Quartz is more than silicon dioxide

Dr Kendal Martyn (UK) This article describes several processes producing the shape of crystals. Such processes are illustrated in the most common mineral from the Earth’s surface, quartz. Quartz or “simple” silicon dioxide, is made up of interlocking atoms of silicon and oxygen, arranged into various symmetrical structures depending on pressure and temperature conditions. Such variation in the structure accounts for most of the different minerals discussed below. The presence of these different SiO2 minerals in rocks gives important information about the conditions those rocks were exposed to. Starting rules Minerals are classified by their distinctive structure (atomic arrangement), as well as their chemistry: all the mineralsdescribed below have the same chemical composition but different physical forms, known as polymorphs.Different physical structures are favoured by specific conditions of temperature and pressure (Fig. 1).If conditions favour a change in structure, the old structure may be preserved or a new structure formed,depending on the amount of energy available, the energy needed to make that change and the time available to make that change. Quenching (very rapid cooling) may “freeze in” the old structure. Kinetics (the physical re- arrangement of atoms and bonds) triumphs over thermodynamics (theoretically the energetically most favourable structure).Fig. 1. Diagram showing the stabilities of some (not all) of SiO2 polymorphs under different temperature and pressure conditions. (From the Cambridge University website.) Breaking and reforming atomic connections, a radical rearrangement, needs more energy and requires material to diffuse across the crystal. Such reconstructive transitions will only happen if there is … Read More

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