found commonly in nodules. A flattened
nodular variety called "Pyrite Suns" or "Pyrite
Dollars" is popular in rock shops. Also massive,
reniform and replaces other minerals and fossils forming
pseudomorphs or copies.
- Cleavage is very indistinct.
- Fracture is conchoidal.
- Hardness is 6 - 6.5
- Specific Gravity is
approximately 5.1+ (heavier than average for metallic
minerals)
- Streak is greenish black.
- Other Characteristics:
Brittle, striations on cubic faces caused by crossing of
pyritohedron with cube. (note - striations on cube faces
also demonstrate pyrite's lower symmetry). Pyrite unlike
gold is not malleable.
- Associated Minerals are
quartz, calcite, gold, sphalerite, galena, fluorite
and many other minerals. Pyrite is so common it may be
quicker to name the unassociated minerals.
- Notable Occurrences include
Illinois and Missouri, USA; Peru; Germany; Russia; Spain;
and South Africa among many others.
- Best Field Indicators are
crystal habit, hardness, streak, luster and brittleness.
