The morphology of {1 1 1} faces grown from water-formamide solutions as well as from pure water solutions was investigated. Surface patterns were examined ex situ and in situ using bright field and differential interference contrast optical microscopy and ex situ atomic force microscopy. It was shown that formamide and urea stabilize the {1 1 1} NaCl faces, whereas larger homologous molecules do not. For the {1 1 1} NaCl crystals growing from water-formamide solutions, it was observed that growth proceeds by monomolecular, stabilized layers of height d f1 1 1g , with most probably Na þ ions on top of Cl À ions. Steps originate from spiral-dislocation growth as well as from 2D nucleation starting from the edges of the crystal. Atomic resolution imaging of NaCl {1 1 1} showed no surface reconstruction. The {1 1 1} surfaces grown from pure water solutions showed developing of shallow growth hillocks with rounded tops. It is presumed that these hillocks are related to dislocation outcrops and growth proceeds close to the roughening temperature. Growth pits develop after a longer period of {1 1 1} surface growth in water solution. Their formation is explained by the presence of a semipermeable particle at the pit bottom, which locally retards the fast {1 1 1} growth.

In this Letter we report a unique fractal aggregation behavior of NH 4 Cl crystallites grown in agarose gel. The fractal branches consist of faceted crystallites with remarkable correlations in their crystallographic orientations, which are formed by successive heterogeneous nucleations. Unlike the conventional fractal growth, this aggregation process occurs at a low growth driving force with strong crystallographic anisotropy, which leads to the ordering in the fractal branches. [S0031-9007(98)

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Three-dimensional colloidal crystals made of ferromagnetic particles, such as magnetite (Fe(3)O(4)), cannot be synthesized in principle because of the strong attractive magnetic interaction. However, we discovered colloidal crystals composed of polyhedral magnetite nanocrystallites of uniform size in the range of a few hundred nanometers in the Tagish Lake meteorite. Those colloidal crystals were formed 4.6 billion years ago and thus are much older than natural colloidal crystals on earth, such as opals, which formed about 100 million years ago. We found that the size of each individual magnetite particle determines its morphology, which in turn plays an important role in deciding the packing structure of the colloidal crystals. We also hypothesize that each particle has a flux-closed magnetic domain structure, which reduces the interparticle magnetic force significantly.

We present a surface x-ray diffraction determination of the ͕111͖ NaCl-liquid interface structure. Using ultrathin water or formamide liquid layers we ascertained that the crystal surface is smooth at an atomic level and is not reconstructed. Our results reveal surprisingly small differences in surface structure between the two cases, which nevertheless lead to dramatic differences in crystal morphology. We determined that the rocksalt ͕111͖ surface is Na + terminated for both environmental conditions. A quarter to half a monolayer of laterally disordered Cl − ions is located on top of a fully ordered Na + crystal surface with occupancy 0.75-1.0. This means that the polar surface is stabilized through the formation of an electrochemical double layer.