Hydrothermal emerald
| Hydrothermal Emerald | |
|---|---|
 Crystals of Emerald | |
| General | |
| Category | Mineral |
| Formula (repeating unit) | Be3Al2Si6O18 |
| Crystal system | Hexagonal |
| Identification | |
| Color | Dark green, blue-green |
| Cleavage | Imperfect |
| Mohs scale hardness | 7,5—8,0 |
| Luster | Glass |
| Streak | White |
| Diaphaneity | Transparent, translucent |
| Specific gravity | 2,69—2,78 |
| Refractive index | nω = 1,564—1,595, nε = 1,568—1,602 |
Hydrothermal emerald or grown emerald (also known as synthetic emerald , lab grown emerald and russian emerald) - emerald, obtained as a result of the process hydrothermal synthesis. From natural emeralds formed as a result of geological processes, it is absolutely impossible for a natural emerald to be clean.
History of hydrothermal emerald[edit]
The emerald is the second precious stone grown artificially (the first were ruby crystals grown in the late 19th century by the French chemists E.Fremi and O.Verneille from the melt of Al2O3 with an admixture of Cr2O3). As in the case of ruby, the stimulus for growing synthetic emeralds was the extremely high price of quality natural material. The history of the synthetic emerald is very complicated - very often crystals, the technique of cultivation of which was developed by one (sometimes anonymous) researcher, began to be grown by a variety of firms. The established fact is that the first emerald of jewelry quality was grown in the early 30s of the 20th century on the German chemical giant IG Farbenindustrie and was given the commercial name "igmerald". From the analysis of these emeralds, it can be concluded that the growth of the crystals was apparently carried out in platinum crucibles at temperatures above 1000 ° C from the solution in the melt of the Li2MoO4 salt. Later this technology (in one way or another modified form) spread throughout the world, becoming the basis for several manufactures of artificial gemstones - firms Nacken, Gilson, Linde, Chamam and several others. The founder of one of them, K. Chamam, in the second half of the 20th century even introduced the special term "created gems" for such crystals. By this term it is meant that the crystal has grown in conditions completely analogous to natural processes, but realized by man. As a result, the crystal itself is similar to the natural one. Note that C. Chamam, using the term "created gems" to his emerald somewhat slukavil. The fact is that in nature, crystals of an emerald do not grow from a melt, but from hydrothermal solutions, i.e. In nature and in the laboratory, fundamentally different processes and growth conditions are realized. The differences in the processes are also reflected in the differences in products (emerald crystals) of natural and laboratory: natural emeralds contain in their composition an admixture of water captured from the hydrothermal solution, whose molecules are located in the cavities of the crystal structure; The crystals grown from the melt are, on the contrary, free of impurities of water. The latter is expressed by the fact that natural emeralds, in comparison with Chamam emeralds, have higher density values, a refractive index and birefringence. This makes clear attempts to implement a hydrothermal growth process to produce crystals of the emerald, to which the term "created gems" could be fully applied. These attempts led to positive results in the US and the USSR. The first results were achieved by the American researcher E. Flanigen in the 1960s, who managed to grow emerald crystals from hydrothermal highly concentrated solutions of NH4Cl at temperatures of ~500 ° C and pressures of about 1000 atm. Unfortunately, the synthesis process quickly faded, because of which, to obtain large crystals, it was necessary to "transplant" the grown emeralds into another autoclave and repeat the growth process. Apparently, this process was implemented in the Austrian company Lehleitner.
In the USSR, work in this direction was conducted in Moscow (the Institute of Crystallography of the USSR Academy of Sciences) and in Novosibirsk (Institute of Geology and Geophysics, Siberian Branch of the USSR Academy of Sciences). If the work of the Moscow researchers was quickly stopped, a group of young Novosibirsk scientists A. Lebedev, A. Ilyin, D. Fursenko (all graduates of the Geological and Geophysical Faculty of Novosibirsk State University), under the direction of the head. the laboratory of hydrothermal growth of crystals V. Klyakhin in the early 70-ies of the twentieth century was able to solve the task - to carry out a stable growth of emerald crystals from hydrothermal solutions. Later in the 80s they were joined by graduates of the same faculty V. Thomas, I. Fursenko, V. Maltsev and S. Demin. Together they formed the backbone of the existing creative team. Since the first crystal hydrothermal emerald was grown at the Institute of Geology and Geophysics, the technology of the growth process has been constantly improved. The main works in this direction were focused on: a) enhanced color saturation of hydrothermal emeralds; b) an increase in the uniformity of the color of the crystals; c) combating the defectiveness of crystals (the goal is to grow a crystal that does not contain a single cracks); d) increasing the reproducibility of the growth process; e) an increase in the yield of the grown material. To intensify technological research and transfer them to self-sufficiency in 1989, the joint venture TAIRUS was founded by the United Institute of Geology, Mineralogy and Geophysics of the Siberian Branch of the Academy of Sciences (after the reorganization of the Institute of Geology and Geophysics of the SB RAS) and the Thai jewelry company Pinky Trading LTD. (Bangkok). The founding fathers of the TAIRUS JV are already the young authors of the hydrothermal method used to grow emerald crystals and their colleagues at the institute O. Holdeyev and V. Efremov, who had the solution of the financial and commercial issues of the enterprise. The emerald crystals grown by this method became the first commercial product of the TAIRUS Company. The founding scheme for growing emerald crystals proved to be so successful that by now such emeralds, known under the brand name "Russian emerald", have commercially practically replaced the emeralds grown by other methods. Moreover, a number of companies appeared that used this technology and began to produce "Russian emerald" on their own, competing with TAIRUS Company. Fortunately for TAIRUS, these companies do not have the main thing: experts - carriers of understanding about the hydrothermal process used, which constantly expands the amount of knowledge about it. Therefore, all the same, emeralds grown in these firms can not compete on an equal footing with the emeralds of TAIRUS. The commercial success of the "Russian emerald" encouraged researchers in other countries to develop original techniques for growing crystals of the emerald using the hydrothermal method. In this direction, certain results have been obtained, which we will consider later.
Hydrothermal method of crystal growth[edit]
The process of crystal growth under hydrothermal conditions occurs at high temperatures and pressures. Therefore, this process is carried out in special thick-walled vessels - autoclaves made of special high-strength corrosion and heat-resistant steel. The autoclave shown in the figure is not large, its working volume is about 250 ml, which allows one to grow in one growth cycle about 100 g of emerald crystals. Autoclaves for hydrothermal growth of quartz crystals, for example, are much larger - up to 6 m3 and they allow growing up to half a ton of crystals per one growth cycle. The essence of the hydrothermal growth of emerald crystals using the "Russian emerald" technology is as follows. At the bottom of the working cavity of the autoclave, charge (4) - crushed natural beryl of non-jewelery quality is filled. Above it is a diaphragm (5) - a steel circular plate with apertures that divides the working cavity of the autoclave into two zones: dissolution (under the diaphragm) and growth (above the diaphragm). In the growth zone, place a frame (6) with one or more seeds (7) - rectangular thin plates cut from the highest quality emerald crystals grown in previous cycles. Then, the necessary amount of water and a mineralizer (a mixture of a certain composition that increases the solubility of beryl in a hydrothermal solution) is added to the inner cavity. As a mineralizer in the technology of cultivation "Russian emerald" salts are used, which are present in real pegmatitic processes, during which the formation of crystals of natural beryl. Thus, either natural substances or completely analogous to natural substances are used as initial products in the technology of Russian emerald cultivation. The charged autoclave is sealed in its upper part and placed in a special vertical oven in which the autoclave is heated to about 600 ° C, and the pressure in it is increased to 1500 atmospheres. Moreover, the lower part of the autoclave (the dissolution zone) is heated to a higher temperature than the upper (growth zone). In the dissolution zone, the hydrothermal solution saturates with the components of the charge (beryl). Saturated solutions from the dissolution zone as a result of natural thermal convection enter the growth zone, where they cool and become supersaturated. The excess substance from the supersaturated solution is deposited on the seed plates - the crystal of the emerald grows. The total duration of a single growth cycle is 1 month, during which one or several emerald crystals can grow up to a total weight of 100 g, with a total length of up to 19 cm and a thickness of up to 2.5 cm. Let us now illustrate with the help of the scheme a close analogy between the realization of laboratory and natural hydrothermal processes. When molten magma is introduced from the depths of the Earth into the Earth's crust, the melt begins to crystallize with the formation, for example, of granite. At the same time, gaseous components begin to separate from the melt, primarily water, in which natural mineralizer additives are dissolved, which is capable of dissolving substances that are insoluble under normal conditions. Such an aqueous gas solution is called hydrothermal. At the end of the magmatic crystallization, a cavity is formed in the thickness of the Earth's crust surrounded by strong walls, composed of crystalline rock, filled with a hydrothermal solution. With further lowering of temperature and pressure, the "excess" substance begins to be deposited from the hydrothermal solution - crystals grow. Comparing natural and laboratory hydrothermal processes, we see almost a complete analogy between them: in a natural even a sealed autoclave is present, only its walls are made of solid granite, not of iron.
Features and comparison of synthetic analogues[edit]
Emerald «Biron» The crystals are grown in strongly acidic chloride solutions. Autoclave steel does not withstand aggressive chloride solutions at all, so crystals are grown in protective containers made of inert material (gold). A juicy green color, close to the color of classical natural Colombian emeralds, is due to the joint entry of impurities Cr3 + and V3 + into beryl. The purity of color is facilitated by the fact that a protective sealed container of gold prevents contamination of the grown crystal by the components of the autoclave steel. The gemological analysis of the Biron emerald shows its close resemblance to natural emeralds (green and yellow-green under the Chelsea filter, lack of luminescence under UV radiation, increased density and refractive index, etc.), in what it is close to the emerald " Russian emerald "and fundamentally differs from crystals obtained from solutions in salt melts. [1] In this emerald, however, it was possible to reduce the contrast of the swills, but because the roughness of the growing regeneration surface is much smaller, the concentration of the swills per unit volume of the faceted stone is much higher. Therefore very beautiful in fine cuts, the emerald "Biron" in large stones begins to lose its game. At the present time, apparently, it is not produced due to a much higher cost price in comparison with the production of the emerald "Russian emerald", but some of its quantities can still be found on the world jewelry market.
Emerald «Chinese»[edit]
Analysis of the morphology of the crystals of this emerald, the composition of inclusions in it, IR spectra and a number of other parameters, allows us to conclude that this emerald was produced using a technology close to the production process of the emerald "Biron". Its coloring, unlike the emerald "Biron", is due to the entry into the beryl of only Cr3 + admixture, because of which there appear red luminescence under ultraviolet and a red color under the filter of Chelsea, which is completely uncharacteristic of natural stones. At the present time, also, apparently, is not produced.[2]
Emerald «Malossi»[edit]
At the turn of the 20th and 21st centuries, the employees of the TAIRUS Company managed to grow an emerald with fundamentally reduced contrast of the swills. The crystals looked exceptionally transparent and allowed to cut large stones of any required size, the game of which did not fall in size. The basis for developing the composition of the mineralizer for growing these crystals was the results of the American researcher E. Flanigen. [3] The process of crystal growth was carried out in platinum containers, which made the crystals free of autoclave steel components, and the emerald itself was called "platinum". The coloring of such emeralds is due to the entry of only Cr3 + impurities into beryl, which, like the "Chinese" emerald, stones look red through the Chelsea filter and are characterized by strong red luminescence in the ultraviolet. The latter fact served as a brake on the wide production of such emeralds *, but small lots through the Italian dealer A. Malossi began to enter the jewelry market under his name: Malossi hydrothermal synthetic emerald. Later, reports appeared that "Malossi emerald" was produced in the Czech Republic, but the analysis of the last crystals of the Malossi emerald showed their complete identity to "platinum", which suggests a complete identity of the growth processes of these "two different species" of hydrothermal emeralds.
"Colombian color emerald"[edit]
In parallel with the work on the "platinum" emerald, a group of employees of the TAIRUS Company under the direction of D. Fursenko tried to grow an emerald that is not inferior in color to the emerald "Biron" (considered in the early 21st century as the standard for hydrothermal emerald color) and as transparent as the "platinum" emerald . More than 7 years of work of this group was spent on obtaining the first positive results and for several years - on the development of the technology. So there was the "Colombian color emerald" - a hydrothermal-grown emerald of an exceptional green color, not inferior in color to the best examples of the emerald "Biron", but because of its transparency (due to the very low contrast of the strands) even surpassing the "platinum" emerald. The color of the Colombian color emerald is due to the presence of V3 + in the beryl, which is the reason for the purely "natural" behavior of this emerald - the green color under the Chelsea filter and the complete absence of luminescence. On other grounds (the presence of water impurities in channels, high density, high refractive index, etc.), the Colombian color emerald is completely analogous to the best natural Colombian emeralds.[4]
At present, the needs of the world market for hydrothermally grown emeralds are almost completely covered by emeralds "Russian emerald" (a niche of relatively small faceted stones) and "Colombian color emerald" (a niche of more expensive large fences). Other varieties of hydrothermal-grown emerald come to this market sporadically and in small lots, which may indicate the absence at the present time of large-scale production of artificial emeralds based on other technological processes.
References[edit]
- ↑ R.E. Kane, R.T. Liddicoat (]r.). The Biron hydrothermal synthetic emerald // GEMS & GEMOLOGY, Fall, 1985, p. 156-170.
- ↑ K. Schmetzer, L. Kiefert, H.-Jür. Bernhardt, Z. Beili. Characterization of Chinese hydrothermal synthetic emerald. // GEMS & GEMOLOGY, Winter, 1997, p. 276-291.
- ↑ Flanigen E.M., Mumbach N.R. Hydrothermal process for growing crystals having the structure of beryl in an acid halide medium. // United States Patent No. 3,567,643; issued March 2, 1971.
- ↑ К. Schmetzer, D. Schwarz, H.-J. Bernhardt, T. Häger. A new type of Tairus hydrothermally-grown synthetic emerald, colored by vanadium and copper. // J. Gemm., 2006, 30, 1/2, p.59-74.
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