The golden color and shine of iron pyrite (which is responsible for its nickname, "fool's gold") may grab all the attention. But, there is another element of its appearance that is every bit as dramatic: It's tendency to form into these incredibly precise, almost machine-like cubes.So how does it do it? The question was raised today in response to this post on the gems that are even rarer than diamonds by commenter mwhite66, who also provided this picture of another excellent example of pyrite setting itself into a gorgeously geometric layout:
It's not at all rare but Iron Pyrite (Fe S2, aka "Fool's Gold") just knocks me out. The molecules somehow "know" how to assemble themselves into nearly perfect cubes. For some reason this amazes me even more than quartz or other crystals, I guess because it looks so artificial but it's not. [Note that the pictured crystal was not shaped; it formed that way naturally.
Pyrite, of course, doesn't always form into such sharply-cut shapes. But, on the occasions that it does, the explanation lies deep in the structure of the of the mineral itself.
Pyrite contains both iron and sulfur. As drawn out in the above diagram from Indiana's Geological Survey, those iron and sulfur molecules join themselves into rigid bonds to create a system of cubic crystals. It's that isometric structure which then forms the sharp planes and lines that you see in the examples above.
Top image: Pyrite cubic crystals on marlstone, from Navajún, Rioja, Spain / CarlesMillan