Reference Work Entry

Mineralogy

Part of the series Encyclopedia of Earth Science pp 469-473

Skeletal crystals

  • Vivien Gornitz

A skeletal crystal is one that develops under conditions of rapid growth and high degree of supersaturation. Atoms are added more rapidly to the edges and corners of a growing crystal than to the centers of crystal faces, resulting in either branched, tree-like forms or hollow, stepped depressions. Branched crystals are considered to have a “dendritic” habit; the hollow stepped crystals are referred to as “hoppers.”

While the concentration gradient of matter to a growing crystal tends to be highest at corners and edges of faces, at low levels of supersaturation the overall growth rate is uninfluenced by local fluctuations in the degree of supersaturation, since flat, polyhedral faces are the general rule among crystals. Only above a critical level of supersaturation will the high corner and edge concentration gradients promote most rapid growth at these positions with the consequent development of skeletal forms.

The term “skeletal growth” may be used in two slightly different ways. During rapid edge growth, internal cavities form on some or all crystal faces. Subsequent growth seals the cavities, but commonly the hollows are filled with fluid, presumably of the same composition as the solution from which the crystal grew. Quartz from Minas Gerais, Brazil, and halite from the Dead Sea, Israel, Illustrate this phenomenon (Figs. 1, 2).
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FIGURE 1

Skeletal quartz (from I. Vanders and P. F. Kerr, 1967. Mineral Recognition. New York: John Wiley. Copyright © by John Wiley & Sons, Inc. Reprinted by permission).

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FIGURE 2

Hopper crystal of halite from the south-western shore of the Dead Sea near Sodom (width of crystal 6 cm).

More generally, the term “skeletal growth” refers to the development of crystals with a branching, dendritic (tree-like) habit (see Crystal Habit). Examples of dendritic crystals include ice on a windowpane, pyrolusite on agate (moss agate), and native gold , silver , and copper .

Hoppers

A hopper crystal is produced when atoms are deposited more rapidly at the edges and corners than at the centers of crystal faces, forming deep, stepped depressions in the centers of the affected faces (Fig. 3). Hopper crystals generally develop under conditions of a high degree of supersaturation (such as at elevated temperatures and high concentration gradients) and, consequently, a rapid rate of crystallization.
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FIGURE 3

(a) Hopper Crystal (from I. Vanders and P. F. Kerr, 1967. Mineral Recognition. New York: John Wiley. Copyright © 1967 by John Wiley & Sons, Inc. Reprinted by permission). (b) Idealized hopper growth (from Crystals and Crystal Growing by Alan Holden and Phylis Singer. Copyright © 1960 by Educational Services Inc. Reprinted by permission of Doubleday & Co., Inc.).

By use of an interferometric technique, the concentration gradient was shown to be greatest near the crystal corners for crystals of the NaCl type in supersaturated solutions (Goldsz-taub and Kern, 1953). Since diffusion of matter to the growing crystal is proportional to the concentration gradient, matter can be transported to edges and corners faster than to the centers of faces.

Under usual conditions of crystallization, the cube face normally develops parallel to itself, even though the level of supersaturation is higher near the apexes. This occurs because the most rapidly growing faces (i.e., [111] or [110]) quickly disappear, whereas the slower-growing cube faces are those that ultimately survive. On the other hand, the presence of crystal imperfections near face centers increases the growth rate there relative to the corners (Chernov, 1974), thus counteracting the effect of diffusion. Above a critical level of supersaturation, however, the importance of diffusion predominates and those defects located near the position of greatest supersaturation (i.e., near the apexes) will then promote rapid deposition. Once hopper-type growth is initiated, the projecting edges may restrict diffusion of solute to the face centers, thus slowing deposition there to an even greater extent. The presence of impurities, such as included mud, may also inhibit development at face centers as seen in the formation of the Dead Sea halite hoppers (Gornitz and Schreiber, 1981; and Fig. 2).

Halite, pyromorphite , and vanadinite are minerals that commonly develop hopper crystals (Figs. 2, 3). Less frequently, hopper crystals may be observed on chalcopyrite , diamond , and gold (Desautels, 1968).

Metallic bismuth, although produced artificially, also exhibits hoppered pseudocubic rhombohedrons. Other synthetic crystals displaying hopper-like habits include ZnS, CdS, CdTe, HgSe, and HgTe (which crystallize with the wurtzite structure). These substances form hollow, sometimes scroll-like hexagonal prisms, in some cases perched on whiskers or thin hair-like growths (Fig. 4). A possible growth mechanism is helicoidal growth along a screw dislocation with growth concentrated along the periphery, developing a pyramid (see Crystal Growth). Upon restriction of continued outward expansion (because of interference with neighboring crystals), the growth becomes predominantly vertical, leading to a hollow hexagonal prism (Simov et al., 1974). In ZnO, clusters of hollow, hexagonal prisms and hoppers grow upward in the c-axis direction, from a substrate crystal that lies parallel to the (2 https://static-content.springer.com/image/prt%3A978-0-387-30720-6%2F19/MediaObjects/978-0-387-30720-6_19_Part_IEq28_HTML.gif https://static-content.springer.com/image/prt%3A978-0-387-30720-6%2F19/MediaObjects/978-0-387-30720-6_19_Part_IEq28_HTML.gif 0) plane. The hoppers are composites of several hollow prisms that have nucleated simultaneously and have grown radially from a common point on the substrate (Iwanaga et al., 1978).
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FIGURE 4

Hollow CdTe crystals.

(a) Hollow hexagonal prism and pyramid perched on whisker. (b) Hollow hexagonal prism with scroll-like opening.

Halite not only forms hoppered cubes; it also occurs in pyramidal hoppers. Pyramidal halite hoppers represent growth in shallow bodies of brine under conditions of high temperatures, high evaporation rates, and, hence, high degree of supersaturation. According to Dellwig (1955), growth is initiated at the surface of the evaporating water. Small, flat, platy crystals form and float on the surface because of the surface tension of the brine. Growth continues downward, as pyramidal hoppers. Eventually plates aggregate into crusts, which sink to the bottom but continue to grow in a cubic habit. The presence of “phantom” pyramidal hoppers is seen in the zoning and banding of natural salt (see Phantom Crystals). The same mechanism applies to grainer salt, produced artificially.

An unusual occurrence of hopper development comes from the mare basalts of Oceanus Procellarum on the moon (see Lunar Minerals). The pyroxenes (see Pyroxene Group ) have an extremely complex crystallization history in which core hypersthene has been epitaxially overgrown by a sheath of pigeonite , and that in turn is rimmed by augite (see Epitaxy ). Electron microprobe traverses suggest, however, that the (001) sector of pigeonite remained hollow and was filled in later by augite in epitaxial overgrowth (Hollister et al., 1971). Similarly, electron microprobe traverses across plagioclase (see Plagioclase Feldspars) crystals from another specimen indicates increases in Na and K toward both the outer rim and the core, implying that plagioclase grew outward, and inward as a hollow crystal, trapping residual liquid that ultimately formed a core of pyroxene and other minerals (Walter et al., 1971).

Dendrites

While most dendritic crystals exhibit irregular branching, a snowflake provides a beautiful illustration of the control exerted by the hexagonal symmetry on the overall shape of the crystal. In a snowflake, the dendrite branches are all parallel to the crystallographic axes a 1, a 2, and a 3. The exact cause for the regularity in dendrite spacing and branching of a snowflake is still a matter of speculation. The even spacing of the branches could result from variations in the level of supersaturation. Growth on the side branches reduces the super-saturation in the immediate vicinity and prevents further growth except beyond a certain distance. According to one point of view (Fletcher, 1973), the overall symmetry of a snowflake derives from anisotropy in surface free energy and kinetics, which leads to the simultaneous development of dendritic growth at all six corners of the ice hexagon. This process is critically dependent on the excess water vapor density.

Experiments have shown that the habit of a snow crystal is a function both of temperature and of vapor density (Fig. 5). Extreme dendritic forms develop in regions of highest vapor density. Hollow hexagonal prisms are restricted to a narrow temperature range between −6° and −8°C. Hexagonal prisms capped by thin hexagonal plates (Tsuzumi crystals) result from transport of the growing ice crystal from the prism thermal regime to cooler conditions that favor growth of flat plates (Knight and Knight, 1973). An unusual type of snowflake is a three-dimensional dendrite, restricted to snow crystals grown from highly supercooled water droplets, below -20°C (Kobayashi and Furukawa, 1978). The angle between the c-axis of the planar snowflake and the secondary branches is close to 70°. This orientation allows the formation of a superlattice in which the misfit of lattice points between the platy crystal and its branches is minimized.
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FIGURE 5

The Nakaya-Kobayashi diagram, illustrating the various crystal habits of ordinary ice as a function of both temperature and excess vapor density (from N. H. Fletcher, 1973. Dendritic growth of ice crystals, J. Crystal Growth, 20, p. 268).

Further reading

Jones B., Renaut R.W., 1996. Skeletal crystals of calcite and trona from hot–spring deposits in Kenya and New Zealand. J Sediment Res 66 (1): 265–274 Part A.

Treivus E.B., 2000. Skeletal growth forms of crystals in terms of variational principles of nonequilibrium thermodynamics. Crystallogr Rep+ 45 (6): 1029–1034.

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© Hutchinson Ross Publishing Company 1981
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