The Golden Rule on April Fools

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Think you know which mineral Fool’s Gold is? You might be fooled...

You’ve probably heard the stories-a miner or prospector (or Olsen Twin) thinks they’ve struck it rich: gold! Eureka! Except when examined, the gold turns out to be a worthless other mineral. Fooled again by Fool’s Gold. But what are these minerals that lead us astray?

There are really three different minerals that are commonly referred to as “Fool’s Gold”: pyrite, chalcopyrite or biotite. Of the three, pyrite, also called iron pyrite, is the most commonly mistaken for gold because of its luster and yellowish hue.

Image shows a cubical sample of pyrite with a quartz crystal extending beneath it

A sample of pyrite and quartz. Iron pyrite, also known as Fool's Gold due to its resemblance to gold, often occurs in quartz veins. Pyrite is an important source of sulfur dioxide, which is primarily used to create sulfuric acid, an important industrial acid. In fact, consumption of sulfuric acid has been regarded as one of the best indexes of a nation's industrial development. More sulfuric acid is produced in the United States every year than any other chemical. For more information about sulfur, visit the USGS Minerals webpage.Sample provided by Carlin Green, USGS. Sample originated from Spruce Claim, Washington, and is 6.2cm in size. (Credit: Carlin Green, USGS. Public domain.)

Pyrite is the most common of the sulfide minerals, meaning it’s based around sulfur. It’s the sulfur that gives it its yellowish hue. Meanwhile, the metallic luster comes from the iron. Interestingly, although iron pyrite is the most famous Fool’s Gold, sometimes it can even fool people who know it as Fool’s Gold, because it can have actual gold in its mineral structure.

Close up of Chalcopyrite

A sample of chalcopyrite from Bingham Canyon, UT(Credit: Scott Horvath, USGS. Public domain.)

Calcopyrite is another type of iron sulfide mineral and closely related to pyrite. However, where chalcopyrite differs is in the presence of copper. In fact, chalcopyrite is one of the most important copper ores and a significant portion of porphyritic copper deposits. USGS assessed porphyritic copper resources as part of its 2014 Global Copper Assessment.

Image shows a macro image of biotite

A sample of biotite. (By Didier Descouens - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=7899984)

Biotite breaks the mold so far in that it is not a sulfide mineral at all, but rather related to mica. It’s not particularly yellowish, but small flakes of it in stream sediments can make a brilliant flash when light shines on it. That flash has fooled people panning for gold in streams.

So now you know, and next time the kids from the Little House on the Prairie need to learn a lesson about family values, you’ll know the end before they do!