216Rock-crystal (quartz)

the growth sectors of or on the surface of z faces. In the banding pattern of r faces, Brazil twin lamellae continue in a zigzag form on one side of triangular banding, whereas on the other side they occur as an alignment of numerous rectangular closed lamellae, as can be seen in Fig. 10.14. In a section parallel to the c-axis, it is observed that Brewster fringes appear by the conjugation of numerous Brazil twin lamellae as the growth of amethyst proceeds.

10.7 Scepter quartz

Since the growth conditions are uncontrolled in the growth of natural quartz crystals, it is probable that the growing crystals experience major conditional changes, such as cessation of growth and regrowth from new solution. Under such circumstances, crystals formed early on may act as seed crystals.

Scepter quartz, which is composed of a trunk portion and a thicker umbrella (cap) portion, is an example of a crystal grown under these conditions (see Fig. 10.15). It is generally observed among groups of quartz crystals occurring in a druse that only the larger crystals show a scepter form, whereas coexisting smaller or shorter crystals do not. In some crystals, the trunk portion is colorless and the umbrella portion is violet amethyst, or smoky, and in other cases the relation is reversed. It is clear either way that the conditions changed at some time between the formation of the trunk portion and the formation of the umbrella portion. It is also generally observed (i) that the surface of the trunk portion and the coexisting non-scepter quartz are covered by precipitation of other minerals, such as mica or clay minerals, whereas a coating of this type is not seen on the surface of the umbrella portion, and (ii) that the density of two-phase inclusions is high in the trunk portion near to the boundary with the umbrella portion. These facts indicate that the growth conditions changed greatly between the formation of the trunk and the umbrella portions, that precipitation of mica or clay minerals occurs at the latest stage of formation of the trunk portion, and that the surface of the trunk is covered by these precipitates. The umbrella portion is formed from a purer hydrothermal solution than the trunk, but with the same orientation, after the surface of the trunk portion is covered by precipitate. Based on these observations, it is understood that the growth of the umbrella portion began from that portion not covered by foreign minerals and that it retains an epitaxial relation with the trunk portion, and overgrows the area covered by foreign minerals. This mechanism is exactly the same as ELO (epitaxial lateral overgrowth) [18], [19], explained in Section 7.4, which is used as a method of dislocation control in liquid phase epitaxy.

Figure 10.16 indicates why both scepter quartz and common quartz coexist in a druse.When quartz crystals are synthesized hydrothermally using a seed, a part of

10.8 Thin platy crystals and curved crystals 217

Figure 10.15. (a) Ordinary scepter quartz with umbrella portion only at the top of the trunk portion. (b) Scepter quartz with many umbrella portions at the top and along the edges of the trunk.

which is masked by silver clay, quartz growing from an unmasked part grows laterally over the surface of the silver clay, and scepter quartz is formed artificially [20]. If the seed surface is covered with outcrops of dislocations and the area has no dislocation outcrops unmasked, allowing ELO to take place, synthesis of dislocationfree quartz crystals may become possible.

10.8Thin platy crystals and curved crystals

There may be crystals of other minerals already present in a vein before the growth of quartz crystals begins from a hydrothermal solution. These crystals may act as heterogeneous nucleation sites for later-forming quartz crystals. In particular, the edges and corners of earlier-formed crystals play an important role as heterogeneous nucleation sites. As a result, many quartz crystals formed later are aligned almost parallel to each other due to preferential nucleation on edges and

218 Rock-crystal (quartz)

(a)

(b)

Figure 10.16. (a) An example of a geode with only part of the crystals showing scepter

form; (b) schematic explanation.

corners, or quartz crystals in sub-parallel intergrowth may be produced, showing macroscopically curved surfaces, controlled by the curved forms of crystal aggregates that are already present. If crystals of other minerals which acted as seeds are dissolved after the growth of quartz crystals is complete, the quartz crystals will show markedly different forms from those of spontaneously formed quartz crystals; for example, see Fig. 10.17. Quartz crystals showing morphologies of this type often occur in Alpine-type mineral fissures, and are referred to as Gwindel-type crystals. The above mechanism was accounted for by Bonev [21].

10.9Agate

Variety names, such as chalcedony and jasper, are given to minute crystalline aggregates of low-temperature quartz. Those showing distinct banding pat-

10.9 Agate 219

Figure 10.17. Gwindel-type quartz.

terns are called agate, which has been used widely as an ornamental stone for centuries. Agate occurs as a lining in the walls of voids in lava flows, or as nodules in sedimentary rocks. Two controversial suggestions have been put forward to account for the genesis of agate in lavas: one assumes that agate banding is due to the so-called Liesegang phenomenon in the crystallization of silica liquid droplets formed by the melting of siliceous rock fragments occluded in the magma; the other explains that the precipitation of quartz from underground water containing SiO2 intruded into the voids of solidified lava. The former hypothesis assumes high-temperature conditions above 700°C, but the second assumption, of precipitation from aqueous solution at temperatures as low as 50°C, is now widely accepted.

Agate that fills the voids in igneous rocks commonly exhibits the following four types of textures; see also Fig. 10.18.

220 Rock-crystal (quartz)

Figure 10.18. Schematic illustration of textures seen in a geode. (1) Agate banding

(including (2) Rundel bundlung); (3) larger crystals; (4) Uruguay band.

(1)Those showing irregularly curved banding parallel to the wall surface of a geode (Fig. 10.19). This is called agate banding. The bands vary irregularly in width, but in general they become gradually narrower towards the inner portion, and eventually they have uniform and very narrow width. Such portions with narrow and uniform banding are called Rundel bundlung; this is explained in (2). Agate banding is composed of a fibrous texture running perpendicularly to the banding. A single fiber with a thickness of micrometer order, which is observable under an optical microscope, is composed of several tens of fibers with width of nanometer order, observable under an electron microscope. A single fiber is composed of an alignment of short-prismatic quartz crystals of nanometer size, whose c-axis is perpendicular to the elongation of the fiber. Fibers of optical microscopic order may sometimes show sheaf, semi-spherical, and spherulitic form, and tend to occur more frequently as the wall of the void is approached.

(2)Uniformly spaced banding of Rundel bundlung, which appears more frequently as the center of a void is approached (Fig. 10.20), is a banding pattern due to a repeated cycle of size distribution of quartz grains (from smaller to larger). The large quartz crystal developing towards the void center corresponds to the final stage of the cycle. Namely, changes from

(1) to (3) can be regarded as representing a series of events.

(3)Large short-prismatic quartz crystals develop toward the center of a void. In many cases, these crystals are amethyst, in which Brazil twins and Brewster fringes are universally observed.

10.9 Agate 221

(a)

(b)

Figure 10.19. (a) Polarizing photomicrograph of agate banding. (b) Transmission-type

electron photomicrograph [16].

(4)In the center of a void after formation of (1)–(3), a pattern with horizontal banding appears, which is called a Uruguay band. This band appears due to grain size variation of minute quartz crystallites of sub-micrometer order, and is formed by gravitational settlement. Quartz particles consisting of this band are idiomorphic, and show ditrigonal dipyramidal form bounded by r and z faces, with no m faces (Fig. 10.21). Brazil twinning is universally observed in these quartz particles.