α-Quartz (SiO₂) 1: introduction

To go directly to the unit cell structure, click the link below to page 2.

Quartz comes in many forms and is extremely abundant. It is the most common mineral on Earth, found in nearly every geological environment and is at least a component of almost every rock type. Indeed SiO₂ is the second most studied chemical compound (after H₂O). Amethyst and rock crystal are examples of gemstones that are based on quartz. Even pure SiO₂ has no fewer than eight other known structures. α-Quartz is the most thermodynamically stable form under ambient conditions. Pure quartz is colourless but samples come in a wide range of colours due to impurities.

Quartz has the property of piezoelectricity: when subject to a mechanical stress a voltage is generated. This is due to an asymmetry in the charge distribution resulting from the stress. This property gives quartz its use as a crystal oscillator in watches. Another interesting property of quartz is optical activity: crystals can rotate the plane of polarisation in plane-polarised light as chiral molecules do. This is because the quartz structure is different from its mirror image. This feature arises from helical chains within the structure that will be explored in the coming pages.

A fragment of the bulk structure of α-quartz is shown to the left. The oxygen atoms are red and the silicons are pale brown. Each oxygen atom in the bulk is connected to two silicon atoms and each silicon to four oxygen atoms. This arrangement satisfies the valency of each element and accounts for the stoichiometry in the formula of quartz.

Go to page 2 to look at the α-quartz unit cell.

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