Inhibited growth of crystals

Inhibited Growth of Crystals

Crystals showing inhibited growth in particular directions are fascinating things. They are in some ways the opposite of crystals formed by dendritic growth, although the resultant crystal forms can be somewhat similar. One example is the "hourglass" form of gypsum, where the crystal is able to push away the sand or clay in which it is growing from certain crystal faces, but must encompass it in others. Finally, the opaque inclusions form an hourglass figure within the otherwise clear gypsum crystal, and trace the sizes and positions of the different types of faces as the crystal grew.

Much prettier (and formed by a different mechanism) are trapiche emeralds such as that shown in Figure 1. (Figure 1 omitted) These crystals have a central core composed of an often tapered, columnar beryl crystal. Surrounding it like spokes of a wagon wheel are an additional six sectors of beryl, and between these sectors are areas of mixed composition; in the crystal shown, the mixture is mostly albite. The exact shape of the sectors, like those in the chiastolite variety of andalusite, varies from crystal to crystal, and along the length of a single crystal. The structure and mechanism of growth of these crystals, which have been found at the Pena Blanca mine near Muzo, Colombia, have been studied by Nassau and Jackson, who reported their results in a short paper in the Lapidary Journal, and in a more detailed report in the American Mineralogist. Briefly, their proposed mechanism for the formation of trapiche emeralds is as follows: First, the central, tapered core grows under hydrothermal conditions. Second, growth may slow or even stop for a while. Next, growth conditions change again, and both emerald and albite are formed. However, the hexagonal prism faces of the core crystal are able to maintain their uniform growth, producing pure emerald, while areas growing from the edges between prism faces are not, and are filled by albite. This results in six sectors of clear emerald, and six of predominantly albite and minor emerald. Thus, the central core and the six surrounding sectors of a trapiche emerald comprise a single, untwinned crystal.

There is a distinct difference between this type of growth and dendritic growth. The growth of these emerald crystals is from crystal faces. That of dendritic crystals is from crystal edges and corners.

The highly altered cordierite crystal from Kameoka, Kyoto Prefecture, Japan (shown in Figure 2) looks very similar to the trapiche emerald. (Figure 2 omitted) Indeed, these crystals vary in pattern from crystal to crystal and along their length like the trapiche emeralds. It is tempting to conclude that they are formed by a similar mechanism. This example is a little more complicated, however. In the case of trapiche emerald, we are asked to accept that renewed growth occurs on the six faces of the hexagonal prism. Cordierite, however, is orthorhombic (although single crystals are often pseudohexagonal), and it can also twin to give pseudohexagonal forms. There are two possibilities, then. First, the six outer segments can be formed by renewed growth of a single, pseudohexagonal crystal by the trapiche mechanism on not one crystal form but two; the two faces of the pinacoid h and the four faces of the prism m. Second, the crystal shown could be the result of renewed growth of a complexly twinned core crystal.

Another example of crystals showing interrupted growth as above, but which results in a completely different crystal shape is the pyrite shown in Figures 3 and 4. (Figures 3 and 4 omitted) This crystal and others like it came from the Westvaco mine in Wyoming, and were found imbedded in trona, from which they could be recovered by dissolving the matrix in water. They were described in 1971 by Pabst (1971), who showed by X-ray methods that the crystals are not twinned. Further, he showed that the striations on the projecting portions of the crystals do not meet at right angles at edges and do not have interfacial angles of the pyritohedron {210}, as do those of normal, striated pyrite. It is probable that these crystals too formed by the trapiche type of mechanism. That is,

normal cube formed; growth was interrupted; finally, further growth occurred, but only on the cube faces, and not the cube edges. The striations must project outward from places where complete coverage of the cube faces did not occur.

Thus far, we have dealt with three opaque minerals, the first two of which must be sectioned to show their internal structural variation. We will now examine two transparent ones, where the internal interruption of growth is visible.

The first is the corundum (var. sapphire) crystal from Badula, Sri Lanka, shown in Figure 5. (Figure 5 omitted) Inside the crystal can be seen a series of voids or liquid-filled cavities forming a true, six-sided, three-dimensional hourglass figure, far better and more symmetrical than those seen in the two dimensional hourglass figures of gypsum. Each six sided step in the voids is bounded by six cavities, each of which appears to have four elongated sides joined at right angles; i.e., each is bounded by pinacoid and hexagonal prism faces. Again, it appears that, as the crystal grew, it was able to continue growth on its {1010} and {0001} faces, but repeatedly had trouble growing on the edges between the two forms. Why did it have no trouble growing on the edges between prism faces? The result, at any rate, is a truly beautiful thing. This, by the way, was the only one of two dozen spectacular Badula corundum crystals which showed the hourglass pattern distinctly. Another beautiful example with great crystal form and numerous inclusions is shown in Figure 6. (Figure 6 omitted) These I obtained from Dick Gaines, a dealer who always has unique and interesting microcrystal material at reasonable prices.