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.
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 .
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.,  or ) 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).
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).
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.
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.