202 Rock-crystal (quartz)

Figure 10.3. The structural form of low-temperature quartz predicted from PBC

analysis (see ref. [3], Chapter 4).

criterion, we may analyze the effect of growth conditions on the growth forms of rock-crystal. This analysis will be useful in understanding various geoscientific problems and in identifying appropriate growth conditions in quartz synthesis.

10.3Growth forms

The growth forms of a crystal are determined by the anisotropy involved in the growth rate, and thus the difference in aspect ratio, i.e. long-prismatic or short-prismatic, is determined by the relative ratio of growth rates Rm: Rr,z , i.e. the

relative normal growth rates of m {1010}, r {1011}, and z {0111} faces. When Rm is much smaller than Rr,z the crystal takes a long-prismatic Habitus. When Rr Rz, six well developed pyramidal faces, r being slightly larger than z, will appear, but crystals with Rr Rz will take a triangular prismatic form with only three r faces at the termination.

All m, r, and z faces grow by the spiral growth mechanism, and so the growth rate

R is determined by the height of the spiral growth layers, their advancing rates, and the step separation; thus, factors influencing these values are the same factors which affect the growth forms of rock-crystal.

Low-temperature quartz, irrespective of whether it is natural or synthetic, grows in hydrothermal solution. In addition to H2O, minor amounts of NaCl, NaOH, NaF2, and Na2CO3 (in synthesis, these are called mineralizers and increase the solubility of quartz) play important roles as solvent components.

The anisotropy in the growth rates of the synthesis of rock-crystal at an industrial scale is depicted in Figure 10.4 [1].* Rm is the smallest, the order being

*Original figures were published in ref. [1], and in most of the later publications Fig. 10.4 is cited. Details are explained in ref. [2]. New data have been published in ref. [3].

10.3 Growth forms 203

growth rate (mm/day)

orientation angle (degrees)

Figure 10.4. Growth rates of crystal faces in synthetic quartz [1]–[3]; ■ normal

growth rate of crystal faces; ● growth rate in weight; ▲ reference data.

Rm Rr Rz R0001. However, the difference between Rr and Rz is small, and the relation may vary depending on temperature difference and the strength of convec-

tion. It has been empirically shown that if these values are large Rr Rz, and if they are small Rr Rz. In the latter case, spontaneously grown crystals, grown without seed in an autoclave, exhibit triangular prismatic form with only r termination, and the z faces disappear.

Whether low-temperature quartz crystals take longor short-prismatic forms depends on the modes of occurrence (the growth conditions). Compared with the long-prismatic or needle-like forms with an aspect ratio exceeding ten shown by spontaneously grown synthetic quartz crystals, the aspect ratio of quartz formed under high-temperature conditions and occurring in pegmatite is two to three at maximum; for example, that of amethyst formed in geodes at lower temperatures is in the range one to two, and it exhibits short-prismatic Habitus. Crystals showing an aspect ratio as high as spontaneously grown synthetic crystals are found only exceptionally in nature; however, very rarely a case in which needle crystals coexist with crystals having a smaller aspect ratio occurs in a druse. In many cases, crystals formed at the later stages take r z.

Since high-temperature quartz occurring as phenocrysts in acidic igneous rocks (igneous rocks with around 70% SiO2 content) exclusively take hexagonal bipyramidal forms with no prismatic faces, it has been assumed that this is the typical