Beautiful Gems

An important characteristic of some mineral species is their tendency to form twinned crystals, which are strictly symmetrical intergrowths of two crystals of the same species. If more than two crystals are joined together according to the same law, the resultant compound crystal is called a repeated twin; and one may talk about trillings andfourlings, depending on the number of crystals involved. If twinning has taken place in accordance with two or more laws, the resultant twin is termed a compound or complex twin.
Twins always begin to form during the initial stages of crystal growth, and the pattern of twinning is determined by the relative positions of the crystal nuclei. The ability to form twins varies greatly from mineral to mineral. Some species, such as andalusite, for example, have not been known to form twins. Feldspar, on the other hand, forms twins in accordance with several twinning laws,
the best known of which are the Carlsbad, Baveno and Manebach laws. Quartz crystals can be twinned according to a number of less well-known laws, but twins are less common than in feldspar. Many minerals are easily recognised because of the characteristic shape of their twins. Staurolite, for instance, forms characteristic cross-like penetration twins (Plates 72, 73). One valuable criterion which is useful for recognising twinned crystals is the presence of re-entrant angles, which are common in twins but unknown in single crystals (Plate 80).

More than half of the crust is, in fact, made up of the various forms of feldspar. The next most abundant mineral groups are the pyroxene and amphibole families, whose most important members are respectively augite and hornblende. These are followed by quartz and then by mica, which includes the dark biotite and the pale muscovite. These relatively few minerals between them
form well over 90 per cent of the earth’s crust. The remainder is made up of olivine and several
18    minerals found in minor quantities in igneous rocks, such as magnetite, apatite, zircon and rutile.

To these can be added the more important minerals of metamorphic rocks, such as chlorite, serpentine and garnet, and the more widespread minerals of sedimentary rocks—the clay minerals and several carbonates, particularly calcite and dolomite—and, in still smaller quantities, haematite, pyrite, limonite, the feldspathoid family, sphene, chromite, corundum, tourmaline, spinel, copper pyrites and pyrrhotite. All these together make up well over 99 per cent of the solid crust. The remaining minerals, which include the beautiful ores that are the pride of many a collection and nearly all the gemstones, form only an exceedingly small, almost negligible, proportion of the crust. The predominant rock types in the crust are the igneous and the related metamorphic rocks, which accounts for the predominance of the minerals of which they are composed.

Thisis surprisingly small: there are at present about 3,000 known mineral species. Every year several new minerals are described, but, as there are also some which are no longer acceptable as separate minerals because they have been found to be mixtures or fine intergrowths of known species, the number of known and accepted minerals increases but slowly. Compared with the number of chemical compounds, of which there are nearly half a million carbon compounds alone, the number of combinations of elements forming natural minerals is exceedingly small. Even the number of known flowering plant or insect species is many times greater than that of all known minerals.

The term species has been borrowed from the realm of biology, where it has been used to define in a loose way a group of individuals with common characteristics which are able to propagate themselves. As no such criterion can be applied to minerals, the definition of a mineral species is of necessity even more arbitrary than that of an animal or plant species.
RELATIVE ABUNDANCE OF DIFFERENT MINERALS IN THE EARTH’S CRUST. Even more surprising than the small total number of known minerals is their uneven distribution in the earth’s crust.

Both closely related and completely different minerals are frequently intergrown with each other, and many of the finest specimens of minerals found in museums consist, not of a single mineral, but of a group of minerals which are to some extent intergrown (Plates 4, 7,8, 33, 42, 58, 68). Quite often minerals are joined together or interlocked in an irregular and unpredictable manner. There are, however, quite a number of regular crystal intergrowths which follow a strict pattern of symmetry. In these cases the minerals concerned are closely related and the pattern of intergrowth is due to the similarities in the internal structure of the minerals.

The great diversity of the roots of mineral names reflects the development of mineralogy. Some names have come to us from ancient civilisations and the orient. Other names, such as blende, kies, spath and glance, are derived from the names first used by German ore miners in the Middle Ages. Yet others are connected with superstitions and myths, and many bear the name of the area in which the ores were first mined. Chemical composition, appearance, and outstanding characteristics have all played their part in the naming of minerals. Other minerals, again, are named after their discoverer or after a famous mineralogist. Many minerals are known by two or even more names, which may have originated in different countries and are now established by long usage. Most mineral names end with -ite or -lite (formerly -hth), which is derived from the Greek lithos = stone.

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).