Quartz

Low (a) quartz allows little ionic substitution into its structure. High (P) quartz allows the charge-balanced substitution of framework silicon by aluminum, with a small cation (Li+) occupying the interstices. In the more open cristobalite and tridymite structures, this charge-balanced substitution can be extensive, with many alkali and alkaline earth ions able to occupy interstitial sites. Such materials are called “stuffed silica derivatives,” with eucryptite (LiAlSiO4), nepheline (Na3K(AlSiO4)4), carnegieite (NaAlSiO4), and kalsilite (KAlSiO4) being examples.

Similar to most other silica structures, quartz has a continuously connected network of (SiO4)4- tetrahedra and an O/Si ratio equal to 2. This characteristic structure is also seen in cristobalite and tridymite. Interestingly, helices have been reported in quartz with two slightly different Si-O distances (0.1597 and 0.1617 nm) and an Si-O-Si angle of 144° [6]. Enantiomeric crystals of quartz are often obtained and separated mechanically. Each enantiomeric crystal of quartz is optically active.

According to Wyckoff [7], the crystalline forms of silica are the largest group of tetrahedral structures. Each of the three main polymorphs of silica formed at atmos­pheric pressure in nature (quartz, tridymite, and cristobalite) has a low and high temperature modification. The unit cell of low (a) quartz has three molecules and similar dimensions as the high ф) quartz structure. The difference between low and high forms of quartz arises from small shifts of atom positions. Table 1 lists structural data for both quartz structures.

The atomic arrangements in high and low quartz are very similar. In fact, when a single crystal of low quartz is carefully heated above 575°C, it is known to gradually and smoothly transform into a single crystal of high quartz, with a shift from a 3- to 6-fold symmetry [7]. The oxygen tetrahedron is almost regular (Si-O distance is 0.161 nm) for low quartz and with each oxygen having six adjacent oxygens (0.260­0.267 nm) and two silicon neighbors. Fourier analysis has provided accurate data for both structures [7]. The low and high forms of quartz are related by a displacive transformation with the former having the higher symmetry. Quartz, hexagonal in structure, is the lowest-temperature form of silica [5].

The structure of quartz has been extensively studied [7-10]. Table 2 summarizes structural data for low quartz obtained with the Accelrys Catalysis 3.0.0 software. The continuous connection of oxygen tetrahedra is apparent from its structure illustrated in Figs. 4 and 5 [11,13].

Figure 5 shows that the linkage of tetrahedra in low quartz is, in fact, a double helix when viewed along the a-axis. This double helix structure was known long before the more celebrated structure of DNA [12,13].

Table 1 Comparison between low- and high-quartz structures [11] (after Wyckoff [7])

Low temperature or a-quartz

High temperature or P-quartz

Bravais lattice

Hexagonal

Hexagonal

No. of ions

9 (3 Si+, 6 O-2)

9 (3 Si+, 6 O-2)

Temperature

<573°Ca

573-867°Ca

ao

0.491304 nm

0.501 nm

co

0.540463 nm

0.547 nm

c/a

1.10b

1.09b

Space group

D34 or D36 (P3I2)b

D^ or D65(P622)b

Si-O

0.161 nm

0.162 nm

O-O

0.260-0.267 nm

0.260 nm

Si-O-Si angle

144°b

LA

::

Symmetry

Threefold

Sixfold

Molecules

3

3

Additional data as indicated from different references: from [1], from [8], and from [6]

Bravais lattice

Unit cell dimensionsa (a, b,c) in nm

Unit cell major

anglesa (a, p,g)

Space group number

Symmetry

number

No. of ions per unit cell

Hexagonal

0.49130,

0.49130,

0.54052

90.0, 90.0, 120.0

P3t21

152

9

Table 2 Structure of low-quartza

aFrom Accelrys software [11]; P = Primitive

Quartz