To imagine what it is like inside a mineral crystal, you can think of "rooms" formed by the crystal's atoms. A room in a copper crystal is formed by 14 copper atoms. The room has an atom at each corner of the floor and ceiling, and an atom at the centers of the floor, the ceiling, and each of the four walls. A copper crystal consists of many of these rooms side by side and one on top of the other. The rooms share copper atoms where they meet. Mineralogists call such rooms unit cells.

Most minerals are composed of more than one kind of atom. Halite, for example, consists of sodium atoms and chlorine atoms. Other minerals may have as many as five kinds of atoms in complicated arrangements. Some unit cells have six walls instead of four, and others have slanted walls. Such differences in the shape of unit cells produce differences in the shape of mineral crystals.

A halite crystal, above left, has four sides and is made up of billions of four-sided unit cells. Each cell, center, contains 14 sodium atoms (shown in black) and 13 chlorine atoms (blue). Halite belongs to the isometric crystal system -- one of six systems into which all mineral crystals are grouped. A general diagram for the isometric system, right, includes three axes (imaginary lines) that show the directions followed by the edges of the crystal.

A corundum crystal, below left, has six sides. Its unit cell, center, is a six-sided "room" containing 21 oxygen atoms (black) and 6 aluminum atoms (blue). Corundum belongs to the hexagonal system, right, which has four axes.

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Copper
Corundum

This page was last updated on 02/22/2010.