Pyrite

Our next step will be learning a bit about the individual minerals that make up the various rocks. The following question gives us a good start.

Q:
Hello. On a recent visit to a museum I came across a crystal called pyrite. I was, and still am astonished by the perfect cubes formed by the pyrite and I'm curious to know how it is possible for a crystal to form a shape as perfectly symmetrical and precise as a cube? I've been doing some searches on Google but I am unable to find any solid answers.
--Brent S.

A:

Pyrite is an important mineral for people looking for metallic sulfide deposits. If you successfully find copper sulfides, you will almost certainly first see a lot of iron sulfides: FeS, or pyrite. These sulfide minerals are usually called "secondary" - that is, minerals that form in the rocks after the rocks themselves are solidified. This often happens because metal-bearing fluids are moving through the rock - and either deposit minerals like pyrite in cracks (which when filled become "veins"), or slowly replace certain minerals already in the rock in a complex chemical process. Pyrite is often called "fools gold" because it fooled many of the early, uninformed people who thought they would go out and make their fortunes as miners. Think of the "49ers" who rushed to California when gold was found in Sutter's Mill near Sacramento in 1849. Pyrite, however, is more brassy than gold. It's is also angular - crystalline - and shiny when found fresh - in other words, when it is unoxidized. Gold is a deeper yellow in color, and found in flakes or occasionally as rounded nuggets in pans. Pyrite forms in rocks where there is little or no oxygen present. 

To explain pyrite's perfect cubic shape, I would have to show you one of those chemistry-class balls-and-sticks models to show how the two ions iron and sulfur fit together. The atoms are only one ionic shell-size different from each other - Fe is one level lower than S on the periodic table, meaning that it has two free electrons in the next shell out, while sulfur is missing two electrons in the nearly-complete next-smaller shell. For this reason, the ions of iron and sulfur when meshed together are virtually the same size. Think of a checkerboard with black being iron and white being sulfur - and then imagine in your mind a 3-dimensional version of this. Crystals grow molecule by molecule, so the next thin, molecular layer added onto the pyrite crystal cube can only form when each iron atom is matched to a sulfur atom. As the individual pyrite molecule in solution approaches the crystal face, the individual atoms will move and re-orient to make a perfect opposites-attract match. This guarantees not just a cube, but a PERFECT cube, at least to within an atom's thickness of all surfaces in contact with the solution where the ions are coming from.

If you actually saw a perfect pyrite cube, you either saw a sample very recently collected, or the sample was kept in a non-oxygen, probably nitrogen-filled glass container. The reason for this is because pyrite will slowly but steadily oxidize in the Earth's present (21% oxygen) atmosphere. It will literally turn to a brownish "rust", and the bright "fools gold" shine will be lost with time. The reason we know that the early Earth did not have oxygen in its atmosphere before about 2,500,000,000* years ago is because some pyrite grains weathered out of a rock in South Africa and were tumbled down an ancient creek, which rounded the cubic grains before they were compressed and consolidated into a sedimentary rock. When unearthed in a deep mine the rounded pyrite grains were still bright and fresh; if they had weathered out of a rock today and washed down a creek somewhere, they would be turned to brownish-looking iron oxides very quickly. 

* NOTE: If you are an American, you would call this number "two point five billion years" - but if you are English you would say this as "two point five thousand million years" - because for many countries where English is spoken, a "billion" means a million million. For this reason, "2,500,000,000 years" is generally abbreviated among geologists as "2.5 Gy" to avoid confusion. We will talk about this interesting oxygen transition-time later.

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