Effects of fluid composition on the precipitation behaviour and shape of minerals

Minerals form by the ordered arrangement of atoms. Each mineral has a characteristic form, determined by the electron configuration of the atoms of its structure and the temperature and pressure of the system. However in nature, where no system is pure, mineral structure is also affected by other atoms in the fluid, be it deposition from a melt, an aqueous solution or a vapour. Minor and even trace concentrations of foreign species can completely change crystal form. Organisms take advantage of this. During their millions of years of evolution, they have developed biomolecules that enhance or inhibit growth on certain crystal sites so they can control mineral growth to precisely equip them for their ecological niche. Whether the trace components are inorganic or organic, the change in mineral form can be dramatic. From an ideal laboratory solution for example, green rust, a layered Fe(II), Fe(III) double hydroxide, precipitates as broad, flat, hexagonal plates. Aluminium, which also forms hydroxide minerals, can substitute for Fe(III) but its smaller ionic radius causes distortion in the green rust structure and crystal radius decreases. Organic impurities can have a dramatic effect, exemplified by biominerals, where bones, teeth and shells look and function quite differently than inorganically formed hydroxyapatite, calcite and aragonite. Polysaccharides, even at parts per million concentration, transform the typical rhombohedral shape of calcite into beautiful, intricate discs, that the coccolithophore algae use to cover their single cell. Learning nature’s tricks for controlling crystal growth provides us with clues for developing new, functional materials.