204Rock-crystal (quartz)
form of high-temperature quartz. When quartz crystals are hydrothermally synthesized at temperatures above 573°C, prism faces appear and the crystals take a hexagonal prismatic form [4]. The hexagonal bipyramidal form of high-tempera- ture quartz simply represents the morphology formed in acidic igneous magmas, and is thus a Habitus representing the growth in this particular ambient phase.
10.4 Striated faces
The prismatic faces of natural rock-crystal are characterized by the development of striations parallel to the edges between m, r, and z faces (perpendicular to the c-axis). Natural rock-crystal showing no distinct striations is almost exceptional. In industrially mass-produced synthetic quartz using NaOH or KOH as mineralizers, no striations are observable on {1010} faces. As shown in Fig. 10.5(a), five-sided growth spiral hillocks are generally observed. However, if quartz crystals are synthesized in hydrothermal solution with NaCl as the mineralizer, the prismatic faces exhibit similar striations to those observed on natural crystals [5].
The ratio Rr,z : Rm is up to ten times higher in NaCl solution than the ratios seen in NaOH or KOH solutions. From this, it is deduced that the striations are due to the remarkable anisotropy involved in the step advancing rate of the growth spirals developing on the m faces The main reason why this anisotropy occurs is understood to be due to the NaCl, which is added as a mineralizer in H2O. The hydrothermal solution in which natural rock-crystal grows is, in general, NaCl aqueous solution.
10.5Growth forms of single crystals
10.5.1Seed crystals and forms
Seed crystals are always used in the synthesis of quartz. Various seed orientations are used depending on the individual requirements. In growing synthetic quartz for industrial purposes, a seed bar parallel to the y-axis (a y-bar) is generally used, whereas for colored quartz, such as amethyst, platy seed parallel to {1011} is sometimes used. (This effectively produces color intensity in the growth sectors, taking into account the difference in partitioning of the impurity element, for example Fe, between the r and z faces.) Experiments using circular disks, holed circular disks, and spheres as seed crystals are also reported [6]. If prolonged growth is achieved on a seed crystal of any form, the crystal will eventually take a polyhedral form in shortto long-hexagonal-prismatic forms, with the aspect ratio determined by Rr,z : Rm corresponding to the growth condition. The forms can be predicted by computer experiments. Figure 10.6 shows a morphodrom prepared by Iwasaki et al. [7]. However, in real synthesis, growth is
10.5 Growth forms of single crystals 205
(a)
(b)
Figure 10.5. (a) Polygonal spiral growth hillocks universally observed on {1010} faces of synthetic quartz. (The arrows indicate the summits of the growth hillocks.) (b) Striation patterns commonly seen on the {1010} faces of natural and synthetic quartz grown in NaCl aqueous solution.
206 Rock-crystal (quartz)
Figure 10.6. Morphodrom of rock-crystal obtained by computer experiments [7];
p Rz/Rr, q Rm/Rr .
terminated before the crystals reach the form shown in Fig. 10.3. As a result, faces that do not appear on natural rock-crystal, such as {0001} or {1120}, do appear on synthetic crystals. These are structurally K faces, corresponding to rough interfaces, and so they exhibit different surface microtopographs from those of {1010} or {1011}, which are F faces. For example, {0001} faces exhibit a cobbled structure, such as that shown in Fig. 10.7. Cobbles are growth hillocks, but not spiral hillocks, and the density is high close to the seed and decreases as growth proceeds by coalescence of smaller cobbles. Since dislocations and impurities principally concentrate at cobble boundaries, the defect structures in the {0001} growth sectors are mainly controlled by the state of development of the cobbles.
Since r, z, and m faces grow by the spiral growth mechanism, spiral growth hillocks or spiral growth steps showing respective characteristic forms can be observed on their surfaces. The {1010} faces, having the smallest normal growth rate, exhibit polygonal forms, whereas on {1011} and {0111} (faces having larger normal growth rate than the former) circular spiral growth layers are observed.
10.5 Growth forms of single crystals 207
Figure 10.7. Cobbled structure observed on {0001} face of synthetic quartz.
Photographed by M. Kawasaki.
10.5.2Effect of impurities
If we consider NaOH, KOH, and NaF2, which act as mineralizers, to be the solvent components, and other minor amounts of elements such as Fe3 and Al3 to be impurity elements, then the partitioning of these impurity elements is controlled principally by kinetics. The impurity partitioning is related to color, or radiation-induced color, and crystal morphology.
The violet color of amethyst is due to the presence of a color center that is induced by the irradiation of point defects caused by the impurity FeOOH, and the smoky color in smoky quartz and black quartz is due to the presence of a color center related to Al, which susbstitutes Si. Since impurity partitioning is affected by growth sectors and growth rates, different color intensities will appear in different growth sectors and will be affected by growth banding. Amethyst is a deeper color in the r growth sector, and a paler color in the z growth sector, resulting in the appearance of a triangular pattern with alternating intense and pale color. Variation in color intensity may also be observed in association with growth banding. Tapered crystals are examples showing the effect of impurity adsorption upon crystal morphology (see Section 10.5.3).
10.5.3Tapered crystals
Some rock-crystals exhibit dog-tooth forms as hexagonal prismatic forms taper off towards the tip of the prism. Some examples are shown in Fig. 10.8. This
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Figure 10.8. Tapered rock-crystals.
phenomenon is called tapering, and has been observed in crystals of ADP and KDP growing from an aqueous solution for piezoelectric materials, and is the subject of extensive investigation [8]–[10]. Crystals of ADP and KDP are bounded by {100} and {111} faces, and tapering occurs in the c-axis direction. It has been understood that tapering occurs because trivalent ions, such as Fe3 , which are present as impurities, are selectively adsorbed in a particular direction that is related to the growth layers on the {100} face, thus retarding the advancing rate of the steps.
The origin of tapering observed in rock-crystal is basically the same as in the cases of ADP and KDP [11]. In the case of natural rock-crystal, it is suggested that the precipitation of clusters formed in the solution on the growing surface has a similar effect to that of impurity Fe3 ions.
10.5.4Solution flow
Rock-crystal occurring in vein-type ore deposits grows in ascending hydrothermal solution through cracks in the strata. The flow of solution causes the solute component to be supplied to crystals growing inclined to or perpendicular to the wall of the crack. In laminar flow, the growth rate of the side facing the flow increases compared with the opposite side. In turbulent flow, the situation will be reversed.
As a result, the section of hexagonal prismatic crystal changes from regular hexagonal (expected when the crystal grows in an isotropic environment) to malformed hexagonal. This variation is also recorded in the crystal as a directional