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Quartz is a form of silicon dioxide, SiO2 and is one of the most abundant minerals on the earth's crust. It represents more than 12% of the volume of the earth's crust (including granite, sand). Quartz has two modifications: α-quartz is trigonal, β-quartz (a higher temperature form) is hexagonal. Silicon dioxide also occurs in many other structures such as α and β-cristobalite, keatite, tridymite, coesite, stishovite, melangophlogite and lechatelierite. Although quartz consists of silicon dioxide, it is often classified as a tectosilicates. The reason for this is that quartz forms a network of SiO4 tetrahedra. Each oxygen atom is shared by two silicon atoms. Individual SiO2 molecules only occur with gaseous silica. Quartz can be very pure, like rock crystal, but it often contains impurities. These impurities can consist of ions that are incorporated into the crystal lattice during the growth of the crystal. Quartz crystals can contain between 13 and 15,000 ppm Al3 +, between 9 and 1400 ppm Na +, between 3 and 300 ppm K + and traces of Fe3 +, Ti4 +, P5 +, H + and Li +. The aluminum and iron, in combination with ionizing radiation, are responsible for the color of a number of quartz varieties. Furthermore, quartz crystals can grow over other minerals, as is the case with rutile quartz. Quartz can form large crystals as well as form microscopic aggregates. The largest quartz crystals are found in Farm Verloren in Namibia. Quartz crystals up to 20 meters, and possibly 50 meters, and dolomite crystals up to 2 meters are found here. When quartz is melted, it often forms a glass on rapid cooling, because the crystallization process takes quite a lot of time. Quartz is a piezoelectric material and the vibrations that can be generated in the lattice of the crystal are used in electronic equipment such as quartz clocks and radios.