Beautiful Gems

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.