10

Rock-crystal (quartz)

Single crystals of quartz (SiO2 ) showing euhedral forms are traditionally called rock-crystals, and interest in their form has continued since the time of Steno. Hexagonal prismatic crystals with two types of rhombohedral faces, {1011} and {0111}, at the tip show short-prismatic to long-prismatic forms or tapered forms determined by the relative growth rates between the rhombohedral and the prismatic faces, as well as various intergrowth forms such as sub-parallel or scepter intergrowth. Twins growing according to the Japan law or Brazil law exhibit characteristic forms (see Sections 10.6.1–10.6.3). It is the aim of this chapter to analyze how various forms of rock-crystals appear. Chalcedony and agate, two polycrystalline aggregates of quartz crystals of micrometer size, exhibit characteristic textures. How these textures are formed will also be explained in this chapter.

10.1Silica minerals

Rock-crystal is a typical mineral crystal, which, in its regular geometric hexagonal prismatic form, has attracted interest since the earliest times. Its chemical composition is SiO2, and its mineral name is quartz. More than six polymorphs are known among minerals of chemical composition SiO2. Within this group, the polymorph belonging to the hexagonal system, crystal group 622, and space group P6222 or P6422, which is stable above 573°C under 1 atmosphere pressure, is called high-temperature quartz, and that belonging to the trigonal system, crystal group 32, is called low-temperature quartz. In low-temperature quartz, there are two types of structures, with space groups P3121 and P3221, called right-handed quartz and left-handed quartz, respectively. In addition to these, high-temperature polymorphs called tridymite and cristobalite and high-pressure polymorphs called coesite and stishovite (as well as a couple of other phases) are known. The general

10.1 Silica minerals 199

name “rock-crystal” has been traditionally applied to clear single crystals of quartz attaining sizes discernible to the naked eye.

Various names, such as amethyst, citrine, and smoky or black quartz, have been used for colored rock-crystal, whereas for the cryptocrystalline aggregate of quartz, names such as chalcedony and jasper are used. Agate and cornelian, for example, are types of chalcedony that have specific textures or colors. In this chapter, we analyze how a variety of morphologies of high-temperature and lowtemperature quartz appear, and how textures of polycrystalline aggregate seen in agate and other crystals are formed.

In both highand low-temperature quartz, the unit of construction is SiO4 (a tetrahedron consisting of one silicon and four oxygen atoms) (Fig. 10.1). A threedimensional network structure is constructed by the sharing of all oxygen atoms at the summits of the SiO4 tetrahedra. Viewed from the c-axis direction, the symmetry axes constructed by connecting SiO4 are either 62 or 64 screw axes in hightemperature quartz, and either 31 or 32 screw axes in low-temperature quartz. We see that slightly changing the angles that are formed on connecting the SiO4 tetrahedra causes the difference between a crystal belonging to a hexagonal system, crystal group 622, and a crystal belonging to a trigonal system, crystal group 32.

Since only an angular adjustment is required to cause the transition between the two phases, it occurs sharply at 573°C under 1 atmosphere pressure. However, we should expect a transitional (precursor) state, corresponding to the energy required for the angular adjustment, to be present during the transition. Since two possible orientations are associated with this transition, Dauphiné twinning occurs. Dauphiné twinning may also occur because of mechanical or electrical stress. In the phase transition from tridymite or cristobalite to low-temperature quartz, structural rearrangement is required, and so the transition is not sharp and these two forms may remain as metastable phases at low temperatures.

The principal form of single crystals of natural rock-crystal is hexagonal prismatic bounded by six prisms m {1010}, terminated alternately by three major r

{1011} and three minor z {0111} rhombohedral faces. The difference between right-handed and left-handed quartz appears in the position of the s {1121} and x

{5161} faces. It is exceptionally rare to observe the appearance of a basal plane {0001} in natural rock-crystal, but the faces appear universally on synthetic quartz grown on seeds. In natural rock-crystal, the r and z faces are flat surfaces on which growth hillocks with triangular pyramidal forms are universally observed, whereas the m face is characterized by the development of striations parallel to the edges with the r and z faces. On synthetic quartz, conical growth hillocks are observed on the r and z faces, whereas polygonal growth hillocks are commonly seen on the m faces. Only when synthesis by NaCl aqueous solution is performed do striation patterns similar to those seen on natural crystals appear.

200 Rock-crystal (quartz)

Figure 10.1. Crystal structure of low-temperature quartz.

Although the principal morphology of rock-crystal is a hexagonal prismatic

Habitus, natural crystals may deviate from this. In Goldschmidt’s Atlas der Kristallformen (see ref. [1], Chapter 9), 855 crystal figures are compiled in 54 plates. A few examples are shown in Fig. 10.2, in which various forms are observed, such as malformed hexagonal prisms, tapered prisms, platy, and scepter forms. (See also Fig. 1.1.)

In contrast to the hexagonal prismatic morphology of low-temperature quartz, it has been assumed that the characteristic morphology of high-temperature quartz is hexagonal bipyramidal where no {1010} faces appear on the crystal

10.2 Structural form 201

Figure 10.2. Various forms of rock-crystal. Selected from Atlas der Kristallformen

(ref. [1], Chapter 9).

which is bounded by {1011} faces. In the following sections we will analyze the origin of the malformation seen in low-temperature quartz and discuss why hightemperature quartz takes on a hexagonal bipyramidal form.

We shall start from an analysis of what sort of morphology we should expect for quartz crystals if we entirely neglect the effect of environmental conditions.

10.2Structural form

If the structural form of rock-crystal is predicted based on the Bravais–Friedel (BF) law, a form is obtained that is entirely different from the naturally observed Habitus, a polyhedral form bounded by nearly equally developed {1011}, {0111}, and {0001} faces, as explained in Section 4.2. From the Donnay–Harker (DH) law, in which a three-fold screw axis in the c-axis direction is taken into consideration, {0001} should be {0003}, and the reticular density reduces to one-third of that predicted by the BF law, leading to the prediction that it is not necessary for {0001} to appear as a Habitus-controlling face. The structural form of rock-crystal based on the DH law is hexagonal prismatic with alternately appearing r {1011} and z {0111} faces at the termination, which is in good agreement with the observed growth forms (see Fig. 4.2). Since there is not a big difference between the structures of the highand low-temperature forms of quartz, we may expect similar hexagonal prismatic growth form bounded by {1010}, {1011}, and {0111} for high-temperature quartz also. The hexagonal, bipyramidal form with no association of {1010}, which is commonly seen as the growth form (Habitus) of high-temperature quartz, is therefore not expected from a structural point of view.

In the PBC analysis of Hartman–Perdok theory, {1010}, {1011}, and {1010} are F faces (see ref. [3], Chapter 4). The predicted structural form of rock-crystal by PBC analysis is hexagonal prismatic (Fig. 10.3). Therefore, using this morphology as a