Atoms, Space Lattices, and Crystals
To understand the properties of minerals, we need to see how atoms, the fundamental building blocks of minerals, fit together.
We might visualize atoms as very small balls, or spheres. The size of an atom depends on the particular chemical element and on the number of electrons in its outermost region. The size of the different atoms affects the way in which they can be fitted together.
The models at the left of the case illustrate the geometrical packing of atoms. Atomic packing and physical properties are also determined by the way atoms are connected, or bonded, to each other. Among the various types of bonding recognized, three types are important in minerals. Metallic bonding occurs in the native metals, such as copper. Ionic bonding is typical of minerals like halite, our common table salt. And covalent bonding is present in the mineral diamond.
The smallest grouping of bonded atoms typical of a mineral is called a unit cell. The unit cells, stacked together in three dimensions, form the mineral as a whole. This stacking arrangements is called a space lattice. The model, consisting of large white atoms and small red atoms, shows a simple grouping of three atoms arranged in a space lattice.
It is the regular repetition of the unit cells in space which make minerals crystalline. Surprisingly, there are only a limited number of possible arrangements that will form a space lattice pattern — 14 in all, called the Bravais space lattices. When space lattices and crystals are examined in terms of symmetry, six main groups, called crystal systems, are found.
In the display on the right, mineral samples are shown adjacent to their appropriate crystal system. The shape of the crystal forms varies greatly, though shape must be consistent with the symmetry of the crystal system. This variation suggests the immense range of forms the can result from the packing of blocks of one shape.
In the lower portion of this case, some strange crystal forms are shown. On the left, a quartz specimen, number 21, from Switzerland, has apparently grown as a twisted crystal. Actually, this is a multiple growth of many crystals, each rotated a small amount from the direction of the previous one, producing the screw shape.
The next four specimens illustrate how two or more crystals of a single mineral are intergrown in a precise relationship because of growth mistakes, a phenomenon called twinning. Some crystals may be deformed, or even curved, like the sample of gypsum, number 27. Departures from the ideal crystal shape are common, as shown by the hopper crystal of salt, specimen number 28, and the dendritic copper crystal, specimen number 29.