Quartz is a hard mineral composed of silica (silicon dioxide). Its atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen atom being shared between two tetrahedra, giving an overall chemical formula of SiO2. Therefore, quartz is classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most common mineral or mineral group in Earth's lithosphere, comprising about 12% by mass. Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold. There are many different varieties of quartz, several of which are classified as gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Europe and Asia. Quartz is the mineral defining the value of 7 on the Mohs scale of hardness, a qualitative scratch method for determining the hardness of a material.
Etymology The word quartz is derived from the German word Quarz, which had the same form in the first half of the 14th century in Middle High German and in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý ('hard'). Some sources, however, attribute the word's origin to the Saxon word Querkluftertz, meaning 'cross-vein ore'. The Ancient Greeks referred to quartz as κρύσταλλος (krustallos) meaning 'crystal', derived from the Ancient Greek κρύος (kruos) meaning 'icy cold', because some philosophers (including Theophrastus) believed the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for transparent, coarsely crystalline quartz.
Early studies Roman naturalist Pliny the Elder believed quartz to be ice, permanently frozen after great lengths of time. He supported this idea by saying that quartz is found near glaciers in the Alps, but not in warm climates. This idea persisted until at least the 17th century. In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that, regardless of a quartz crystal's size or shape, its long prism faces always meet at a perfect 60° angle, thereby establishing the law of constancy of interfacial angles.
Crystal habit and structure
Quartz can form as two distinct polymorphs depending on the temperature and pressure: α-quartz (also called low quartz or normal quartz) and β-quartz (also called quartz-beta or high quartz). α-quartz crystallizes in the trigonal crystal system, while β-quartz has greater symmetry and crystallizes in the hexagonal crystal system. The transition from α-quartz to β-quartz occurs abruptly at 573 °C (1,063 °F; 846 K) at ambient pressure; the transition temperature is greater at higher pressures. β-quartz is unstable at room temperature; therefore, all quartz at room temperature is α-quartz regardless of which polymorph it formed as. Both polymorphs of quartz can occur in two different space groups depending on the chirality. Above the transition temperature, α-quartz in P3121 (space group 152) becomes β-quartz in P6422 (space group 181), and α-quartz in P3221 (space group 154) becomes β-quartz in P6222 (space group 180). These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO4 tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without a change in the way they are linked. However, there is a significant change in volume during this transition, and this can result in significant microfracturing in ceramics during firing, in ornamental stone after a fire and in rocks of the Earth's crust exposed to high temperatures, thereby damaging materials containing quartz and degrading their physical and mechanical properties. The ideal crystal shape for quartz is a six-sided prism terminating with six-sided pyramid-like rhombohedrons at each end. In nature, quartz crystals are often twinned (with twin right-handed and left-handed quartz crystals), distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples. The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum.
Varieties
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