Abstract
Thin plates, slender needle-like crystals, skeletal plates and skeletal linear forms can be produced in the free growth of cubic tetrahydrofuran hydrate from the melt, with growth rates differing by orders of magnitude on adjacent, crystallographically equivalent {111} faces. These anisotropic growth modes are distinctly different from each other and from the normal octahedral form. The growth of the solid plates and needles is unsteady, in that the fastest growth rates decrease with time, becoming nearly equal to those on the slower-growing faces. The skeletal plates and columns have steadier growth rates and are more common at higher values of supercooling. The most extreme solid needle and plate shapes occur at very low values of supercooling. While some or all growth forms that lack the cubic symmetry of the crystal structure may arise because of crystal imperfections that stimulate growth on some faces but not others, aspects of their growth are not straightforward to explain in this way. Possible alternative explanations are conjectured. Very fragile thin growths are initiated by collisions at low values of supercooling, providing an effective crystal breeding mechanism. As far as we know, there is no comparable set of observations of a fast-growing cubic vicinal crystal from its melt, but extensive studies of AgBr grown from gels have revealed the solid needle and plate forms with some similar difficulties of explanation.
