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Encyclopedia Britannica - Main :: TUM-VAN |
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TURQUOISE , a mineral
mineral
Much discussion has arisen as to the chemical composition of turquoise. It is commonly regarded as a hydrous aluminium phosphate having the composition 2Al203P205.5H2O or rather Al2HPO4(OH)4, coloured with a variable proportion of a copper phosphate, or perhaps partly with an iron phosphate. Professor S. L. Penfield, however, has been led by careful analysis of turquoise from Nevada to propose the general formula
Turquoise is usually cut as an ornamental stone in circular or elliptical form, with a low convex surface. In the East, where it is used not only for personal ornament but for the decoration of dagger-handles, horse-trappings, &c., the pieces are not unusually of irregular shape; and when worn as amulets the turquoise is often engraved with Oriental inscriptions , generally passages from the Koran, the incised characters being gilt or inlaid with gold wire. The turquoise has always been associated with curious superstitions, the most common being the notion that it changes colour with variations in the state of the owner's health or even in sympathy with his affections. It is commonly held to be a " lucky stone."In Persia, where the finest turquoise is found, the mines have been worked for at least eight centuries. The workings have been described by General Houtum Schindler, an Austrian, who was at one time in charge of the mines. The principal locality is north-west of the village
porphyry . It is found also in some other localities in Persia and in Turkestan. Jean Baptiste Tavernier (16051689) states that the best turquoise, reserved for the sole use of the shah, was obtained from the Vieille Roche, whilst inferior stones were got from the Nouvelle Roche. These terms still survive, for turquoise of fine colour is sometimes said in trade to be from the " oid rock," and that of pale tint or of unstable colours is described as from the " new rock." The latter is sometimes not true Oriental turquoise, but the material called " bone-turquoise " or odontolite, and known also as " occidental turquoise." This is merely fossil bone or ivory coloured by iron phosphate (vivianite) or perhaps stained in some cases by cupriferous solutions, and is readily distinguished from true turquoise by showing organic structure under the microscope. Bone-turquoise occurs in Europe; and it may be noted that mineral turquoise also is known from certain localities in Saxony and Silesia, but the quantity is very limited and the quality poor, so that it has no commercial importance. Chrysocolla
In 1849 turquoise was found by Major C. Macdonald in Wadi Maghara and Wadi Sidreh in the Sinaitic Peninsula; and a large series of the specimens was shown in the Great Exhibition of 1851. According to H. Bauerman, who described the locality geologically, the turquoise occurs in a red sandstone, in the form of embedded nodules and as an incrustation lining the joint-faces. The turquoise was worked for some time by Macdonald, and many years afterwards workings were resumed on a systematic scale by an English company, but without great success. Relics of extensive ancient mining operations for turquoise show that the rock was at one time worked with flint implements. The locality was examined by Professor Flinders Petrie in 1905.In ancient Mexico much use was made of turquoise as an inlay for mosaic work, with obsidian, malachite, shell and iron pyrites. Such work is illustrated by fine specimens in the ethnographical gallery of the British Museum and elsewhere. Relics of extensive workings are found in the mountains of Los Cerillos near Santa Fe in New Mexico, where mining for turquoise is now actively carried on. One of the hills in which old workings occur has been called Mt Chalchihuitl, since it is believed that the turquoise was known by the name chalchihuitl, which in some places was applied also to jade. Another of the Cerillos hills in which workings have been opened up is called Turquoise Hill. The matrix at Los Cerillos is described by D. W. Johnson as an altered angite-andesite, in which the turquoise occurs in thin veins and in small nodules in patches of kaolin
Turquoise is sometimes termed by mineralogists callaite, since it is believed to be the callais of Plinya stone which he describes as resembling lapis lazuli, but paler, and in colour more like the shallow sea. The callaina of Pliny was a pale green stone from beyond India, whilst his callaica was a kind of turbid callaina. The name callainite was suggested by Professor J. D. Dana for a bright green mineral which was found in the form of beads, with stone hatchets, in ancient graves near Mane-er-H'roek (Rock of the Fairy), near Locmariaquer in Brittany, and which A. Damour sought to identify with Pliny's callais. The mineral in question seems to be identical with variscite, a hydrous aluminium phosphate named by A. Breithaupt, and occurring as a beautiful green amorphous mineral, sometimes polished as an ornamental stone; fine examples occur in Utah. Somewhat allied to turquoise is the blue mineral called lazulite (to be distinguished from lazurite, see LAPIS LAZULI), which has the formula
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