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Chemical gardens in laboratory chemistries ranging from silicates to polyoxometalates, in applications ranging from corrosion products to the hydration of Portland cement, and in natural settings ranging from hydrothermal vents in the ocean depths to brinicles beneath sea ice. In many chemical-garden experiments, the structure forms as a solid seed of a soluble ionic compound dissolves in a solution containing another reactive ion. In general any alkali silicate solution can be used due to their high solubility at high pH. The cation should not precipitate with the counterion of the metal salt used as seed. A main property of seed chemical-garden experiments is that initially, when the fluid is not moving under buoyancy or osmosis, the delivery of the inner reactant is diffusion controlled. Another experimental technique that isolates one aspect of chemical-garden formation is to produce precipitation membranes between different aqueous solutions by introducing the two solutions on either side of an inert carrier matrix. Chemical gardens may be grown upon injection of solutions into a so-called Hele-Shaw cell, a quasi-two-dimensional reactor consisting in two parallel plates separated by a small gap.

Categories of the generalized crystallography, where structures are tiled into a large number of identical cells, include quasi-identity and quasi-equivalence. The hierarchy of organization levels including the n-dimensional space is considered. In the theory of proportions, irrational numbers, such as e, π, τ, etc., play an important role. The golden ratio τ is basic to the geometry of structures with five- or tenfold symmetry that are called quasi-crystals. The irrational nature of these numbers probably means that in the Euclidean space E3, we deal with projections of the fundamental polyhedra from a higher-dimensional space. Thus, the structures of quasi-crystals in the three-dimensional space represent slices of much more complex assemblies in the higher-dimensional space.

The principles of algebraic geometry have been applied for the modelling of substances of different natures. A local packing approach provides a tool for constructing not only the special icosa-geodesic 2D tilings of a sphere but also the 3D tilings of spherical layers. The structures of the pseudo-T = 7 polyoma and papilloma viral capsids can thus be modelled. This general approach is applicable to inorganic, organometallic, and biological nanoparticles.

In icosahedral carbon nanoparticles, the diamond-like core can undergo a reversible topological transition into and coexist coherently with the onion shells., The general approach for describing and designing complex hierarchical icosahedral structures is discussed. Structural models of icosahedral carbon nanoparticles in which the local arrangement of atoms is virtually identical to that in diamond are derived. It is shown that icosahedral diamond-like particles can be transformed into onion-like shell structures (and vice versa) by the consecutive smoothing (puckering) of atomic networks without disturbance of their topological integrity. The possibility of coherent coexistence of icosahedral diamond-like core with onion shells is shown.

In nature there are numerous layered compounds, some of which could be curved so as to form fascinating nanoshapes with novel properties. Graphite is at present the main example of a very flexible layered structure, which is able to form cylinders (nanotubes) and cages (fullerenes), but there are others. While fullerenes possess positive curvature due to pentagonal rings of carbon, there are other structures which could include heptagonal or higher membered rings. In fact, fullerenes and nanotubes could display negative curvature, thus forming nanomaterials possessing unexpected electronic and mechanical properties. The effect of curvature in other nano-architectures, such as in boron nitride and metal dichalcogenides, is also discussed in this account. Electron irradiation is a tool able to increase the structural complexity of layered materials. In this context, we describe the coalescence of carbon nanotubes and C(60) molecules. The latter results now open up an alternative approach to producing and manipulating novel nanomaterials in the twenty-first century.

Extrapolating from human history, science is seen as having ever enhancing responsibility in surviving the future.

By the materials of the lecture delivered in the School on Electron Crystallography, Erice, Sicily, 1997 and published in Proceedings of NATO Advanced Study Institute on Electron Crystallography, Erice, Sicily, 1997, Series E, vol. 347, p. 1.

The bulk of the 'silica garden', made by seeding a solution of sodium silicate with crystals of aluminium nitrate, is an amorphous aluminosilicate. Transmission electron microscopy reveals a hierarchic substructure involving fine tubes f 44 . nm in diameter composed of amorphous silica rods. It is also possible that the macroscopic fibres seen by scanning electron microscopy contain periodic arrays of microtubules f 4.4 mm in diameter. q 1998 Elsevier Science B.V.

Two samples of hydrated silica from the bamboo plant (‘tabasheer’ and ‘bambusa’) were examined by X-ray fluorescence and 1H, 27Al and 29Si solidstate NMR, X-ray diffraction and thermogravimetric analysis. Silicon is the major elemental component of both samples, with much smaller amounts of aluminium (about 1·2 wt%), iron (0·3—0·4 wt%), calcium and magnesium, and traces of phosphorus. The maximum theoretical water loss calculated from the OH/Si ratio derived from the 29Si NMR spectra (4·92 and 3·82 wt% for the two samples) agrees with the weight loss measured by thermogravimetric analysis. Adsorbed water is evolved at temperatures below about 250°C, which is followed by gradual dehydroxylation. The water content of tabasheer is greater than that of bambusa, so that the initial rate of weight loss in both in tabasheer is higher. However, above about 250°C the rate of weight loss in both samples is very similar and the thermogravimetric curves are virtually parallel. The weight loss on dehydroxylat...

With the discovery of C60 or Buckminsterfullerene a new kind of materials with important applications has emerged. Orginary graphite is composed of flat hexagonal layers. If pentagons are introduced, the graphite sheets start to curve in such a way that 12 pentagons are needed to close the structure and form fullerenes. Starting from the properties of 2D manifolds or surfaces, we have found that by introducing rings with more than six carbon atoms, periodic structures with the same topology as triply periodic minimal surfaces, we have found that by introducing rings with more than six carbon atoms, periodic structures with the same topology as triply periodic minimal surfacds can be constructed. The D, G, P, H and I-WP type surfaces have been obtained from graphite-like sheets. In terms of the Gaussian curvature K, ordinary and cylindrical graphite have K=O, Fullerenes have K>O and triply periodic surfaces decorated with graphite have K

We have extended the use of the invariants of spherical harmonics for the characterisation of the angular distribution of coordinating atoms about a central atom. These invariants are suitable indices for use in clustering algorithms for automatic crystal chemistry. Invariants for coordination polyhedra in intermetallic compounds and Ca-O silicates are considered. A study of invariants in coordination polyhedra distorted by the Jahn-Teller effect is presented. In addition, Invariants from small clusters and the cuboctahedron-icosahedron transition are analyzed.

Assuming that rings of 5, 7 and 8 carbon atoms are allowable (as well as hexagons) in graphitic sheets, a great variety of finite and infinite structures can be postulated. Besides the shells of the fullerenes, which have positive Gaussian curvature and the cylinders which have zero Gaussian curvature, infinite periodic networks with negative Gaussian curvature are possible. The latter promise to have lower energies than the convex fullerenes.

Conventionally, crystals are infinitely repeating lattices of atoms or molecules, in which every lattice site is identical to every other. Clusters of clusters are an alternative to strict crystalline arrangements, and could form an entirely new family of condensed matter. Such a structure has now been created: hierarchically clustered icosahedral particles of boron sub-oxide can be grown to sizes several micrometres across. As well as being of theoretical interest, this substance is very hard, and could be a useful engineering material.

SUMMARY The Penrose tiling is indeed a geometrical model appropriate for the description of the phenomena of icosahedral quasicrystals, although it is probably a related example, rather than the central theory itself, but many aspects of the theory remain to be developed. The transition from X-ray crystal structure analysis (XRCSA) to high resolution electron microscopy (HREM) as the paradigmatic method for the investigation of crystal structures has facilitated this development and promises other advances from classical crystallography.

SummaryA new mineral, akaganéite, has been found in limonite deposits and has been shown to be identical with synthetic β-FeOOH. It contains no chlorine. The micro-morphology of β-FeOOH as it occurs as akaganéite and as it is produced by a solid-state transformation from FeCl2.4H2O , by the hydrolysis of dilute FeCl3 solutions, and by the hydrolysis of very concentrated FeCl3 a solutions, has been studied, chiefly by electron microscopy and diffraction. Some modes of colloidal association of the particles of β-FeOOH are described.

SummaryThe crystallographic properties of the hydrated iron oxide obtained by the hydrolysis of FeCl3 are described. The characterizing X-ray powder pattern can be indexed on a tetragonal unit cell with a 10·48 and c 3·023 Å. This cell is shown to contain eight formula units of FeOOH but Cl- or other anions are necessary for the crystallization of the material The structure is that of hollandite or α-MnO2. The unit cell has been confirmed by electron diffraction from single crystals having dimensions about 5000 × 500 × 500 Å. and twinning, which produces hexagonal stars, has been studied. On heating, the structure breaks down and recrystallizes as α-Fe2O2, probably with an intermediate stage involving a spinel phase.

The composition and morphology of magnetite in CI carbonaceous meteorites appear incompatible with a nebular origin. Mineralization on the meteorite parent body is a more plausible mode of formation. The iodine-xenon age of this material therefore dates an episode of secondary mineralization on a planetesimal rather than the epoch of condensation in the primitive solar nebula.