Quartz is a widely distributed mineral composed primarily of silicon dioxide. It has a crystalline structure consisting of silicon and oxygen atoms arranged in repeating patterns. The basic building block of quartz is the silicon-oxygen tetrahedron, with four oxygen atoms surrounding a central silicon atom. These tetrahedra are linked together in a three-dimensional network structure. Specifically, the tetrahedra form threefold and sixfold helical chains running parallel to the c-axis of the unit cell. Six of these helical chains are connected to form a ring surrounding a central channel along the c-axis. This channel structure is important as it can accommodate small cations within the quartz structure.
2. Introduction
• quartz, widely distributed mineral of many
varieties that consists primarily of silica, or silicon
dioxide (SiO2). 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 SiO4 group, in
which four oxygen atoms surround a central silicon atom to form a tetrahedron. Since each oxygen
is member of two SiO4 groups, the formula of quartz is SiO2. The SiO4 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 SiO4 tetrahedra
that run parallel to the c axis. Figure 1 shows two representations of a threefold SiO4 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 SiO4 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 cations.
