QUARTZ mineral.pptx

Structure of quartz

1. QUARTZ

2. Introduction quartz , widely distributed  mineral  of many varieties that consists primarily of  silica , or silicon dioxide (SiO 2 ). Minor impurities such as lithium, sodium, potassium, and  titanium  may be present. Quartz has attracted attention from the earliest times; water-clear crystals were known to the ancient Greeks as  krystallos —hence the name  crystal , or more commonly  rock crystal , applied to this variety. The name  quartz  is an old German word of uncertain origin first used by  Georgius Agricola  in 1530.

3. Structure Of Quartz The structure of quartz was deciphered by Bragg and Gibbs in 1925 (for a review of the structure and symmetry features of quartz, see Heaney, 1994). Its basic building block is the SiO 4  group, in which four oxygen atoms surround a central silicon atom to form a tetrahedron. Since each oxygen is member of two SiO 4  groups, the formula of quartz is SiO 2 . The SiO 4   tetrahedra form a three-dimensional network and many mineralogy textbooks classify quartz as a network silicate or tectosilicate . Quartz can be thought of as being made of threefold and sixfold helical chains of SiO 4   tetrahedra that run parallel to the c axis. Figure 1 shows two representations of a threefold SiO 4  helix and its relationship to the quartz unit cell: to the right a ball model with red oxygen and white silicon atoms, to the left a tetrahedral model, with the corners of the tetrahedra at the position of the oxygen atoms. Six of such helices are connected to form a ring that surrounds a central channel which runs parallel to the c-axis, sometimes called "c-channel". The SiO 4   tetrahedra around the central c-channel form two independent sixfold helices. Figure 2 shows two views of the corresponding structure: looking in the direction of the c-axis in the top row, and looking in the direction of an a-axis in the bottom row. Like quartz crystals, the ring is six-sided but has a  trigonal symmetry . The large channels are an important structural feature of quartz because they may be occupied by small catio ns .