First, the basic properties of tourmaline
(a) name of the mineral
Gemmological name is tourmaline and mineralogical name is tourmaline, which belongs to tourmaline family. (2) Chemical composition
The chemical formula of tourmaline gem is (Na, K, Ca) (Al, Fe, Li, Mg, Mn) 3 (Al, Cr, Fe, V) 6 (BO3) 3 (SI6O 18) (OH, F) 4, which is an extremely complex borosilicate characterized by containing B.
Tantalite namg3al6b3 (Si6O27) (OH) 4
Black tourmaline NaFe3Al6B3(Si6O27)(OH)4
Lithium tourmaline (aluminum tourmaline) Na (Li, Al) Al6b3 (Si6o27) (OH) 4
Sodium manganese tourmaline (TSILAISIT) namn3Al6B3 (Si6O27) (OH) 4
There are two complete isomorphic series between magnesium tourmaline and black tourmaline, and between black tourmaline and lithium tourmaline, there are incomplete isomorphic series between magnesium tourmaline and lithium tourmaline. People with bright colors and clear transparency can make gems.
(3) Crystal system and crystallization habit
Tourmaline belongs to compound cubic single cone crystals. Crystals are often columnar, and the common crystal forms are triangular column m{0 1 10}, hexagonal column d{ 1 120} and triangular single cone R {101. Longitudinal stripes are developed on the cylinder, and the cross section is spherical triangle (see Figure 3- 1-329). The aggregate is radial, beam-shaped, rod-shaped, or dense or aphanitic. It can be used as a good ornamental stone.
Fig. 3- 1-328 Crystal Morphology of Tourmaline
Figure 3- 1-329 tourmaline crystal
(4) Optical characteristics
1. color
Pure is colorless, but it is generally rose or pink, red, green, dark green, light blue, blue, dark blue, blue gray, purple, yellow, green yellow, brown, yellow brown, light brown orange, black and other colors, with rich and colorful colors (see Figure 3- 1-330). The inside and outside of the same crystal or different parts can be bichromatic or polychromatic (see Figure 3- 1-33 1).
Fig. 3- 1-330 Tourmaline faceted gems with various colors
Figure 3- 1-33 1 Bi-color tourmaline crystal
The color of tourmaline varies with its composition. Tourmaline rich in iron is dark green, dark blue, dark brown or black. Tourmaline rich in magnesium is yellow or brown; Tourmaline rich in lithium and manganese is rose red or light blue; Tourmaline rich in chromium is dark green. Tourmaline ribbon is developed, which can form a color ring around the Z axis from the inside, or form a parallel ribbon perpendicular to the Z axis. As a gem, tourmaline has three main colors.
Red series includes red, pink, magenta, rose and pink, and its color is mainly caused by Mn2++.
Blue series blue, purple blue.
Green series blue-green, yellow-green, green.
There are yellow tourmaline, purple tourmaline, black tourmaline and colorless tourmaline.
2. Gloss and transparency
Glass luster; Transparent to opaque.
3. luminosity
Uniaxial crystal, negative optical properties.
4. Refractive index and birefringence
The refractive index is1.624 ~1.644 (+0.01,-0.009). The refractive index varies with the composition, and increases when the composition is rich in iron and manganese. The refractive index of black tourmaline can be as high as 1.627 ~ 1.657. The birefringence is 0.0 18 ~ 0.040, usually 0.020.
5. Polychromaticity
The multicolor intensity of tourmaline varies from medium to strong, and the multicolor varies with the body color, showing different shades of body color.
Step 6 emit cold light
Ultraviolet fluorescent tourmaline generally has no fluorescence, and pink tourmaline has weak red to purple fluorescence under long-wave and short-wave ultraviolet irradiation.
X-ray fluorescence is only lavender fluorescent pink tourmaline, and nothing else.
7. Absorption spectrum
The green areas of red and pink tourmalines have a broad absorption band, sometimes a narrow band of 525nm, and absorption lines of 45 1nm and 458nm can be seen (see Figure 3- 1-332). Green and blue tourmalines generally have absorption in the red zone, with a strong absorption band of 498nm, and blue zone sometimes has an absorption line of 468nm (see Figure 3- 1-333).
Fig. 3- 1-332 absorption spectra of red and pink tourmalines
Fig. 3- 1-333 absorption spectra of green and blue tourmalines
(5) Mechanical properties
1. Split
No cleavage; Shell fracture.
2. Difficulties
Mohs hardness is 7 ~ 8.
3. Density
The density is 3.06 (+0.20, -0.06) g/cm3, which is closely related to the composition. When the contents of Fe and Mn in the composition increase, the density increases.
(6) Electrical characteristics
1. piezoelectric
Tourmaline gem is a mineral with asymmetric center. When tourmaline gemstone is stressed in a specific direction, it can produce charges with equal magnitude and opposite sign on both sides of vertical stress, and the amount of charges is proportional to the pressure.
2. Thermoelectricity
Tourmaline gemstones will generate opposite charges at both ends of the Z axis when the temperature changes, which is easy to adsorb dust, so it is also called "vacuum stone".
(VII) Internal and external microscopic characteristics
Tourmaline contains typical irregular linear and tubular inclusions (see Figure 3- 1-334) and flat thin-layer cavities parallel to the Z axis, which can be filled with gas and liquid (see Figure 3- 1-335) or a little iron. A large number of parallel fibers can be seen in some tourmalines, which can lead to cat's eye effect. There are often red, blue and green tourmaline cat eyes.
Figure 3- 1-334 Tubular inclusions parallel to Z axis in tourmaline
Fig. 3- 1-335 gas-liquid two-phase inclusions in tourmaline
There are many cracks parallel to the long axis of the crystal in the red tourmaline, which are often filled with gas-liquid inclusions and may have specular reflection. Red tourmaline also contains filiform liquid inclusions.
Green tourmaline rarely contains cracks parallel to the long axis of the crystal, but is characterized by many slender and irregular filiform, "teardrop-like" gas-liquid inclusions, which can be evenly distributed throughout the gem, also known as "ground crystal" (see Figure 3- 1-336).
Figure 3- 1-336 "Frosted Crystal" in Tourmaline
(viii) Special optical effects
Cat's eye effect is common, and discoloration effect is rare.
Second, the variety of tourmaline
Tourmaline is very rich in color, and the gemstone industry divides tourmaline into different varieties according to color and special optical effects.
1. by color
1) Red tourmaline. Pink to red tourmaline.
2) Green tourmaline. Yellow-green to dark green, blue-green and brown-green tourmalines.
3) Blue tourmaline. Light blue to dark blue tourmaline.
4) Polychromatic tourmaline. Due to the good development of tourmaline ribbons, red and green two-color ribbons or three-color ribbons often appear on single crystals (see Figure 3- 1-337). Ribbon can also form a ring around the Z axis from the inside (Figure 3- 1-338), and the one with green infrared is called "Watermelon Tourmaline" (See Figure 3- 1-339).
Figure 3- 1-337 Multicolor Tourmaline
Figure 3- 1-338 tourmaline ring belt
Figure 3- 1-339 "Watermelon Tourmaline"
2. According to the special optical effect.
1) tourmaline cat's eye. When tourmaline contains a large number of fibrous and tubular inclusions arranged in parallel, it can show cat's eye effect by grinding into arc gems, which is called tourmaline cat's eye. Common tourmaline cat eyes are green, and a few are blue and red (see Figure 3- 1-340).
2) Discolored tourmaline. Tourmaline with obvious discoloration is rare (see Figure 3- 1-34 1).
Figure 3- 1-340 Tourmaline cat's eyes in various colors
Figure 3- 1-34 1 color-changing tourmaline
Three. Identification of Tourmaline from Similar Gems
Generally speaking, tourmaline is not easy to mix with other gems as long as it is carefully observed and tested. Tourmaline is different from other gems because of its rich color, obvious polychromatic, high birefringence (leading to obvious ghost phenomenon on the ridge of faceted tourmaline) and typical inclusions. It is particularly worth mentioning that tourmaline is pyroelectric, and its surface can be charged when it is heated or irradiated by the sun. These charges will attract foreign charges in the air, that is, these charges can adsorb dust with foreign charges in the air. Therefore, in the exhibits of shops, the surface of tourmaline often absorbs more dust than other gems, and experienced jewelers can make a preliminary judgment on tourmaline from this phenomenon.
Tourmaline with different colors has different similar gems. Red tourmaline is easily mixed with pink topaz, red spinel and andalusite. At this time, tourmaline can be selected as long as there is a bottle of heavy liquid with a density of 3.06g/cm3. In the heavy liquid with the density of 3.06g/cm3, red tourmaline suspends or slowly sinks, while red topaz and red spinel sink faster, and andalusite also sinks, and the sinking speed is slightly slower than topaz and spinel.
Green tourmaline is easily mixed with green sapphire and green diopside. Compared with green sapphire, green tourmaline has a larger birefringence, and the two shadow boundaries in the refractometer are obviously separated. Compared with diopside, the birefringence of the two is larger, and the ghost on the back facet is clear and difficult to distinguish. But carefully observing the shadow boundary on the refractometer, only the shadow boundary with low refractive index moves up and down, and the other one does not move, which is the characteristic of uniaxial gemstones. In diopside, however, two dark boundaries can move up and down, which is the characteristic of a typical biaxial gem. In addition, diopside has a higher refractive index than tourmaline.
Emerald is very similar to green and emerald tourmaline in appearance. However, the refractive index and birefringence of emerald are obviously lower than tourmaline, and the ghost of the back facet of gem is not obvious, while tourmaline can see obvious ghost. Emerald is weak in dichroism and tourmaline is strong in dichroism. According to this, the two can be distinguished. The density of emerald is obviously lower than tourmaline. Emerald contains characteristic three-phase or two-phase inclusions, while tourmaline inclusions are irregular linear, flat thin-layer cavities, which are often filled with liquid. Accordingly, they can also be distinguished.
4. Synthetic tourmaline and its identification characteristics
Judging from the current data, synthetic tourmaline has appeared in foreign markets. Tourmaline is synthesized by hydrothermal method under the pressure of 200Pa and the temperature of 300 ~ 700℃ in an environment rich in magnesium and calcium.
Synthetic tourmaline has the general characteristics of hydrothermal synthetic gem, which is very similar to natural tourmaline, but its color is uniform and pure, giving people a perfect feeling. In addition, the density of synthetic tourmaline is low, ranging from 2.9 to 3.0g/cm3, while that of natural tourmaline is generally between 3.06 and 3.1g/cm3.
5. Optimization of tourmaline processing
When the quality of natural tourmaline is poor or not, manual methods are often used to improve its quality. The optimized tourmaline surface will always leave traces of treatment. Not only that, the color produced by treatment often disappears gradually with the passage of time. Commonly used methods are as follows:
1. heat treatment
Heat treatment refers to heating tourmaline with deep color (such as dark blue, dark green, dark yellow green, dark purple, etc.). ) to dilute their colors, thereby enhancing their transparency and improving their gem grade. There are many methods, such as vacuum heat treatment, sealing heat treatment and redox heat treatment. Protective gas plays an important role in heat treatment. Coloring is carried out in oxidizing environment, and fading is carried out in reducing environment. During operation, dark tourmaline can be heated at low temperature, then placed in a sealed container under redox conditions, and filled with protective gas (ne or he), and the protective medium is Al2O3, quartz sand, etc. Then choose different temperatures to slowly raise and lower the temperature of the gem, and you can get satisfactory colors. For example, the dark blue, dark green and dark yellow-green tourmalines in Xinjiang have been transformed into blue (light blue), green (light green) and yellow-green (light yellow-green) tourmalines after heat treatment, and the transparency has also been greatly improved.
2. Irradiation treatment method
Refers to those colorless or light-colored, multicolored tourmalines irradiated with high-energy rays. Different irradiation time and dosage can make it appear different colors (such as red, pink, purple, reddish purple, reddish green, etc.). ). There are many kinds of high-energy rays, among which gamma rays (such as Co60) are commonly used. Generally, the cumulative radiation dose above 106crem can change its color. The tourmaline produced in Xinjiang was irradiated, and the pale pink tourmaline turned red and crimson after treatment. Light green tourmaline turns pink, red and crimson; Two-color tourmaline turned into red, green and purple; Colorless tourmaline turns pink, red and crimson.
Electron bombardment can also turn colorless or pink tourmaline into better red, but it will produce a lot of cracks at the same time. The color of green tourmaline will not change under electron bombardment.
Step 3 coat tourmaline
Colorless or nearly colorless tourmaline can form various colors and bright colors after coating (see Figure 3- 1-342). The coated tourmaline has a wide range of refractive index and no characteristic absorption spectrum. The characteristic inclusions are colorless transparent crystals, needle-like inclusions, fingerprint-like inclusions and cracks. The luster of the coated tourmaline is greatly enhanced, reaching the sub-metallic luster. Most of the coated tourmalines have only one refractive index on the refractometer, and the refractive index ranges vary greatly, even exceeding 1.70.
Fig. 3- 1-342 coated tourmaline
Sixth, the quality evaluation of tourmaline
The evaluation of tourmaline can be carried out from several aspects such as weight, color, clarity and cutting. Among them, tourmaline with good transparency and large volume is the top grade, and color is the most important factor to judge. In addition, the special optical effect of tourmaline can also improve its value.
1. color
Tourmaline is required to be bright and pure, and evenly distributed among gems. There are pure and uniform requirements for ribbons and color rings, and the boundaries between ribbons and color rings are clear. The colors of high-quality tourmaline are rose red and purple red, both of which are very expensive, while pink is of low value. Emerald is the best green tourmaline, followed by yellow and green. Pure blue and dark blue tourmalines are rare, so they also have high value. The price of a good red tourmaline is 2/3 higher than that of a green tourmaline of the same size. Tourmaline of various colors is bright in color, and pure tourmaline is of high value.
Step 2 be clear
It is required that the internal contents should be as little as possible, and the crystal clear tourmaline has a higher price. Tourmaline contains many cracks and gas-liquid inclusions, and is usually used as jade carving material.
Cut work
Cutting should be regular, proportional and well polished. Tourmaline can be cut into various shapes: emerald, oval, standard round diamond and mixed. Among them, emerald can best reflect the beautiful color of tourmaline, is the best cutting, and the relative price is also higher. The section of tourmaline perpendicular to the Z-axis direction is dark, and the color parallel to the Z-axis direction is light, which is prone to dichroism. Therefore, it should be oriented according to the actual situation when processing.
Seven. Brief introduction to the origin and producing area of tourmaline
Tourmaline is rich in volatile components B and H2O, which mainly exist in granite pegmatite and gas-generated hydrothermal deposits. Generally, black tourmaline forms at a higher temperature, while green and pink generally form at a lower temperature. In addition, tourmaline also occurs in metamorphic deposits. Tourmaline is a mineral component of granite pegmatite, so tourmaline mineralization should also be in the most widely distributed area of granite pegmatite.
Tourmaline is abundant in many countries in the world, such as Brazil, Sri Lanka, Myanmar, the former Soviet Union, Italy, Kenya and the United States. Among them, colored tourmalines produced in Minas Crass, Brazil account for 50% ~ 70% of the world's total output, while rare purple and blue tourmalines have also been found in Paraiba, Brazil. The high-quality blue transparent tourmaline produced in Brazil is known as "Brazilian sapphire". Brazil is famous for producing red and green tourmalines and tourmaline cat's eyes. The United States is famous for producing high-quality pink tourmaline; The high-quality rubies produced in the Urals region of Russia are known as "Siberian rubies"; Italy is famous for producing colorless tourmaline.
Tourmaline in China is mainly produced in Altai, Ailaoshan, Yunnan, Inner Mongolia and other places in Xinjiang, with rich varieties of colors and excellent quality.
Xinjiang is the most important producing area of tourmaline in China, mostly occurring in granite pegmatite deposits in Altay and Fuyun, followed by Kunlun Mountain and the hinterland of South Tianshan Mountain. Xinjiang tourmaline is bright in color, and red, green, blue and multicolor tourmalines are all produced, with large crystals and good quality. Xinjiang also produces "Watermelon Tourmaline", which is distributed in a ring shape, with dark green outer ring and nuclear red, or black outer ring and pink inner ring.
Inner Mongolia is one of the important producing areas of tourmaline in China, which is distributed in Urad Zhongqi, Wrestling Tiger and other places. Pure is colorless and transparent. There are usually green, green, blue-green, light green, yellow-green, grass green, sky blue, dark blue, black, pink, rose red, light yellow, orange, brownish yellow and multicolor tourmalines. The transparency of a crystal is related to its size. Generally speaking, the smaller the crystal, the higher the transparency. The research and processing show that the tourmaline produced in this area has excellent texture, especially the green tourmaline.
Yunnan tourmaline is mostly produced in the form of single crystal, and some tourmalines are rod-shaped, radial and massive aggregates. Red tourmaline is pink, rose and peach, transparent to translucent, produced in Fugong and Yuanyang. Green tourmaline has many colors, such as green, green, dark green, yellow-green, basket green, grass green, light green, apple green, etc. It is transparent to translucent, and its crystals are mostly self-shaped and semi-self-shaped long columns, with relatively complete appearance and good texture. It is produced in Gongshan, Fugong and Baoshan. Blue tourmaline has blue, green, dark blue and other colors, transparent to translucent, produced in Gongshan, Baoshan and other places. Polychromatic tourmaline crystals have many cracks and poor transparency, and are produced in Fugong area.