Pyrite (#59)

Lustrous cubes to ½".
North America Park City, Utah 1 ¾" x 1 ½" x 1"

 

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Pyrite is FeS2, and its nickname is fools' gold. Because none of us wants to be a fool, let's consider how to tell the difference between gold and this impostor.

Specimen #46 is gold, so compare this specimen with that one. First, note that the colors are different. The color of gold is . . . well, gold. The color of pyrite is usually described as brass-yellow. The streaks are different, too. Streak is the color of the powder residue left by rubbing the mineral on a piece of unglazed porcelain. The streak of gold is shiny gold - just like the bulk mineral; the streak of pyrite is green-black. The hardnesses are different, too: 2.5-3 for gold, and 6-6.5 for pyrite. Your fingernail would scratch pure gold, but your knife blade would probably fail to scratch pyrite.

Notice the beautiful cubes of pyrite in this specimen. You don't see any nice crystal forms in specimen #46. Crystals of gold are uncommon, but where they occur, they are typically quite crude. Besides cubes, pyrite may occur in other forms and in combinations of forms ("form" here is used in its technical sense: a group of equivalent faces). The most common is the pyritohedron, obviously named for pyrite:


Pyritohedron

 


Cube + Pyritohedron