Учебное пособие Иркутск 2006 Рецензент

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Malachite

Malachite has long been prized as a semi-precious stone. Beautiful ornaments and table tops are made from banded forms of malachite. It was popular among the ancient Greeks and Romans, and it later became fashionable in the courts of the Russian Czars. Malachite is also a copper ore. One major deposit is at Shaba (formerly Katanga) province, Congo. Crushed malachite has been used as a pigment. It is known as green copper carbonate, in contrast to azurite, known as blue copper carbonate. Malachite is named after the Greek for ‘mallow’, as it is similar in colour to a mallow leaf.

Apatite

Apatite is a widely distributed mineral occurring as fluorapatite, chloroapatite and hydroxylapatite. Its name comes from the Greek apatao, meaning ‘deceit’, because it may be mistaken for other minerals. It is distinguished from beryl and quartz by its lustre and lower hardness. Clear, well-coloured apatites are sometimes used as gemstones, such as ‘asparagus stones’, but they are rather soft. Apatite is the main constituent of bones and teeth, and in fluorine-bearing forms, is resistant to decay, which is why drinking water is fluoridized. Apatite rocks are used in the manufacture of fertilizers and in the chemical industry.

Turquoise

Turquoise is a semi-precious blue gemstone. It is often mottled, with brown or black vein lets. It can be distinguished from the mineral chrysocolla by its greater hardness. Care must be taken when handling it, because specimens can alter to an unattractive green if heated too much – due to friction when polishing – or on exposure to sunlight. It is also sensitive to soap, water and grease. Turquoise was used in Ancient Egyptian jewellery. American Indians also use it, typically in silver jewellery. Its name comes from the French Pierre turquoise, meaning “Turkish stone”. It was so called because it was brought into Europe via Turkey.

Garnet

Garnet is the name for a family of minerals. Chemically, they are divided into two groups: pyralspite garnets and ugrandite garnets. Pyralspite garnets are pyrope (magnesium and aluminium), almandine (iron and aluminium) and spessartine (manganese and aluminum). Garnets are used as gemstones in jewellery and are often given incorrect names, such as ‘Cape ruby’ or ‘Arizona spinel’. The beautiful deep red pyrope is one of the most popular. Almandine, one of the most widespread varieties, often occurs as large, perfect crystals. Spessartine, one of the most precious gemstones, is popular for its yellowish-orange colour.

Ugrandite garnets

The ugrandite garnets are uvarovite (calcium and chromium), grossular (calcium and aluminium) and andradite (calcium and iron). Most crystals are mixed, so that there is a partial replacement of some elements by others. The beautiful, deep emerald-green uvarovite is one of the rarest gemstones. Grossular gets its name from the botanical name for ‘gooseberry’, which it resembles in colour and shape. It is unusual among garnets in that it luminesces strongly in ultraviolet light. The green variety of andradite used as a gemstone is called demantoid, after the Old French word diamant, meaning diamond. Demantoid has a high dispersion comparable with diamond. Garnet is derived from the Latin granatus meaning ‘like a grain’.

Zircon

Zircon is an ore of the metal zirconium, which is used in the construction of nuclear reactors. It has long been mined as a gemstone, though it did not become fashionable until the 1920s. The reddish-brown variety is called hyacinth or jacinth. Its main shortcoming is its brittle nature, and so the edges of cut stones are easily damaged. Zircon has variable properties. Scientists now know that these result from the presence of radioactive elements – thorium and uranium – substituting for zirconium. This radiation alters the arrangement of atoms in the crystal, giving a weakened ‘metamict’ form. The name zircon is thought to be derived from the Persian zar meaning ‘gold’, and gun, meaning ‘colour’.

Tremolite - actinolite

Tremolite and actinolite are amphiboles that form a chemical series. Tremolite contains little or no iron and is almost colourless. The iron content in actinolite produces a green colour, the most attractive variety being nephrite, or greenstone, the more common form of jade. Nephrite is tough and takes a good polish. Beautiful nephrites in New Zealand are called Maori stone, because of their widespread use in ancient Maori art. Tremolite and actinolite often occur in fibrous masses. They were the first such materials to be called asbestos. Fibrous tremolite is used for insulating and fire-proofing. They both occur in relatively low-grade metamorphic rocks. Actinolite is characteristic of the green schist faces. Tremolite was discovered in Tremola, a valley in the Alps.

Serpentine

Serpentine is the name for a group of common secondary minerals. The group includes Chrysolite, a type of asbestos that forms fibrous yellow-white or green aggregates with a silky lustre, a compact, whitish variety made up of tiny plates, and antigorite, which is lamellar and occurs as tough, compact, dark green masses. Serpentine minerals are formed by low-temperature hydration of ultramafic rocks, including dunite and pyroxenite. Coloured varieties are used for ornaments and internal decorations, because their low hardness makes them easy to work. The name refers to the snake-like appearance of the massive form.

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Gem localities

Emerald

Major expansion has occurred in the sources of this very important gemstone. Colombia, long recognized as the principal supplier of high-quality material, must now compete with new localities in Brazil, Zambia, Zimbabwe, Madagascar, Pakistan, Afghanistan and Ural (Russia). However, important new mining developments helped Colombia retain a 30% share of the world output. For example, under the direction of lessee companies Tecminas and Coesminas, the Muzo mine is now fully mechanized and has introduced a sophisticated tunneling operation. Prospecting in the region continues to provide new date on the geology and origin of these Colombian deposits, and there is every indication that they will remain an important source of high-quality emerald.

Even so, newer sources now account for most of the emeralds on the world market. Deposits of major significance came into full operation in Brazil. A major deposit found in 1981 at Santa Terezinha in the state of Goias, was highly productive. The Santa Terezinha crystals are generally small (less than 1 cm) and range from pale to very dark green, with a distinct bluish green tone. The Nova Eva deposit, which may be an extension of the Itabira deposit discovered in the late 1970s, is noted for relatively large emeralds, with many cut stones exceeding 5 ct.

Important emerald deposits were found on other continents as well. In Africa, the Kitwe district in Zambia has become a major source of good, strongly bluish green, material. The Mberengwa area of Zimbabwe continues to produce fine material, particularly from the older Sandawana and more recently discovered Machingwe and Adriadne mines. The emerald deposits of Madagascar occur in the southeast part of the island, at Ankadilalana, in biotite schists.

Several deposits in the Swat Valley of Pakistan produced material of good to excellent quality, although some stones are extremely dark. Emeralds are also mined in the Panishir Valley northeast of Kabul in Afghanistan. Meanwhile, sparse information has appeared on emeralds recently recovered from Russia. It is interesting to note that most of these recent emerald discoveries occur in metamorphic rocks, in environments very different from the classic hydrothermal vein-type deposits found in Colombia.

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