For biomineralization processes, the interaction of the surface of calcite crystals with organic molecules is of particular importance. Especially, biologically controlled biomineralization as in exoskeletons of mollusks and echinoderms, e.g., sea urchin with single-crystal-like spines and shells,1-3 requires molecular control of seed formation and growth process. So far, experiments showing the obvious influence of organic molecules on the morphology and habit of calcite crystals have demonstrated the molecular dimension of the interaction.4-7 Details of the kinetics of growth and dissolution of mineral surfaces influenced by additives are available,8,9 but other experimental data about the structure of the organic/inorganic interface on the atomic scale are rare. On the other hand, complicated organic macromolecules which are involved in biomineralization are numerous, with only a small fraction solved in structure and function so far.10-13 Therefore, model systems have to be designed to provide a basic understanding for the interaction process.14 Using grazing incidence X-ray diffraction combined with molecular modeling techniques, we show that glycine molecules order periodically on the calcite (104) face in competition with the solvent water when exposed to an aqueous solution of the most simple amino acid. In contrast to the general concept of the charge-matching fit of organic molecules on mineral surfaces,4,14 glycine is not attached to the calcite surface directly but substitutes for water molecules in the second hydration layer.

The structure of the fluorapatite ͑100͒ surface in humid ambient N 2 with grazing incidence x-ray diffraction is investigated and compared with results on the same surface in dry ambient conditions. Measurement of specular and nonspecular crystal truncation rods provided atomic scale information about the surface structure and the adsorption sites of the water molecules. In humid environment ͑75% relative humidity͒, a laterally ordered monolayer of four water molecules per unit cell is formed at about 1.8 ͑1͒ Å above the relaxed surface reducing the magnitude of atomic relaxations observed on surface in dry conditions.

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