Beautiful Gems

The faces of perfect crystals are arranged in a regular pattern. This regularity is termed the symmetry of the crystal. Any plane which divides a crystal into two halves so that these two halves are mirror images is called a plane of symmetry. A cube, for instance, has nine such planes. A crystal has a centre of symmetry if its every face has a corresponding face which is a mirror image on the opposite side. The third element of symmetry of a crystal is termed the axis of symmetry. This is an axis passing through the crystal around which it can be rotated so that it occupies the same position in space at least twice in one complete turn. An axis of symmetry may be two-, three-, four- or six-fold according to the number of times the crystal occupies its first position in a complete turn, and the angles of rotation which are required to place the crystal into this position are respectively 180, 120, 90 and 60 degrees. The axes of symmetry are expressions of rectangular, triangular, square or hexagonal cross-sections of the simple form of a crystal. We find, for instance, that a flat surface such as a floor can be completely covered with identical tiles only if these are rectangles (two-fold symmetry), cubes (four-fold symmetry), equilateral triangles (three-fold symmetry) or hexagons (six-fold symmetry).

A careful look at a well-formed crystal with many faces gives us some idea of the complexity of the laws of crystal symmetry, which are among the features distinguishing crystals from living organisms. Another difference between the members of the mineral kingdom and living things is the range in size of members of any one species. The adult size of any member of a species of animal or plant is confined to fairly narrow limits, but there is practically no limit to the possible size of a crystal of any one mineral. This is governed by the supply of liquid from which the mineral crystallised, the temperature and pressure prevailing at the time, and, of course, the space available. The size of single crystals of a given mineral may vary from a mere fraction of a millimetre to enormous masses weighing more than a hundred tons.

We are able to determine the system of any given crystal by examining its faces, their number and positioning; the crystal’s over-all proportions tell us its habit. The habit of a crystal may be acicular (needle-shaped), columnar, prismatic or tabular. Barytes, for example, can form either thick or thin tabular crystals, but both types of crystals must have the same arrangement of faces as they belong to the same system. On the other hand, two minerals can have the same habit but belong to different crystal systems. Two mineral species may, for instance, both form crystals with prismatic habit, yet one may belong to the hexagonal and the other to the tetragonal system. In some minerals, the system and habit of the crystal are a guide to the conditions under which the mineral was formed.

CRYSTAL FORM. Well-developed crystals are bounded by a number of faces, which are usually flat. Any two adjacent faces meet at a straight edge, and three or more edges meet at a point, which is known as a solid angle. If all the faces of a crystal are alike, as in a cube (Plate 4) and an octahedron (Plate 78), the crystal is termed a simple form. If a crystal contains elements of two or more simple forms, it is called a combination (Plate 33). If in the combination the faces of the constituent simple forms are developed to the same extent, the combination is said to be in equilibrium, but if one form is dominant it may be called the primitive figure, whose edges and corners have been modified by the smaller faces of the other figure.
CONSTANCY OF THE INTERFACIAL ANGLE. In a growing crystal there is a continual change in the relative size of adjacent faces, but the angle between these faces, known as the interfacial angle, remains constant. Crystals of different sizes belonging to the same mineral and having the same crystal habit are therefore always found to have a similar shape. In every mineral species there are certain characteristic angles formed by corresponding faces of all its crystals. These angles occur not only in natural crystals but also in artificially produced crystals of die same substance. Constant angles are not only present between faces of the actual crystal but also between its internal cleavage surfaces. The angle between cleavage faces of the rhombohedron of calcite, for instance, is always 1050 5 regardless of where the crystal was found or how it was formed.

Large single crystals with fully developed faces are relatively rare. Usually numerous crystals are formed at the same time, and thus interfere with each other’s development. Masses of closely packed minerals which consist mainly of one mineral species are known as mineral aggregates. According to their structure and appearance such aggregates may be described as: radially fibrous (natrolite, Plate 19, wavellite, Plate 49), concentrically banded (malachite, Plate 9), granular (lapis lazuli, Plate 67), botryoidal (blende, Plate 38, azurite, Plate 55, chrysocolla, Plate 52, adamite, Plate 62), nodular (smithsonite, Plate 61), encrusting (antimony ochre, Plate 63), stalagmitic (limonite, Plate 48), fibrous (malachite, Plate 54), sheaf-like (calcite, Plate 14), scaly (chlorite, Plate 76, 80), matted fibres (amianthus, i.e. actinolite asbestos, Plate 79), and stellate, i.e. fibres radiating from a centre to produce star-like forms. There is an almost limitless variety of forms amongst mineral aggregates. Many joint surfaces are lined with innu¬merable small closely packed minerals. The form of native metals, such as copper, silver or gold, may take the shape of thin wires, foils or sheets, or be tree-like or moss-like (Plates 10, 34, 36).

Minerals which have been allowed to develop their own crystalline shape are called euhedral or idiomorphic. Those minerals which crystallised in the spaces between earlier formations and are bounded not by their own but by pre-existing surfaces, are said to be anhedral or allotriomorphic. Yet others which have replaced a pre-existing crystal and thus taken on its shape, are termed xenomorphic. Completely euhedral crystal forms are not common, as most crystals grew up from a base and so are only partially idiomorphic (Plate 6).

Most crystals found in nature are to some degree imperfect, as they usually stand on a surface, such as the wall of a druse, from which their growth started. The crystals which are completely perfect are the ‘floating’ crystals which grew in a soft medium, as, for instance, gypsum crystals in soft clay. The uneven supply of solution during crystallisation leads to distortions and generally the perfect growth of the crystal is prevented by the presence of adjoining crystals or the host rock itself. Due to such external influences, the shape of natural crystals frequently falls far short of the ideal shape. Apart from obvious structural distortions, there are other defects, such as small inclusions of other minerals, which may locally disrupt the orientation of the crystal lattice. Crystals of fairly large size are thus hardly ever completely uniform. Usually they consist of a mosaic of small blocks of homogeneous crystals which are variously orientated and may frequently interlock.

MINERALS AND CRYSTALS. The word mineral is derived from the Latin minare, to mine, and was originally used to include all rocks which were obtained through mining. Nowadays, the word mineral is used to describe those materials of the earth’s crust formed by the inorganic processes of nature, which have a definite chemical composition and whose constituent atoms are arranged in a consistent pattern. Rocks, on the other hand, are defined as aggregates of one or more minerals. Most rock types are composed of several different minerals; of those made up of only one mineral, the best known is marble, which is formed of interlocking crystals of calcite.
The name crystal, which was first used to describe rock-crystal, the clear, transparent form of quartz, has its root in the Greek word krystallos, meaning ice. In the early Middle Ages and before, it was thought that rock-crystal was made of ice which had been so intensely frozen that it could never again melt.

Crystals are now defined as bodies which have definite geometrical forms bounded by flat faces and straight edges. The crystalline state, of course, is not only found among natural minerals, but also occurs in many artificially produced substances, such as refined salt and sugar, and metals, and among waste products of living organisms. The crystalline form is really the external expression of the internal atomic structure of the mineral. The constituent atoms of any given mineral species are arranged in a definite geometric pattern known as the crystal lattice. This lattice determines the mineral’s physical properties, which include hardness, specific gravity, the ease and direction of splitting, and the effects on light transmitted through it .Though the internal structure of any given mineral is always the same, the shape of the crystal is often imperfect, since the growth of crystals in nature is always dependent on the space available to them. The name crystal, however, is used not only for the perfect form, but also for any part of a crystalline substance which has at least some crystal faces and edges. Not all minerals are crystalline. Some are completely structureless and are termed amorphous, but these are relatively rare among natural minerals, the best known being opal and amber. Amorphous minerals are never bounded by flat surfaces, and never have an ordered lattice structure.

They usually have roundish, indefinite shapes (Plate 15). Under certain conditions, an amorphous mineral may, in the course of time, become finely crystalline. Opal, for instance, tends in due course to change into the minutely crystalline {cryptocrystalline} mineral chalcedony.

The number of crystal forms which any one mineral can produce varies enormously. Some minerals never produce any crystal with shaped faces, and some ateonly rarely found in crystalline form or produce only simple crystals. There are others, again, such as topaz, garnet, potash-feldspar, galena, tetrahedrite, pyrite and copper pyrites, which can give rise to a large number of crystalline shapes. The minerals of hydrothermal origin, that is, those which crystallised from dilute solutions at moderately high temperatures, can occur in a particularly large variety of crystal shapes. Barytes, for example, has over 200 distinct crystal forms; the mineral with the largest number is calcite. Several hundred distinct face positions relative to the crystallographic axes have been observed in this mineral, and the combinations of these faces can produce several thousand separate forms. It is quite common to find one mineral in the same deposit, or even specimen, occurring in several crystal forms.

The actual shape of a mineral is determined by its environment during growth. As in the case of a living being, whose inherited characteristics are to some extent modified by its environment, the shape of a crystal depends not solely on its atomic structure but also on such environmental factors as the temperature, confining pressure and concentration of the crystalline fluid, interference from adjacent crystals and, last but not least, the speed of crystallisation.