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Quartz belongs to the trigonal crystal system at room temperature, and to the hexagonal crystal system above 573 °C (846 K; 1,063 °F). The ideal crystal shape is a six-sided prism terminating with six-sided pyramid-like rhombohedrons at each end.
This chapter introduces the crystal structure of quartz and its relation to the symmetry and the physical properties of quartz crystals. All renderings are based on a single data set of quartz unit cell coordinates downloaded from the now orphaned site www.molecules.org.
The structure of beta-quartz is hexagonal, with either a left- or right-handed symmetry group equally populated in crystals. The structure of alpha-quartz is trigonal, again with either a right- or left-handed symmetry group.
Due to the helical arrangement of the SiO 4 tetrahedra, the atomic lattice of quartz possesses the symmetry properties of a helix: Quartz forms left- and right-handed crystals, whose crystal structure and morphology are mirror-images of each other.
Quartz Crystal Habit and Structure. Quartz belongs to the trigonal crystal system. The ideal crystal form is a six-sided prism terminating with six-sided pyramids at every cease.
Quartz (trigonal, low-temperature α-quartz) is the most important polymorph of the silica (SiO 2) group and one of the purest minerals in the Earth crust. The mineralogy and mineral chemistry of quartz are determined mainly by its defect structure.
The crystal structure of Quartz is a very complicated. As a result of a changeover from alpha to beta Quartz, crystals form as hexagonal prisms with modified crystal face s. Click here for a detailed explanation on the crystal structure of Quartz.
— By means of the Lewis theory, a structure has been obtained for quartz which accounts in a satisfactory way for the crystalline form, crystal symmetry, optical rotation, hardness, high melting point, insolubility, and x-ray spectra of the substance, It is in strict accord with the conclusions of W. H. Bragg in regard to the structure.
There are two forms of quartz: α-quartz, stable below 573 °C at ambient pressure, and β-quartz, stable from 573 to 870 °C. β-quartz has an hexagonal crystal lattice with ‘rings’ of 6 SiO 4 tetrahedrons linked to each other.
Pure quartz is made of crystallized silica (silicon dioxide, SiO 2), but in nature it occurs in many varieties based on the presence of small amounts of other substances. Historically, different varieties of quartz—such as agate, amethyst, onyx, and tiger's eye—have been used as gemstones.