These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Above 573 ☌ (846 K 1,063 ☏), α-quartz in P3 121 becomes the more symmetric hexagonal P6 422 (space group 181), and α-quartz in P3 221 goes to space group P6 222 (no. Α-quartz crystallizes in the trigonal crystal system, space group P3 121 or P3 221 (space group 152 or 154 resp.) depending on the chirality. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum. The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples. 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. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. Quartz belongs to the trigonal crystal system at room temperature, and to the hexagonal crystal system above 573 ☌ (846 K 1,063 ☏). Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz.
The Ancient Greeks referred to quartz as κρύσταλλος ( krustallos) derived from the Ancient Greek κρύος ( kruos) meaning "icy cold", because some philosophers (including Theophrastus) understood that the mineral to be a form of supercooled ice. 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").
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 to abrasion. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia. There are many different varieties of quartz, several of which are classified as gemstones. 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. The transformation from α-quartz to β-quartz takes place abruptly at 573 ☌ (846 K 1,063 ☏). Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar. The atoms are linked in a continuous framework of SiO 4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO 2. Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). Lattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic) Insoluble at STP 1 ppm mass at 400 ☌ and 500 lb/in 2 to 2600 ppm mass at 500 ☌ and 1500 lb/in 2
181) Ĭolorless through various colors (pink, orange, purple, dark brown)Ħ-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massiveĬommon Dauphine law, Brazil law and Japan lawħ – lower in impure varieties (defining mineral)Ģ.65 variable 2.59–2.63 in impure varietiesġ670 ☌ (β tridymite) 1713 ☌ (β cristobalite)
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