Crystals

Abstract

The beauty and regularity of crystals impressed people to such an extent that, in the past, crystals were regarded as products of nature with mysterious properties. Scientific investigation of crystals started in 1669, when Nicolaus Steno, a Dane working as a court physician in Tuscan, proposed that during crystal growth, the angles between the faces remained constant. For a given crystal form, individual crystals might differ in shape (i.e, in the development of their faces), but they always have identical angles between the same faces (Figure 3.1). The specific morphology might depend on factors such as the supply of material during growth, on the presence of certain substances in the mother liquor, or on the mother liquor itself. For a single crystal form, the angles between the faces are constant, but this is not true if the crystals belong to different crystal forms. Figure 3.2 shows four different crystal forms of deoxyhemoglobin from the sea lamprey Petromyzon marinus. Their appearance depends on the buffer and on the precipitating agent, although, occasionally, two different forms appear under the same conditions.