The basic structure of quartz consists of spiral chains (helices) of tetrahedra around three- and six-fold screw axes. At left below is one of the three-fold helices, at right is a diagram showing the helices connected into a framework
The diagram at left below shows the quartz structure with one of the sixfold helices shown in color. The diagram at right shows the relationship between the threefold and sixfold helices. The threefold helices are shown by different colors, and the sixfold helix is indicated by darker shades of the same colors.
Quartz has two polymorphs. Alpha quartz is trigonal and stable below 573 C. Above 573 C thermal agitation becomes vigorous enough to overcome the slight skewness of the chains and the structure inverts to beta quartz, which is hexagonal. The transformation involves no atomic rearrangement, and all quartz at surface conditions is alpha quartz.
Below is a polyhedral view of the quartz structure viewed along the c-axis. Several rotation axes are shown. Tetrahedra are viewed along twofold symmetry axes (actually, if the tetrahedra were isolated, they'd be fourfold inversion axes). Yellow, green and blue denote increasing distance from the plane of the diagram.
Below is the arrangement of oxygen atoms around the symmetry axes in quartz. Atoms are at elevations of 0, 1/3, 2/3 and 1 in terms of the c-axis, with darker blue representing lower elevations and light green the highest. One six-membered ring is shown. Two three-membered rings are shown, one a close-packed model and the other with smaller spheres so the vertical superposition of atoms at elevation 0 and 1 can be seen. Note that the two rings are differently oriented.
Below is the arrangement of oxygen atoms in quartz. Atoms are at elevations of 0, 1/3, 2/3 and 1 in terms of the c-axis, with darker blue representing lower elevations and light green the highest. At lower left is the lowest set of tetrahedra, with progressively higher oxygens added toward upper right. We can see that there are three kinked chains of oxygen atoms each oriented at a 60 degree angle to the chain below.
Created 6 March, 2002, Last Update 14 December 2009
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