Beautiful Gems

Many crystals enclose a certain number of foreign particles during their growth. These inclusions often make a clear mineral appear cloudy, or may change the colour or some other characteristic of the mineral. Occasionally, gaseous or liquid remnants from the mother liquor are enclosed in the crystal. Quartz, for instance, may appear milky, due to minute liquid inclusions which are invisible to the naked eye. Other varieties of quartz, such as rock-crystal, may contain quite large, clearly visible crystals of actinolite, tourmaline or rutile.

The properties of any particular mineral are determined not so much by its chemical composition as by the arrangement of its smallest particles. This regular arrangement in space of atoms and ions is called a lattice, and the smallest complete unit of the pattern is known as the unit cell. Every crystalline substance is made up of such a lattice of atoms and ions, and by combining the basic lattice structures according to the laws of symmetry one obtains 230 distinct lattice types into which all the many different crystal lattices can be grouped. The mineral with the simplest crystal lattice is rock-salt (sodium chloride). In its lattice the ions of sodium and chlorine are arranged at the corners of a series of cubes, so that every chlorine ion is linked to six sodium ions, and every sodium ion to six chlorine ions. The unit cell thus consists of four sodium and four chlorine ions, and the distance between the centres of adjacent ions of the same type is 5- 64 x 10-8 cm or 5.64 ten-millionths of a millimetre. Along the edge of a cube of rock-salt one cubic millimetre in size there are thus almost two million unit cells. The determination of the atomic structure of minerals by X-rays is now one of the most important fields of crystallography.

If the supply of the crystalline fluid is not even on all faces of a growing crystal, owing perhaps to currents within the mother liquor, the resultant crystal is to some extent distorted: certain crystal faces are abnormally large, while others are comparatively small. Fluorspar, for example, whose usual crystal shape is a cube, may produce elongated four-sided prisms (Plate 42). Distorted crystals often take on a shape which suggests a symmetry-system other than that to which the mineral actually belongs.
SKELETAL CRYSTALS. If the edges and corners of a crystal grow at a much greater rate than the intervening faces, the resultant structure is a mesh or lattice which is known as a skeletal crystal. The best known skeletal crystals are snowflakes, and the nearest approach among natural minerals is the skeletal quartz illustrated in Plate 75. The so-called dendrites, which are brown or black “skeletal structures,  are formed  from ‘iron- or manganese-bearing solutions which penetrated along thin cracks or planes in the rock and crystallised only after the solution had largely dried up. In shape, dendrites often resemble ferns, mosses, trees or the ice-ferns on window panes.

It can be proved mathematically that among the innumerable possible crystal shapes there are only 32 distinct types of symmetry, known as symmetry classes. These fall into six major groups called crystal systems, the cubic, tetragonal, hexagonal (with the trigonal subsystem), orthorhombic, monoclinic and triclinic systems. The most common minerals of the earth’s crust, the feldspars, crystallise in the monoclinic and triclinic systems.

Rounded or kidney-shaped minerals with a smooth, polished-looking surface usually start their life as small masses of glutinous mineral-gel. These gels contain minute particles of mineral matter; in the process of drying up they become gradually harder and in many cases acquire a definite crystalline atomic structure before finally solidifying. Of the minerals which originated as gels, only a few are still amorphous. Opal (Plate 15) is the best known of these. Minerals which were originally gels, but became crystalline in the course of time, are recognised by their smooth, rounded or kidney-like (reniform) shape and their concentrically layered or radically fibrous internal structure. Apart from reniform and hotryoidal (grape-like) masses, those minerals formed from gels may be elongated and thus look like stalactites (Plates 11, 48).

By far the most abundant element in the earth’s crust is oxygen, which accounts for nearly half its total weight. Silicon, the next most abundant, makes up nearly a quarter. These are followed in quantitative order by aluminium, iron, calcium, sodium, potassium, magnesium and titanium. These nine elements together make up practically the entire earth’s crust, while all the remaining elements form just under one per cent of its weight.

We have only to consider the distribution of elements to realize that silicates and quartz are by far the most common minerals of the earth’s crust. The amount of any element in the crust is not, however, always directly related to the abundance of the minerals in whose composition it forms a significant part. There are, for instance, some relatively abundant elements which do not form part of any known mineral. Rubidium, for example, is the seventeenth most important element, yet the only mineral of which it forms an appreciable part is the rare pegmatite mineral, rhodozite. Hafnium, which is far more common than, say, antimony or bismuth, forms no separate mineral at all. On the other hand, silver, which forms only o.ooooi per cent by weight of the crust, and is amongst the least abundant of elements, is present in a considerable number of minerals. Another relatively rare element is lead, yet it forms the very common mineral galena. The ability or inability of elements to form distinct minerals depends upon their chemical properties. Some elements, whose total amount within the crust is quite significant, are found only as traces or impurities in the minerals of other elements. Zircon, for instance, which is present as an accessory mineral in most igneous rocks, almost always contains some traces of hafnium and thorium. Some elements are thus more or less ‘hidden’ in minerals, and it is not surprising that for a long time they went unnoticed. Other elements again, whose total percentage in the crust is low, form a surprisingly large number of mineral species, owing to their chemical properties. The large number and great variety of antimony minerals, for instance, seem out of proportion to the relatively minor quantity of antimony present in the crust.

Equilateral five-, seven- or eight-sided tiles would not do. In the same way, five-, seven- or eightfold symmetry is unknown in crystallography, but may be found among certain animals such as radiolaria, corals, molluscs, sea-urchins and starfish, and also among many flowering plants. The degree of symmetry of the crystal depends upon the number of axes and planes of symmetry which it contains. The least symmetrical crystals are those belonging to the triclinic system which have no elements of symmetry; and the most symmetrical is the cube, which possesses nine planes of symmetry, thirteen axes of symmetry—of which three are of four-fold, four of three-fold and six of two-fold symmetry—as well as a centre of symmetry. Another aspect of crystal symmetry is that corresponding faces have certain properties in common. Thus, in many crystals, dull or rough faces may alternate with smooth, gleaming faces, while striated faces may lie next to smooth ones. If striated or grooved faces are present in a crystal the orientation of the lines is governed by the symmetry pattern (Plate 12); and if foreign particles are included within the crystal their alignment is controlled by the same factor)

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.