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Garnet

GARNET, a name applied to a group of closely-related minerals, many of which are used as gem-stones. The name probably comes from the Lat. granaticus, a stone so named from its resemblance to the pulp of the pomegranate in colour, or to its seeds in shape; or possibly from granum, "cochineal," in allusion to the colour of the stone. The garnet was included, with other red stones, by Theophrastus, under the name of , while the common garnet seems to have been his . Pliny groups several stones, including garnet, under the term carbunculus. The modern carbuncle is a deep red garnet (almandine) cut en cabochon, or with a smooth convex surface, frequently hollowed out at the back, in consequence of the depth of colour, and sometimes enlivened with a foil (see Almandine). The Hebrew word nophek, translated in the Septuagint, seems to have been the garnet or carbuncle, whilst bareketh ( of the Septuagint), though also rendered "carbuncle," was probably either beryl or, in the opinion of Professor Flinders Petrie, rock-crystal. Garnets were used as beads in ancient Egypt. Though not extensively employed by the Greeks as a material for engraved gems, it was much used for this purpose by the Romans of the Empire. Flat polished slabs of garnet are found inlaid in mosaic work in Anglo-Saxon and Merovingian jewelry, the material used being almandine, or "precious garnet."

Garnets vary considerably in chemical composition, but the variation is limited within a certain range. All are orthosilicates, conformable to the general formula R"3R"'2(SiO4)3, where R" = Ca, Mg, Fe, Mn, and R"' = Al, Fe, Cr. Although there are many kinds of garnet they may be reduced to the following six types, which may occur intermixed isomorphously: -

1. Calcium-aluminium garnet (Grossularite), Ca3Al2Si3O12.

2. Calcium-ferric garnet (Andradite), Ca3Fe2Si3O12.

3. Calcium-chromium garnet (Uvarovite), Ca3Cr2Si3O12.

4. Magnesium-aluminium garnet (Pyrope), Mg3Al2Si3O12.

5. Ferrous-aluminium garnet (Almandine), Fe3Al2Si3O12.

6. Manganous-aluminium garnet (Spessartine), Mn3Al2Si3O12.

These are frequently called respectively: - (1) Lime-alumina garnet; (2) lime-iron garnet; (3) lime-chrome garnet; (4) magnesia-alumina garnet; (5) iron-alumina garnet; (6) manganese-alumina garnet.

The types are usually modified by isomorphous replacement of some of their elements.

All garnets crystallize in the cubic system, usually in rhombic dodecahedra or in icositetrahedra, or in a combination of the two forms (see fig.). Octahedra and cubes are rare, but the six-faced octahedron occurs in some of the combinations. Cleavage obtains parallel to the dodecahedron, but is imperfect. The hardness varies according to composition from 6.5 to 7.5, and the specific gravity in like manner has a wide range, varying from 3.4 in the calcium-aluminium garnets to 4.3 in the ferrous-aluminium species. Sir Arthur H. Church found that many garnets when fused yielded a product of lower density than the original mineral. The colour is typically red, but may be brown, yellow, green or even black, while some garnets are colourless. Being cubic the garnets are normally singly refracting, but anomalies frequently occur, leading some authorities to doubt whether the mineral is really cubic. The refractive power of garnet is high, so that in microscopic sections, viewed by transmitted light, the mineral stands out in relief.

Garnets are very widely distributed, occurring in crystalline schists, gneiss, granite, metamorphic limestone, serpentine, and occasionally in volcanic rocks. With omphacite and smaragdite, garnet forms the peculiar rock called eclogite. The garnets used for industrial purposes are usually found loose in detrital deposits, weathered from the parent rock, though in some important workings the rock is quarried. The garnets employed as gem-stones are described under their respective headings (see Almandine, Cinnamon Stone, Demantoid and Pyrope). Most of the minerals noticed in this article are of scientific rather than commercial interest.

Grossularite or "gooseberry-stone," is typically a brownish-green garnet from Siberia, known also as wiluite (a name applied also to vesuvianite, q.v.), from the river Wilui where it occurs. It is related to hessonite, or cinnamon-stone. A Mexican variety occurs in rose-pink dodecahedra. Romanzovite is a brown garnet, of grossularia-type, from Finland, taking its name from Count Romanzov. Andradite was named by J.D. Dana after B.J. d'Andrada e Silva, who described, in 1800, one of its varieties allochroite, a Norwegian garnet, so named from its variable colour. This species includes most of the common garnet occurring in granular and compact masses, sometimes forming garnet rock. To andradite may be referred melanite, a black garnet well known from the volcanic tuffs near Rome, used occasionally in the 18th century for mourning jewelry. Another black garnet, in small crystals from the Pyrenees, is called pyreneite. Under andradite may also be placed topazolite, a honey-yellow garnet, rather like topaz, from Piedmont; colophonite, a brown resin-like garnet, with which certain kinds of idocrase have been confused; aplome, a green garnet from Saxony and Siberia; and jelletite, a green Swiss garnet named after the Rev. J.H. Jellet. Here also may be placed the green Siberian mineral termed demantoid (q.v.), sometimes improperly called olivine by jewellers. Uvarovite, named after a Russian minister, Count S.S. Uvarov, is a rare green garnet from Siberia and Canada, but though of fine colour is never found in crystals large enough for gem-stones. Spessartite, or spessartine, named after Spessart, a German locality, is a fine aurora-red garnet, cut for jewelry when sufficiently clear, and rather resembling cinnamon-stone. It is found in Ceylon, and notably in the mica-mines in Amelia county, Virginia, United States. A beautiful rose-red garnet, forming a fine gem-stone, occurs in gravels in Macon county, N.C., and has been described by W.E. Hidden and Dr J.H. Pratt under the name of rhodolite. It seems related to both almandine and pyrope, and shows the absorption-spectrum of almandine. The Bohemian garnets largely used in jewelry belong to the species pyrope (q.v.).

Garnets are not only cut as gems, but are used for the bearings of pivots in watches, and are in much request for abrasive purposes. Garnet paper is largely used, especially in America, in place of sandpaper for smoothing woodwork and for scouring leather in the boot-trade. As an abrasive agent it is worked at several localities in the United States, especially in New York State, along the borders of the Adirondacks, where it occurs in limestone and in gneiss. Much of the garnet used as an abrasive is coarse almandine. Common garnet, where abundant, has sometimes been used as a fluxing agent in metallurgical operations. Garnet has been formed artificially, and is known as a furnace-product.

It may be noted that the name of white garnet has been given to the mineral leucite, which occurs, like garnet, crystallized in icositetrahedra.

(F. W. R.*)

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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