In search of the crystal structure of low quartz

Abstract

A comparison of numerous determinations of unit cell dimensions and of crystal structure refinements of low quartz, SiO2, shows that there exists no study at a defined temperature and at ambient pressure of a chemically well characterized sample, for which we have precise unit cell lengths and at the same time positional coordinates of high precision for the silicon and oxygen atoms. An analysis of the available data from 18 carefully selected measurements of cell lengths and 25 determinations of positional coordinates results in an averaged structure at 291 K in space group P3121 with unit cell dimensions: a = 4.9130(1) Å, c = 5.4047(1) Å; and standardized coordinates: x(Si) = 0.5301(2), x(O) = 0.4139(5), y(O) = 0.1466(4) and z(O) = 0.1188(3); the silicon atom is in Wyckoff position 3a (x, 0, 1/3) and the oxygen atoms are in Wyckoff position 6c (x, y, z). Error estimates for cell dimensions and coordinates are taken as the mean of the observed e.s.d.‘s in the various original structure refinements. The bond lengths within one coordination tetrahedron are, twice each, Si–O,a 1.604(2) Å and Si–O,b 1.613(2) Å; the angles around the silicon atom are O,a–Si–O,a 109.0(1)°, O,b–Si–O,b 109.5(1)°, and twice each O,a–Si–O,b 110.4(1)° and O,b–Si–O,a 108.8(1)°; the Si–O–Si angle between two tetrahedra is 143.7(1)°. For the time being this is the best currently available estimate of the geometry of the crystal structure of low quartz.

The Floppiness of It All: Bond Lengths Change with Atomic Displacement Parameters and the Flexibility of Various Coordination Tetrahedra in Zeolitic Frameworks. An Empirical Structural Study of Bond Lengths and Angles

“…At that point, one can wonder whether or not in all tetrahedral frameworks of SiO 2 composition, including those that are denser than zeolites, the Si−O bond lengths are on average just under 1.600 Å? In low cristobalite, 59 the mean Si−O distance is 1.605 Å, in low tridymite 60 it is 1.597 Å, in coesite 61 1.611 Å, in low quartz 62 1.609 Å. The mean for all four of these crystal structures is 1.606 Å, which is higher than the averages given for the SiO 2 only zeolites in Figure 1b2 (1.594 Å) and for the SiO 2 zeolites compiled by Wragg et al 41 (1.597 Å), but is lower than the grand mean of all Si−O distances considered in Table 2, line 14.…”

Creation of glass-characteristic point defects in crystalline SiO2 by 2.5 MeV electrons and by fast neutrons

“…Both materials consist of corner-shared SiO4 tetrahedra of nearly the same sizes. SieO bond lengths are 160.4 and 161.3 pm in α-quartz [1] and ≈160 pm in silica [2]. The amorphous/glassy disorder is characterized by distributions of Si-O-Si angles, inter-tetrahedral torsional angles and medium-range Corresponding author.…”

Mineralogy and mineral chemistry of quartz: A review

“…The left-handed or right-handed helices in the quartz structure correspond to morphologically right and left crystals, respectively (Donnay and Le Page, 1978; Glazer, 2018), which can be distinguished by the relative position of the trapezohedron face x to the positive main rhombohedron face r. The c-axis in the trigonal symmetry is identical with the optical axis and results in the optical uniaxial character of alpha-quartz. The silicon atom in the quartz structure resides on a two-fold axis, whereas all oxygen atoms are at general positions (Le Page et al , 1980; Kihara, 1990; Baur, 2009). Each SiO 4 tetrahedron has two long and two short Si–O bonds at 1.613(2) Å and 1.603(2) Å, respectively, in quartz measured at 291 K and ambient pressure (Baur, 2009).…”

“…The silicon atom in the quartz structure resides on a two-fold axis, whereas all oxygen atoms are at general positions (Le Page et al , 1980; Kihara, 1990; Baur, 2009). Each SiO 4 tetrahedron has two long and two short Si–O bonds at 1.613(2) Å and 1.603(2) Å, respectively, in quartz measured at 291 K and ambient pressure (Baur, 2009). Le Page et al (1980) noted that the two Si–O bond distances are 1.611(1) Å and 1.606(1) Å at 94 K, and they do not change in the temperature range from 94 K to 298 K, however the Si–O–Si angle increases from 142.69(4)° to 143.65(5)°.…”