How are the lattices of minerals effected by impurities?

Question

For example Pyrite mostly consists of FeS2 but it often contains impurites such as Nickel and Cobalt. What I don't undertand is how such impurites "interconnect" with the FeS2 crystal lattice. In other words what effect do impurities have on a mineral major chemical make up in terms of its lattice structure?

Answer 1

Actually, impurities in the crystal lattice have little to do with crystal inperfections. In order for an impuritiy to be accepted, an element must have similar properties to the element it is substitutuing. For example, Fe can be replaced by Mn, Mg, Cr in the +2 state, among other elements. The presance of these elements have little to do with imperfections.

Imperfections occur naturally and randomly. Simply put, it is a situation in which the growth of the crystal goes in a different direction when it is not supposed to, like a round peg in a square hole. This causes the imperfection.

Answer 2

You’ve chosen a good example, with pyrite. It has a cubic crystal structure. The best way to understand this is to imagine each molecule as a little sphere, and we will represent the crystal by stacking them in a box.

Say we start with golf balls. They stack up regularly to form a rectangular solid. Now, suppose we stick in a molecule that is too large or too small. (a different molecule) We could put in either a baseball or a marble.

In our box, gravity is trying to get the balls to settle down to the lowest level. As we stack more layers of the golf balls, we will soon see that there is a ‘discontinuity’, an error in the crystal structure. In actual practice, it might not be visible on the ‘macroscopic’ scale. We might have to put in a half a layer of baseballs, or marbles, before the mismatch became visible.

In the actual crystal, it is the property of the atoms wanting to share their outermost (valence) electrons that cause them to bind together. But the principle is the same: the crystal will be deformed by the impurities. Typically, we will see smaller crystals growing out of the main one, or the crystal will not take on a perfect cubical structure. It might take an octohedral shape, instead.

When we make semiconductors, we start with purified silicon and introduce carefully controlled AMOUNTS of impurities. But the amounts are so small, that they do not disturb the structure enough to stand out. Only one out of every several ten thousand atoms of silicon is replaced by an impurity atom.

If the SIZE of the impurity atoms is very small, they might not cause any distortion in the lattice. Heavy metals such as palladium are supposed to absorb large amounts of hydrogen, because when the metal is heated, the lattice expands just enough to allow hydrogen atoms to slip in between the palladium atoms. When it cools, the hydrogen atoms are trapped inside between the palladium atoms because the lattice contracts enough to hold them. We must re-heat the palladium once more to get the hydrogen out.

Natural crystals are rarely pure, of course, so it is common to see pyrite crystals with little deformities, smaller crystals growing out of the sides, etc. Those are due to lattice irregularities. It is not at all unusual for iron nickel and cobalt to occur together. They are the ferromagnetic minerals, and they are relatively commonly found together in meteorites, suggesting that the composition of our own planet (like that of the one that broke apart to form the asteroids) is mostly iron inside, with some nickel and some cobalt. Look up ‘transition elements’ to find more information on them, and their relative abundance in parts per million, compared to oxygen, silicon, and carbon.

24 DEC 06, 0208 hrs, GMT.