crystals and crystal growth

crystals and crystal growth Crystals are solid bodies bounded by naturally formed plane faces with a regularity of external form which reflects the regular arrangement of the constituent atoms.

Externally, crystals are rarely perfectly developed; some faces are usually distorted or even suppressed, or have striations or etch pits. Internally, what appears to be a perfect crystal may have millions of defects in a specimen a few millimetres across. This is perhaps not surprising when a garnet 10 mm in diameter consists of at least 10²⁰ atoms. What is believed to be the biggest crystal ever discovered, a microcline 50 m × 36 m × 14m calculated to weigh 15 900 tonnes, might have 10³⁵ atoms. Under tightly controlled conditions of growth in the laboratory crystals of much greater perfection are grown.

The growth of a particular crystal depends on the chemistry of the local environment, the presence of fluids, the degree of supersaturation, and the temperature and pressure conditions. Well-formed crystals can grow from a melt (e.g. orthoclase feldspar from a granite magma; quartz from aqueous solution in, say, a hydrothermal vein; garnet in a solid rock during metamorphism; gypsum in a clay deposit). In all cases small groups of atoms are forming and dispersing but as the physical conditions change or the chemical concentration increases these groups will grow larger and reach a critical size, at which stage a new mineral can nucleate and the atoms assume regular positions. In ionic crystals every cation is completely surrounded by anions, and vice versa. The numbers are determined by the ionic charges and the relative size of the ions; the total crystal will be electrically neutral. Slow growth with few nuclei will lead to large crystals; rapid growth with a high level of supersaturation will produce many small crystals.

The bonds of the atoms on the margins of the crystal will be unsatisfied and will latch on to appropriate atoms in the vicinity. Once a crystal face begins to grow, it is comparatively easy for a layer of atoms to be completed, but it is much more difficult to initiate a new layer. However, spiral growth means that there is no new layer to initiate and the face continues to grow spirally. These spirals on the faces of crystals, particularly those growing from aqueous solution, are readily visible under a microscope.

Even though the atomic structure remains the same, the presence of impurities in the growing medium and variations in the physical conditions affect the growth of particular crystal faces, resulting in a variety of external forms. Thus, for one structure many different habits may result—at least 200 in the case of calcite.

Already-formed, small ‘foreign’ minerals and globules of fluid may adhere to the growing face and then become trapped by the further addition of atoms. These inclusions provide valuable information about the environment in which the mineral grew.

Crystals may grow as composite solids in two or more parts, the individual parts being related in a quite specific geometrical way according to the nature of the lattice. These twins can be of contact or interpenetrating type and may be simple or repeated. A plagioclase feldspar crystal may have 200 or more individuals, all closely related.

The large microcline crystal has already been mentioned. It may not in fact be a single crystal. Other large specimens include garnet, beryl, and phlogopite crystals, weighing 38, 370, and 334 tonnes, respectively.

R. Bradshaw