1902 Encyclopedia > Garnet


GARNET (German, Granat; French, Grenat), a mineral the name of which is derived from the Latin granatum, the pomegranate, or, as Lydgate calls it, "garnet appille"(see Halliwell, Dict., i. p. 392), on account of the resemblance of its granular varieties to the seeds of that fruit.

Several sorts of garnets, with other stones, seem to have been included under the terms anthrax (Gk.) and carbunculus, employed by Theophrastus and Pliny.

Garnet occurs in crystals, mostly dodecahedral or trapezohedral, very rarely octahedral, of the isometric, regular, or cubical system, also in pebbles and grains (as in alluvial deposits), and massive, with a granular or coarse lamellar structure. It varies in diaphaneity from transparent to nearly opaque; is red, red-brown, or black in color, less frequently white, yellow, pink, or green has a vitreous to resinous luster, a white streak, dodecahedral cleavage, hardness of 6-5 to 7.5, specific gravity of 3.15 to 4.30, and an uneven sub-conchoidal fracture; and is brittle and sometimes friable, or, in the compact crypto-crystalline varieties, tough.

Before the blowpipe it gives a brown, green, or black (often magnetic) glass, which hydrochloric acid decomposes, with the separation of gelatinous silica. Previous to melting, the mineral is but little affected by the acid. The least fusible forms are the lime-iron garnets. It has been shown by Professor Church that, although unaffected by exposure to a full red heat for a quarter of an hour, iron garnet may by fusion have its specific gravity lowered from 4.059 to 2.204. By almost complete fusion a specimen of almandine garnet examined by him had its specific gravity increased from 4.103 to 4.208. Long continued ignition effected only a slight increase in the density of various specimen of lime garnet (see Journ. Chem. Soc., vol. xvii. P. 388).

Garnets, which through the isomorphism of their constituents are extremely variable in chemical composition, are silicates of the general formula R"3Riv2. Si3O12, or 3R",O,Riv2O3.3SiO2, in which R" = calcium, magnesium, iron, and manganese, and Riv = aluminium, iron, and chromium. Occasionally rarer metals are present; yttrium, for instance, has been found in garnets from Brevig, Norway.

Three principal groups have been recognized, called, according to their chief sesquioxide basic components, alumina, iron, and chrome garnets, which have the general formula R"3Al2.Si3O12, R"3Fe2.Si3O12; and R"3Cr2.Si3O12, respectively.

These are further classed, by the predominance of one or other of their contained protoxides, into numerous subordinate groups, as lime-alumina garnet, Ca3Al2.Si3O12, e.g., grossularite, topazolite, and essonite; magnesia-alumina garnet, comprising pyrope, the typical specimens of which contain a small percentage of chromium; iron-alumina garnet e.g., almandite, common garnet in part, and allochroite; manganese-alumina garnet, as spessartite and romanzovite; lime-iron garnet, which includes andradite, melanite, or black garnet, which may be titaneferous, as at Frascati, and pyreneite, aplome, and common garnet in part; lime-magnesia-iron garnet (CaMg)3 Fe2.Si3O12, or bredbergite; and lime-chrome garnet, or ouvarovite. Colophonite, a yellow-brown to honey-yellow or almost pitch-black mineral, with a resinous luster, commonly considered to be a lime-iron garnet, according to Wilchmann and Des-Cloiseaux must be regarded as for the most part granular vesuvian.

Garnet is a wide-spread mineral, and is found in micaceous, talcose, chloritic, and hornblendic schists, and in syenitic gneiss, syenite, granite, dolomite, and crystalline limestone; sometimes as pyrope, in serpentine; also in felspar-porphyry, and in volcanic rocks. In Cornwall it is met with chiefly in greenstone, or in close proximity thereto. It is an essential ingredient of the rock eklogite. Grossularite, a greenish to gre-green-garnet, is found at Rezbanya in Hungary, and the Wilui river, Siberia; topazolite and essonite at Mussa, Piedmont, the latter also in Ceylon, Piedmont, and Elba; pyrope in Bohemia, and at Zoblitz in Saxony; and almandite in Ceylon, Pegu, Brazil, and Greenland. Spessartite is obtained at Haddam, Ct., and elsewhere; melanite in Vesuvian and other lavas; aplome at Breitenbrunn and Schwarzenberg in Saxony; the fine green garnet ouvavorite chiefly at Saranovskaja, 14 versts from Bissersk in the Urals, and at New Idria in California; and white garnet in the Urals. Numerous other localities for garnet might be mentioned.

Precious garnet, almandite or almandine (so termed, it is said, from being cut at Alabanda in Caria, whence the appellation alabandicus employed by Pliny), essonite or cinnamon-stone, grossularite, grossularia, or gooseberry stone, and pyrope or Bohemian garnet are the varieties of the mineral employed as gems. They are shaped by means of garnet powder or enemy on a copper, wheel, and polished on lead with Tripoli.

Carbuncles are almadine garnets cut en cabochon; when of large size, and free from black spots, they may be worth as much as £20 apiece.

The deep red or precious garnet often has a density close to that of the ruby, for which stone it has been sold. The Syriam or Pegu garnets, possibly the amethytizontas of Pliny (Nat. Hist., xxxvii. 25), commonly designated amethystine or oriental garnets, vary in color from a deep red to a violet-purple, and may occur 3 inches in diameter. They are usually cut with four large and four small facets, and may fetch very high prices, a single specimen, of a fiery-red hue, measuring 1 inch by 6/10 inch, having been sold for £40, and another, of octagonal form, from £140.

Pyrope is a dark hyacinth-red to blood-red gem, much esteemed in Austria Transylvania, and Turkey. Viewed by transmitted light is appears of a yellowish-red tint, more especially at the edges., essonite, yellow to hyacinth-red in color, is a softer and more fusible garnet than the other kinds used in jewellery. It is commonly called hyacinth , and has frequently been mistaken, as also, for true hyacinth or jacinth, which is a zirconium silicate, and may be distinguished by its density of 4.05-4.75, that of essonite being about 3.60-3.66.

The garnet was much used as a jewel in ancient times. Antique intaglios on garnet are recognized by their usually fragmentary condition, due to their brittleness, and by a softness of color imparted to them by time, which defies imitation by even the ablest artists (castellani). The bust of Hadrian in the Odescalchi museum, the Venus Genetrix in the cabinet of abbe Pullini at Turin, and the representative of Sirius on the celebrated Malborough stone, are among the finer examples of engraving in garnet.

Garnet, where abundant, has been used in the smelting of iron ores. For polishing purposes it is sometimes substituted for enemy. The large dull-colored "carbunculus of India," according to Pliny (l.c), used to be hollowed out into vessels that would hold as much as a pint. Garnet has been obtained as a furnace-product, and otherwise artificially. What is known as "white garnet" is the mineral leucite.

See Bischof, Chemical Geology, vol. ii chap, xxxiii., and vol iii. p. 348; C. E. Kluge, Hdb. d. Edelsteinkunde, Leipsic, 1860; Emanuel, Diamonds and Precious Stones, 3d ed., 1867; A. Schrauf, Hdb. d. Edelsteinkunde, Vienna, 1869; A. Castellani, Gems, 1871 J. D. Dana, A System of Mineralogy, 5th ed., pp. 265-72, New York, 1874; C. F. Naumann. Elemente der Mineralogie, 10th ed., by Dr F. Zirkel, pp. 532-5, Leipsic, 1877. On so-called garnets from the river Bobrowska, Urals, see Church, Mineralog. Mag. Ii., 1879. p. 191. (F. H. B.)

The above article was written by F. H. Butler, Associate of the Royal School of Mines.

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