About Hydrothermal Quartz 

Rock crystal (natural colorless quartz) and its varieties of different colors, especially amethyst and citrine, have always been successfully used as gemstones and as ornamental stones. The need to create synthetic quartz did not arise, since quartz crystals are plentiful. But when the 'piezoelectric' effect of quartz was discovered, quartz has become an important technical material as well.

When compression is applied to the direction of the crystal axis (x axis), the electric change generates on the quartz crystal plate, and, on the contrary, when the electricity is applied to the quartz crystal plate, the distortion occurs inside the crystal plate. The phenomenon is called the piezoelectric effect or piezoelectric reverse effect.

This property of quartz opened up a lot of possibilities for industrial applications of quartz. It turned out that the colorless rock crystal was particularly valuable due to its optical uniformity. However, natural crystals of quarts are rarely suitable for industrial purposes, whereas synthetic quartz is. Quartz crystals were first synthesized in the research laboratory, and since the middle of 1950s the growth has become commercial.

PROPERTIES

Hardness (Mohs' scale): 7
Density: 2.65
Refractive index: 1.544 – 1.553
Dispersion: 0.013
Crystal system: trigonal
Chemical composition: SiO2 (with various additives to produce colour)
Dielectric constant: 4.5
Transition temperature: 573°C

HISTORY

The very first experimental studies on the synthesis of quartz were conducted with a mere purpose to determine the conditions of natural quartz formation. In 1845, C.E. Schafhautl first succeeded in crating a tiny quartz crystal. After that, small quartz crystals were created from silica dissolved in alkali water solutions at high temperatures and pressures.

The experiments were continued until two scientists, A.C.Walker and E.Buchler, arrived at the best combination of different conditions like temperature, pressure and other particulars to create synthetic quartz crystals. In their experiment, the crystals weighing 100-300 g were grown at a speed of 2.22 mm/day. Since then, the technology for creating synthetic quartz crystals has been developed to allow mass production.

SYNTHESIS

Synthetic Quartz crystals are grown by hydrothermal method based on the temperature differences. In general, this process closely mimics how many crystals in nature originate from high temperature, high pressure water solutions. Hydrothermal method is also used in manufacturing high quality synthetic emeralds, rubies and sapphires. The ingredient materials are enclosed in a partially filled, sealed container of water solution called autoclave.

Heated to high temperatures the feed materials are dissolved under high pressure and at point of cooling crystal growth is induced.

To grow quartz crystals an autoclave is filled with alkali solution of material such as Na2CO3 or NaOH. The high pressure required for growing is obtained by filling the alkaline solution up to the approximate 70-85% of the capacity of the autoclave and by rising the temperature. The ordinary growing temperature is approximately 350-360ºC and the growing pressure is approximately 90-145Mpa. When the autoclave is put into service, the manufacturer performs a 'conditioning' run which allows a crystalline coating to form on the inside of the vessel. This coating is stable and prevents interaction between the autoclave and the chemical solution used in the vessel.

In the autoclave, the upper growing area and the lower dissolution area are partitioned by the perforated panel called the baffle. This baffle allows the bottom temperature and the top temperature to be significantly different and to help each zone have an isothermal characteristic. The baffle also restricts the convection flow and channels the convection flow into a desirable geometry.

The seed crystal is suspended in the upper growing area, and the raw material, Lasca, a fragment of the natural quartz crystal is placed in the lower dissolution area. A cap is screwed on and the autoclave suspended vertically, the lower half being inserted into an industrial furnace. The temperature in the lower part of the autoclave is higher than in the upper part. As a result, materials that dissolved in the lower part under high temperature and pressure are carried to the upper part by convection to deposit on seed crystals, creating synthetic quartz crystals. The growth cycle may vary from 40 to more than 200 days, time being a function of the size of the stone being grown. Different impurities are added to the growing solution to provide color to the quartz. In laboratories, smaller autoclaves for experimental work are used.

The main parameters that are monitored in the process are the temperature at various points on the vessel and the pressure of the vessel. These parameters are used to control the growth process. They have to vary during the process because at startup the majority of the quartz material is at the bottom of the autoclave. The top only contains the seed material. At the end of the process, most of the quartz in the bottom has dissolved and recrystallized on the stones in the upper growth zone. Moving this much material around greatly affects the hydrothermal flow conditions and the thermodynamic state of the vessel. For this reason, the control algorithms are designed to make continual changes during the process to compensate for these changing conditions.

Heating of the autoclave is accomplished by means of electric heaters fastened to the outside of the vessel. There are heaters throughout the length of the vessel with a higher density at the bottom. The temperature of both the bottom dissolving zone and the top growing zone has to be carefully controlled.

Around the autoclave and the heaters there is an exterior insulation package. This insulates the autoclave and helps isolate it from external temperature changes. There is usually an air space between the insulation package and the vessel. There are also vents at strategic places to allow the air flow to be controlled. Changing these vents and the power delivered to the heaters determine the thermodynamic condition of the autoclave.

COLOURS AVAILABLE

At present, we are offering a wide range of synthetic quartz varieties as, for instance, the following:
GREEN QUARTZ (PRASIOLITE) (SiO2 : Fe2+)

The colors and hues available: green, yellowish- or bluish-green. Some varieties correspond to green tourmaline color.
2;±crystal’s dimension (mm): 200x75x28
AMETHYST
Synthetic amethyst is the same violet to purple color as its natural counterpart. Few other synthetic gemstones so closely emulate their natural counterparts as synthetic amethyst does natural amethyst. The colors and hues available: purple, reddish- or bluish-purple. The crystals correspond to natural Ural amethyst color. The main physical characteristics of crystals are:

2;±crystal’s dimension (mm): 200x75x30
thickness of grown layer: 12-14 mm
AMETRINE
The colors and hues available: two-colored, with clear limits between citrine and amethyst zones. Some varieties correspond with Brazilian natural amethyst.
dimension of citrine`s zone (mm): 28x30
width of amethyst grown layer: 8 - 10 mm;
CITRINE (SiO2 : Fe3+ Li+)
The colors and hues are: yellow, orange-yellow. Their color corresponds to the best hues of natural citrine. The characteristics of crystals are:

2;±crystal’s dimension (mm): 200x75x30
Thickness grown layer: 12 - 15 mm;
BLUE QUARTZ (SiO2 : Co3+)
The colors and hues available: light blue, dark blue hue, medium light to moderately strong to strong saturation (ч 4-6/4-6).

2;±crystal’s dimension (mm): 200x75x28
Thickness grown layer: 12 - 14 mm;
Synthetic quartz, especially its colored varieties, is still widely used as a gemstone and an ornamental stone. Apart from jewelry, the scope of application of optic and piezoelectric synthetic quartz crystals includes:

Computer hardware
Photo and telemetry
Digital photo and video cameras
Radio communications, telecommunications, and remote control
Automatic and control systems
Radar and radio navigation technology
Top quality substrates
Watches and clocks


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