Tourmaline - original gems of all colors and shades

Tourmaline
The chemical formula (generalized): Na (Mg, Fe2 +, Mn, Li, Al, Fe3 +) 3 {(BO3) 3, [Si6O, 8] (OH, F4)}.
The chemical composition is unstable due to the wide manifestation of isomorphous substitutions.
Tourmaline contains 35-38% of silica, 8-11% of boron oxide; 0.6-5.4% hydroxyl substituted with fluorine.
Impurities are potassium, chromium, titanium, vanadium, beryllium, rubidium, cesium, zinc.
Singonia: trigonal, the habit of crystals is prismatic, columnar, acicular.
There are one- and two-headed crystals, the sizes of which are from several to tens of centimeters in length and up to 6-10 cm in diameter. On the faces of the prisms, vertical hatching and the shape of spherical triangles in the cross section are often observed. The usual sizes of crystals of a noble tourmaline are 1-5 x 0.5-2 cm.
Color is very diverse. Insignificant fluctuations in the content of chromophore elements cause a change in the color of crystals rarely colored uniformly. They are usually polychrome, zonal (two-, three- and even five-color). The color of red and pink tourmaline is caused by Mn2 + ions, green - Fe2 +, blue - Fe2 + and Fe3 +.
Among tourmalines are distinguished: sherlock - black, glandular; Dravit - brown, honey, yellow, magnesium; Elbaites (achroit - colorless, rubellite - pink, red, purple, verdelit - green, indigolite - blue) - alkaline, contain sodium, lithium, fluoride; Tsaileisite - yellow, manganese; Chrome-turbine - green, chrome. Tourmalines can be of different shades of green, blue and yellow.
Hardness: 7-7,5.
Density: 2.92-3.26 g / cm3, depends on the chemical composition: rubellite 3,012-3,049 g / cm3, verdelite 3,047-3,075 g / cm3, sherley 3,1-3,2 g / cm3. For minerals with insignificant density, respectively, a low refractive index.
Cleavage: absent.
Fracture: uneven.
Glitter: glass.
Transparency: from transparent to opaque.
Light refraction: n0 = 1.639- 1.692, ne = 1.620-1.657.
Birefringence: 0.019-0.035; For pale green tourmaline 0.019, for dark 0.035.
Dispersion: 0.017.
Pleochroism: for colored tourmalines, pronounced dichroism is characteristic.
Absorption: red - 456.0; 451.0 nm; Green - 497.0; 461.0 nm (the brightest lines).
Luminescence: absent.

The name tourmaline comes from the Sinhalese turmali - a stone that attracts ash. This is explained by the ability of tourmaline to electrify when heated. Under this name, he was brought in 1703 with Sri Lanka to Amsterdam.

Tourmaline is a polygenic mineral, but its colored varieties are associated only with granite pegmatites with lithium mineralization. According to AE Fersman, tourmalines crystallize into geo-phases F and G from supercritical gaseous fluids in the following sequence: black - blue - green - pink - colorless.

Sherles are formed in the first potassium stage of the pegmatite process. Blue and green tourmalines are associated with rare metal pegmatites and appear in the sodium stage, which refers to the stage of metasomatism. Pink, red, colorless and polychrome tourmalines formed in the late stage of pegmatite development, which was accompanied by a change in the ionic form of iron and manganese. Along with colored tourmalines, lepidolite, sparrowite, kunzite and others are formed at this stage.

The deposits in which the tourmalines are marked are numerous. Particularly widespread is black tourmaline, large opaque crystals of which often reach considerable sizes. However, transparent jewelry color varieties are very rare and their dimensions are insignificant. A unique crystal of pink tourmaline was found in 1848 in the Borshchovochny ridge (Transbaikalia); He is in the museum of the Leningrad Mining Institute. Polychrome tourmalines were mined in the pegmatite veins of the Urals (Shaytanka, Murzinka, Yuzhakova, Sarapulka and Lipovka).

Particularly appreciated are carmine-red tourmaline Sarapulki, pink, blue and green Lipovka, crimson northern slope Borshchovochnogo ridge in Transbaikalia. In the Middle Urals in the Nizhne-Isetsky region, relatively large crystals of chromatic tourmaline from dark green to black were found, ranging in size from several centimeters in length to 1 cm in diameter. In the area of ​​Murzinka, there were fine crystals of cherry-pink tourmaline (in the 18th century the French called them siberite), with which no tourmaline of the world can compete in transparency and beauty of tone. From the famous pit of Mora near Shaytanki in 1810-1820. They extracted rare brown-red tourmalines, which were then sent to museums in Europe. In 1910, light pink, and then violet, polychrome and other colors of tourmaline were found in Lipovka. In recent years, polychrome and green tourmalines have been found in the Pamirs (pictured).

Tourmaline unlike minerals with perfect cleavage (topaz, etc.) easily accepts polishing without cracking and clouding with usual thin pastes. Of the defect-free differences, inserts are made, cut with a stepped, diamond, rose, chess and combined shapes. Fractured translucent tourmalines are processed in the form of cabochons.

Most valued in the world market are dyed-colored pink, purple and crimson rubellites (like rubies), as well as bright blue indigolites (like blue sapphire). Polychrome tourmalines are valued below, although they are very interesting. A good collection material are druses, coats of crystals of colored tourmaline with other minerals. In addition to the jewelry industry, tourmaline is used for technical purposes due to its piezoelectric properties.

Currently, an insignificant number of tourmalines are mined in the complex development of pegmatite veins in the Urals. Research works are conducted on the Borshchovoe Ridge in Transbaikalia and in other regions.

Popular gems, which are represented on the territory of the CIS (former USSR)