Expert: Dr Thomas Bell - 4/28/2009

Question
I would like to perform a lab in school in which I make Iron Pyrite, more commonly known as Fools' Gold. I would like to know how it is formed naturally, if that method is safe and usable, and, if not, how I could make it artificially. If you know anything, I would by much obliged.

Thank you.

Answer
Stephen,

There is no simple way to make pyrite in the lab without expensive equipment and hazardous materials.  Pyrite is a very common mineral and there are many geologic environments where it forms.  The environment that is most accessible to you might be a pond or swamp.  The natural raw ingredients for pyrite are sulfur and iron.  

Iron can be dissolved in water in the absence of oxygen.  The mud in a pond or swamp is saturated with water that has been depleted in oxygen by bacteria as they digest organic debris like pieces of waterlogged wood or other plant material and dead animals.  

Some types of bacteria also metabolize sulfate, a soluble sulfur compound that is found in most natural waters.  You may have noticed a very strong smell of rotten eggs when you have disturbed the black mud at the bottom of a pond or swamp.  This is hydrogen sulfide gas, the waste product of sulfate reducing bacteria.

These two types of bacteria create the conditions for pyrite formation.  Aerobic bacteria digesting organic debris deplete the water in oxygen allowing iron to go into solution.  Sulfate reducing bacteria transform sulfate into sulfide.  The sulfide and iron in solution can then combine to form pyrite.  This step is probably aided by other bacteria as well.  The pyrite that forms is initially amorphous which means it is not in crystalline form.  With age, this amorphous iron sulfide forms ordered crystals of the minerals pyrite or marcasite.

Coal also forms in swamps and commonly has pyrite embedded within it.  When we burn this coal, the pyrite also burns and can release sulfur to the atmosphere as pollution.  Coal fired power plants in the United States have a 'scrubber' on their smoke stack to remove the sulfur before it escapes to pollute the air.