Mineralogia

Minerals explained 26: The garnets
Early man soon became aware of garnets. Their warmth of colour and durability made them well suited to forms of adornment. No doubt the development of trade routes made them widely available from such sources as central Europe and the Indian continent. Evidence of such activity has been found in Roman, Celtic, Etruscan and Byzantine excavations. In Britain, themost outstanding example of such jewellery was that found in the ship burial of an early Anglo-Saxon king at Sutton Hoo in Suffolk. This was rich in cloisonné-type jewellery of superb workmanship, consisting essentially of garnetencrusted gold. Apart from their use as adornment, garnets were also known, especially in medieval times, for their supposed medicinal value, such as being efficaceous inthe treatment of bleeding and inflammation. While such folklore is now discredited, other tales have survived, such as the symbolism of garnet for constancy and fidelity. The fact that it is recommended by the National Association of Goldsmiths (GB) as the birthstone for January has obviously a financial motive. Garnet is a group name and is a far more diverse mineral than its name suggests.There are many varieties, but only the principal ones can be examined here. These are: almandine, pyrope, spessartine, andradite, grossular and uvarovite. The name garnet is from the Latin granatus, meaning ‘like a grain’. The substantive granatum was later applied to the pomegranate, as its many seeds supposedly resemble the common colour and shape of garnet. There is an alternative etymology; theLatin granatum means both grain and cochineal-red, similar to the colour of many garnets. The name almandine is derived from the locality Alabanda, an ancient gem-cutting centre in Turkey. The name pyrope (the e is silent) is from the Greek for fiery, in allusion to its fiery-red colour. Spessartine is taken from the type locality Spessart in Germany. Andradite is named in honour of J. B. d’AndradaSilva, a Brazilian mineralogist. Grossular is derived from the Latin botanical name for the gooseberry, Ribes grossularium, because of its common colour. The honey-coloured variety of grossular is sometimes known as hessonite (less correctly essonite), from the Greek for inferior, as it was thought to possess inferior physical features. Uvarovite is named in honour of Count S. S. Uvarov(1786–1855), a former president of the Academy of St Petersburg. There are many synonyms of the above members, but some are purely gemmological and at best confusing. Others will be referred to when examining individual members of the group.

Chemistry and structure of the garnets
The garnet group consists of a multicomponent solid solution with the general formula A2+B3+(SiO4)3, where A and B refer,respectively, to 3 2 8- and 6-coordinated cationic sites. The A sites are occupied by large divalent cations, such as Ca2+, Mg2+, Fe2+ and Mn2+, while the B sites house smaller trivalent cations such as A13+, Fe3+ or Cr3+. The structure of the group consists of alternating SiO4 tetrahedra and BO6 octahedra that share corners, thus forming a continuous three-dimensional network, the configuration ofwhich is cubic (Fig. 1). While obeying cubic symmetry, the planes {100} and {111} are poor in atoms and the cube and octahedron are poorly represented. The common forms are the dodecahedron and trapezohedron and modifications of both. Usually distinctly crystallized, the garnets may be grains rounded through dynamic metamorphism or through erosion. They may also be massively

Fig. 1. Part of thegarnet structure projected on (001), showing SiO4 tetrahedra, octahedra, 8-coordinated polyhedra and triangular dodecahedra (outlined by parallel hatching).

granular. The atomic packing is dense, producing high density and hardness. Because the SiO4 tetrahedra are not linked to chains or sheets, the structure is equidimensional and strong cleavage directions are lacking. The garnets are...