Turquoise is one of the four famous jade in China, and it is a traditional jade material deeply loved by people. According to archaeological excavations, it was known to the ancients as early as the Neolithic Age in China. Among the cultural relics unearthed from Yangshao culture (6500 ~ 4000 years ago) in Dahe village, Zhengzhou, Henan Province, there are turquoise fish-shaped ornaments. The English word Turkish comes from French pierre tourques, which means Turkish stone. This is because the turquoise from Persia and even Egypt entered Europe through Turkey in history. At present, most people in the jewelry industry in Taiwan Province Province still call turquoise Turkish stone or Turkish jade.

Second, the mineral composition and chemical composition

The main mineral of turquoise jade is a kind of copper-aluminum phosphate containing water, and its chemical composition is cual6 [PO4] 4 (OH) 8 4h2o, in which aluminum can be partially replaced by iron. The water content of turquoise is generally 15% ~ 20%, and it exists in three states: structural water, crystal water and adsorbed water. Turquoise belongs to triclinic system. There are many cryptocrystalline texture and oolitic, pea-shaped, kidney-shaped, nodular, tuberculous, veined and dense massive structures. Under the electron microscope, we can see that turquoise crystals are in the form of large flakes, small plates, short columns, needles and hairs, and they appear in the form of closely superimposed, sporadic, bundled, radial and frame-shaped aggregates. The minerals associated with turquoise mainly include halloysite, kaolinite, Yingshi, mica, limonite, hectorite and some carbonaceous inclusions.

Third, physical properties.

Mohs hardness is 5 ~ 6, but the hardness varies greatly with density and porosity, and the loose and porous hardness can be reduced to about 2. The density is 2.40-2.90g/cm3. The refractive index of single crystal turquoise is α 1.6 1, β 1.62, γ 1.65, the birefringence is 0.040, and the biaxial crystal is positive. The refractive index of turquoise is between 1.6 1 ~ 1.65, but usually there is only a vague shadow boundary at the refractometer 1.62. The colors are mainly blue, green and their transition colors, often with white spots or brown-black iron wires; There are also variegated turquoise such as yellow, khaki and grayish white. It is basically opaque, waxy, and occasionally called "johnite" (scaly turquoise with glass luster), and some light grayish white can have earthy luster. Turquoise is a kind of self-color mineral, and its sky blue primary color is caused by divalent copper ions, while the green tone is closely related to trivalent iron. With the increase of iron ions, turquoise changes from sky blue to green or khaki. Moisture increases the brightness of turquoise, and when the moisture decreases due to weathering or air drying, the color will also change from blue to grayish green or even grayish white. The absorption spectrum is characterized by fuzzy bands at 460 nm and 420 nm and absorption lines at 432 nm. There is weak green-yellow to bright blue fluorescence under long-wave ultraviolet light. Turquoise will fade and burst when heated and dehydrated. When it comes to alcohol, soap bubbles or some organic matter, it will also fade and change color.

Fourthly, the texture and variety of turquoise.

Turquoise is usually divided into the following varieties according to texture, mainly structural structure:

1. Crystal turquoise

Very rare. The only case of turquoise with obvious crystals was discovered in Virginia, USA in 19 12, and has not been reported since.

2. Large turquoise

A massive and nodular turquoise aggregate with a shell. Some blocky aggregates are pure and dense, and the hardness is between 5 and 6. One of them is called porcelain pine, which is bright and hard, and its fracture is like porcelain and shell-shaped. Some massive aggregates have loose texture and low hardness, which may be related to different weathering degrees. A kind of light color, loose texture, hardness less than 3, and sometimes can be crushed by hand is called foam, which can only be used after treatment.

3. Reticular turquoise

Turquoise with irregular fine lines or meshes can be iron, carbon, kaolinite, etc. Also known as "wire turquoise".

4. mottled turquoise

Turquoise with variegated appearance and cement, such as turquoise distributed in spots in kaolin gangue.

5. Optimization of turquoise treatment

Include waxing, dye and casting.

1. Waxing

Soak turquoise products in paraffin for a period of time to improve color and luster. At present, waxing has been accepted by the jewelry industry and has become an indispensable treatment. Waxing is often the last process of turquoise dyeing and pouring to improve the surface polishing effect.

dye

Soak turquoise products in inorganic or organic dyes for dyeing. Dyed turquoise is often dark blue green or dark green, which is very uniform and obviously different from natural color. However, due to the limited penetration depth of dyeing, light primary colors are often exposed at the peeling parts or pits on the sample surface.

pour into

Also known as stable treatment. Oil, resin and plastic are all used to pour turquoise, and sometimes inorganic substances are used to pour turquoise, such as soaking it in water glass (sodium silicate) and then treating it with concentrated hydrochloric acid to form silica colloid in pores. Colorless perfusion is widely accepted in trade and does not need to be made public. Dyeing is rarely done alone. If dyeing is needed, it is often done at the same time as pouring, that is, the dye is mixed into the pouring material. This is colored perfusion and must be made public. Pouring can improve the stability, transparency and color of turquoise. The density and hardness of turquoise treated by perfusion are low. If it is filled with plastic, you can smell the pungent smell of plastic melting by probing with a hot needle.

Six, imitation turquoise

1. blue iron stained bone fossils (tooth collophanite)

The blue iron-stained skeleton fossils consist of the bones and teeth of prehistoric mammoths. Although rare, it may be confused with turquoise because of its color. Due to the penetration of mineral salts, it is naturally colored and sky blue. It retains the remnants of the pipes that seal nerves and blood vessels as evidence of organic origin. Many primitive organic substances have been replaced by apatite. The hardness is about Mohs hardness 5, the density is about 3.0g/cm3, and the refractive index is 1.57 ~ 1.63.

2. "Synthetic" turquoise

Production started in gilson from 1972. It is believed that the purified compound is used as a raw material. One kind of raw materials contains materials used to imitate the common brown texture in natural turquoise, and the other kind is used to imitate "pure" turquoise. A remarkable feature of this material is that a large number of regular angular blue particles can be seen embedded in the white tone matrix under 50 times magnification. Although it is called "synthetic", it is not a true copy of its natural counterpart, so it is actually an imitation.

Reconstructed turquoise

Regenerated turquoise is formed by combining turquoise powder with resin or silica compound. This makes its density value lower than the normal range of natural turquoise density value. There are no features that can be easily distinguished from natural turquoise. The presence of the adhesive material can be detected by an infrared spectrophotometer.

4. Dye xonotlite

Xonotlite is a borosilicate of calcium with Mohs hardness of 3.5 and density of 2.58g/cm3. The undyed xonotlite is white, sometimes with light gray spots. The blue xonotlite aggregate shows obvious particle structure under the microscope, and the color is concentrated between particles and cracks. Odorless patches can be seen, especially in the drilled holes of beads. Compared with natural turquoise, the luster is closer to that of glass. Its absorption spectrum has a broad band in the green region. Some have reddish-brown plaque fluorescence under long-wave ultraviolet light. Mohs hardness is 3.5, refractive index 1.59 and density is 2.50 ~ 2.57 g/cm3, which is obviously different from turquoise. Xonotlite used for dyeing mainly comes from California, USA.

Dyed magnesite

Magnesite is magnesium carbonate. Its Mohs hardness is 4 and its density is 3.00 ~ 3. 12g/cm3. Green-dyed magnesite is often filled with black asphalt and other substances to imitate turquoise wire. Magnification inspection shows that the green color is concentrated in magnesite particles. It should be pointed out that most of the "dyed xonotlite" sold now are actually dyed magnesite, because magnesite is obviously heavier than xonotlite, so it is not difficult to distinguish.

Seven, turquoise quality evaluation

It should be comprehensively evaluated from the aspects of color, hardness, blockiness and pattern.

1. color

Turquoise for jewelry is preferably sky blue, followed by dark blue and blue-green, and the color is required to be uniform. Light blue and gray blue turquoise can only be carved.

2. Difficulties

The hardness of high-grade turquoise should be around 6.

Step 3 split

Jewelry and sculpture have different requirements for raw materials. For example, according to the requirements of the United States, the original stone size of light blue and light green turquoise should not be less than 10mm, and the weight should be 7-28g. The sky blue turquoise can be smaller, but not less than 4 g. According to the carving requirements, Yunyang turquoise in China is mainly divided into four grades: more than 3kg, about 3kg, 3kg to 1kg and small materials.

model

For reticulated turquoise or when preparing to cut turquoise and surrounding rock together, the beauty of the pattern is also very important, and it is best to have a clear pattern. Some people especially like patterned turquoise.

Eight, turquoise origin introduction

Turquoise deposit belongs to exogenetic leaching and is related to sulfide mineralized rocks containing phosphorus and copper. The main producing areas of turquoise are Iranian, Egyptian, American, Russian and China.

Iranian turquoise, also known as Persian turquoise, is produced in NiShahpour district, west of Mashhad, Iran. Turquoise in this area is produced in the upper part of weathered porphyritic trachyte breccia zone, which is formed by the interaction of feldspar, apatite and chalcopyrite in trachyte and coexists with kaolinite and limonite. Many turquoise have the illusion of feldspar, and tiny white material spots can be seen on the cut turquoise. Persian turquoise is usually beautiful sky blue, with dense texture, density of 2.9g/cm3 and good luster. At present, turquoise mining in NiShahpour district has stopped, but there are still small-scale mining in other parts of Iran. It should be pointed out that at present, Persian (Iranian) turquoise is often said to have similar qualities and characteristics with Iranian turquoise.

Egyptian turquoise is produced in Sinai Peninsula with a mining history of over 3,000 years. In the arid area of Sinai Peninsula, turquoise is produced in the middle-upper fractured zone of sandstone closely related to copper mines and volcanic rocks, which belongs to weathering and leaching. Unlike the sky blue of Iranian turquoise, Egyptian turquoise is mostly blue, but some of it is blue. It is not difficult to see from the polished surface that this is because there are small blue dots distributed in it. The density of Egyptian turquoise is 2.7 ~ 2.9g/cm3, and the density of blue turquoise is higher. The turquoise in Sinai Peninsula has been basically mined.

American turquoise occurs in the cracks of porphyry (rhyolite), altered trachyte and adamellite in Nevada, Arizona, California, Colorado and New Mexico. The reason is that precipitation leached alkali feldspar and its associated copper ore and apatite, and then precipitated in cracks. American turquoise has sky blue, green and turquoise. Its main characteristics are light color, high porosity, chalky shape and density of 2.6 ~ 2.7g/cm3. Turquoise is still one of the favorite gems in southwest North America, and it is also the first choice for American Indian jewelry. At present, commercially, turquoise produced in the United States with similar characteristics and quality is often called American turquoise.

The turquoise mines in China are mainly distributed at the junction of Hubei, Shaanxi and Henan provinces, with Yunyang turquoise mine in Hubei being the most famous. The formation of turquoise here is mainly related to the siliceous slate rich in copper and phosphorus in the Lower Cambrian, which belongs to the secondary leaching deposit. The ore bodies are mainly lenticular, disseminated or cystic. Turquoise is mostly nodular and veined. Colors are blue, light blue, blue-green, apple green, light gray and grayish yellow. The mining of Yunyang turquoise has a history of hundreds of years, and it is famous for its good quality and large output. Due to the long mining time, large-size high-quality materials are rare at present.

In addition to the border areas of Hubei, Shaanxi and Henan, China's turquoise producing areas also include Aoshan in Anhui, Hami in Xinjiang, Xichuan in Henan and Anning in Yunnan. The turquoise of Aoshan Iron Mine in Anhui Province is an iron hat turquoise produced in the oxidation zone of copper-bearing and phosphorus-bearing altered surrounding rock in the upper part of Xiuyan Iron Mine. The ore bodies are mostly irregular nodules and inclusions, and the associated minerals are apatite, actinolite and magnetite.

Think about a problem

1. Mineral composition and chemical composition of turquoise.

2. Physical (including optical) properties of turquoise.

3. The structure and structural characteristics of turquoise.

4. What are the main varieties of turquoise?

5. What are the best treatments for turquoise?

6. What are the imitations of turquoise? How to identify these imitations?

7. What is gilson's "synthetic" turquoise?

8. How to evaluate the quality of turquoise?

9. What is the geological origin of the turquoise deposit?

10. What are the famous turquoise producing areas at home and abroad? What are the main characteristics of turquoise produced?