Devils Dice a Modification of PyritePyrite is an ore of iron, in fact the sulfide of iron. One atom of iron (or ferrum, Fe) combines with two atoms of sulfur (S), to form FeS₂. It can assume cubic, pyritohedra, or octahedral crystalline forms. Devils Dice earned its name because it is the cubic variety of pyrite. Interestingly, during crystallization, the competing of the crystal types can introduce facial lines, facial striation.
Cubic FormMolecules of iron pyrite connect in such a way that a cube (or a cuboid) easily results. In the schematic layout shown, notice the cube in the lower left quadrant. At the center of that cube are two sulfur atoms. The rest of the layout follows naturally. Iron atoms are seen at each corner and in the center of each face of the cube.
Pyritohedron FormHow is it that pyrite can form a number of crystal shapes that are as different as the cube and the pyritohedron? It is because they are not actually that different. A cube is divided into a pyritohedron by bisecting all its edges and faces in alternate directions.
Octahedral FormThe above explanation of why a pyritohedron doesn’t represent a great departure from the cubic is quite valid. And the same can be said of the octahedron, as any iron atoms are at the corners and/or faces of the cubic crystal lattice. Yet the reason the three different forms (including the very rare octahedral) form has to do with concentration during the time of formation. Nucleation depends upon that concentration, and the manner of nucleation determines the crystal structure formed (see reference, below).
Transformation by WeatheringMany metals tarnish. We might be inclined to think the presence of air is all that is required to produce a tarnish. In fact, the presence of water, and not air alone, greatly hastens tarnishing. Cubes (and other shapes) of pyrite react with these to form a chocolate brown tarnish or coating called limonite. What is the chemistry of limonite? The chemical formula of Limonite is FeO(OH)·nH₂O. In other words, the oxygen and hydrogen of the air and moisture have replaced the atoms of sulfur in the outer layer. The limonite remains bonded to the pyrite at their interface. It’s rather like a skin has formed over the pyrite. An overall reaction equation could be written,
4 FeS₂ + 10 H₂O + 15 O₂ → 4 FeO(OH) + 8 H₂SO₄But where are the n molecules of water that form part of the limonite structure? They can easily be accounted for by including an excess of water on the left side of the chemical reaction.