1911 Encyclopædia Britannica/Pyrope

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PYROPE (pronounced pĭrōp), a deep red variety of garnet, named from the Gr. πυρωπός (fiery) in allusion to its colour. It is used, like almandine (q.v.), as a gem-stone, but may be distinguished by the absence of any tinge of violet in its colour and by its lower specific gravity (3.7 or 3.8, while that of almandine is 4.1 to 4.3). The typical colour of pyrope is blood-red, though sometimes a trace of orange gives rise to a hyacinthine hue: occasionally the mineral becomes nearly black, as seen in the pyrope of Arendal in Norway. Crystals are rare, but cubic forms have been observed. Pyrope may be regarded as a magnesium-aluminium garnet (see Garnet), but it usually contains more or less calcium, iron, manganese and chromium; and the rich colour of the mineral seems due to the presence of some of the last three metals, though their exact condition in the mineral has not been determined.

Pyrope generally occurs in grains embedded in peridotites (olivine rocks) or in serpentine resulting from their alteration, or it is found as loose grains in detritus due to the disintegration of the matrix. The grains may be surrounded by a chloritic rind, or by a crust of a fibrous mineral called by A. Schrauf kelyphite (from the Gr. κέλυφος, a nut-shell), which seems in some cases to be an amphibole. In the serpentine of Zöblitz and of Greifendorf near Leipzig, in Saxony, pyrope is characteristically developed; and the Saxon garnets, found loose in gravels, were referred to by G. Agricola as far back as 1546. Several localities in Bohemia are famous for yielding pyrope, and from its characteristic occurrence here it is often known, even when found elsewhere, as Bohemian garnet. The garnet-bearing district is a tract of about 70 square kilometres in the north of Bohemia, the chief locality being Meronitz near Bilin. It is notable that the pyrope is found at Meronitz in a clayey calcareous tufa or conglomerate, with opal and serpentine, products of the decomposition of a peridotite. It occurs also in sands and gravels near Chrastian, Lobositz, Triblitz, Podseditz, Chodolitz, and at several other localities in the Mittel Gebirge, between Teplitz and Leitmeritz. It is believed that the original pyrope-bearing rocks resulted from the eruptive activity which gave rise to Linhorka Hill, near Starrey. The garnets in the detritus are accompanied by zircon, spinel, corundum, cyanite, tourmaline, olivine, &c. Though generally very small, they are abundant, and are used not only as ornamental stones, but as a counterpoise in delicate weighing and as an abrasive agent. To obtain the stones the detritus is washed, and the garnets picked out by hand and then sized through sieves. The pyrope is generally rose-cut or step-cut, and often mounted with a foil. Beads are faceted all over. Some pyrope is cut en cabochon, forming, like almandine, carbuncle, and if very dark the stone is hollowed at the back so as to form a “ garnet shell." The industry of cutting Bohemian garnets is centred in Turnau on the Iser, near Reichenberg; but there are also works at other localities. Large stones are very rare, but a Bohemian pyrope as large as a hen's egg is preserved in the Imperial treasury at Vienna; and another the size of a pigeon's egg in the Grüne Gewölbe of Dresden.

Pyrope occurs in many localities in the western part of the United States, especially in Colorado, Arizona and New Mexico, where it is often called " ruby." it is found loose in sand accompanied by olivine, and has resulted from the alteration of a peridotite. The Navajo Indians of New Mexico collect the garnet from the sands of the ant-hills and scorpion-holes. Very fine pyrope occurs in the diamond-fields of South Africa, having been derived from olivine-bearing rocks. It occurs in the blue-ground and in the detritus of the river-diggings. The Cape garnets have usually a rich colour, but some stones incline to an orange hue. The finest pyrope is often cut as a brilliant, and passes under the misleading name of “ Cape ruby.” A pyrope-bearing rock, rather like that of South Africa, occurs in Elliott county, Kentucky, U.S.A.; it is notable, too, that pyrope is found near Elie in Fife, in Scotland, where it occurs in volcanic agglomerates and in basaltic dikes. Sir A. Geikie has pointed out the suggestive resemblance of the occurrence there to that in South Africa.

See “ Bohemian Garnets," by G. F. Kunz, Trans. Amer. Inst. Mining Eng. (1893), xxi. 241; and “ Die böhmischen Granatlagerstatten," by Dr Hans Oehmichen, Zeit. f. prakt. Geol. (1900), viii. 1. Both papers contain bibliographical lists. (F. W. R.*)