Crystal structures of metals and minerals:

To see these pictures, you need the MDL Chime plug-in viewer. If you have not already done so, you can download this viewer for free. If you have the viewer, you can rotate, resize, or change the display characteristics of these structures.  Several of the structures have buttons that allow you to highlight specific atoms in the crystal lattices.

Metals

Pure metal crystals are relatively simple, because they contain only one kind of atom.  The unit cells are frequently easy to see in these lattices.  Recall that metal atoms in the solid are joined by electron deficient bonds that are similar to the covalent bonds found in organic molecules.

Ionic compounds

Ionic salts are a little more complex, because their lattice must contain both cations and anions.  Remember that the total of the positive and negative charges must cancel out, so that the unit cell and the overall crystal are electrically neutral.

Network Covalent Solids

Network covalent solids include two polymorphs of pure carbon, diamond and graphite, as well as pure silicon and the silica (SiO₂) minerals such as quartz.

The silicate minerals have structures based on the SiO₄ tetrahedron.

They may have the simple orthosilicate structure, in which the SiO₄^4- is found as a discrete anion.  Examples include zircon (ZrSiO₄) and the aluminosilicates sillimanite, andalusite and kyanite (Al₂SiO₅).

Silicates can also consist of polymeric structures in which SiO₄ groups are joined by sharing oxygen atoms in a covalent fashion to make large polyanions.

• The pyroxenes (e.g. diopside) are linear single chain structures.  The cyclosilicates are based on star-shaped rings having the formula Si₆O₁₈^12-.  An example is the semi-precious stone beryl.
• The amphiboles consist of cross-linked double chains.  If you look closely at these double chains, you can see the ring structure of the cyclosilicates hidden within them.  Amphibole structures form linear strips as in the asbestos minerals (e.g. tremolite), or extended covalent sheets as in the micas (e.g. biotite).

Three-dimensional aluminosilicates are known as zeolites.  These structures resemble cages, and small molecules can enter and be trapped within their pores.  Zeolites are also known as molecular sieves for this reason.  They are important in water and gas purification, and also as catalysts in the petroleum industry.