Stone of the Week - Pyrite

Pyrite is a metallic mineral that is comprised of iron and sulfur with the chemical formula FeS2. It belongs to a group of minerals called “sulfide minerals” and is the most common mineral of the group. It can often be found with other sulfide minerals (as well as other mineral types), often in quartz veins, coal, sedimentary or metamorphic rocks. The name pyrite comes from the Green word “pyros”, meaning fire, due to the fact that pyrite can spark if struck. It looks so much like gold to the untrained eye, that it has also earned the nickname “fools gold”. In fact sometimes gold can be found in the same locations as pyrite, making it even harder to tell which is which! However they can be told apart, as pyrite is lighter than gold, yet pyrite can’t be scratched with a fingernail or pocket knife like gold can because it is harder.

Pyrite is a metallic mineral that is a bright, brass yellow color, and found in a variety of forms.


Pyrite can be found in a variety of forms, from massive accumulations to various crystal forms. The most recognizable is the cubic form, which often have striations running across the surface. It has a hardness of about 6–6.5, actually making it softer than quartz; additionally it can also be brittle and break in an uneven or somewhat conchoidal manner. However pyrite is heavier than many common minerals, with a specific gravity of almost 5. When pyrite is exposed, it can react with both oxygen and water, resulting in the formation of acid.


Pyrite can be found in massive forms, sometimes readily seen along surfaces of rocks, such as this rock outcrop of metamorphic gneiss located in Canada.


There are several minerals that are very similar to pyrite. Marcasite is what is called a ‘polymorph’ of pyrite, meaning that while it has the same chemical formula, it is structurally different, and thus a different mineral. Chalcopyrite has copper added to it (CuFeS2), and is also a brassy yellow color, but is softer and lighter than pyrite (can you think of why this might be?). Bornite has even more copper and added sulfur (Cu5FeS4); you may know this mineral as “peacock ore” as when it tarnishes it turns from a copper-red to iridescent blues and purples.


Peacock ore is a technically called Bornite, and has a similar chemical composition to Pyrite. When it oxidizes these amazing colors can form as a result!


Pyrite has been used in the past for use in fire arms due to its ability to spark when struck. In more modern times it has been used to produce sulfur dioxide or sulfuric acid for industrial use, though this is not as common as it used to be. Pyrite in recent years has become available for use as beads or pendants for jewelry work. Care must be taken when using it – be sure to NEVER store it completely sealed without something to absorb moisture. Many a collector has stored pyrite in air tight containers or zippy bags, only to find a slushy mess a few months later. This is because the pyrite will react with the moisture and air trapped with it, resulting in the disintegration of it and the production of acid in its place (hence if you come across this situation handle with care!)

Rarely, fossils can undergo a process called "replacement", where the fossil is replaced with a mineral (while keeping its original form); in this case an ammonite found in Russia has been replaced with pyrite.



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Pyrite Facts:
Chemical composition: FeS2
Crystal System: Isometric
Color: Pale brass-yellow
Habit: Massive, Crystals (often cubic)
Fracture: Uneven, Slightly Conchoidal
Cleavage: Poor
Luster: Metallic
Transparency: Opaque
Hardness: 6–6.5
Specific Gravity: ~5g/cm3
Streak: Greenish-black
Occurrence: Worldwide



References

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Pyrite. http://en.wikipedia.org/wiki/pyrite

Mindat – Pyrite. http://www.mindat.org/min-3314.html