Geoscience Reference
In-Depth Information
7.9.2 Beryl, Be 3 Al 2 (SiO 3 ) 6
Beryl is one of the most common minerals of hydrothermal origin. It occurs as
well-developed crystals of large size. In fact, beryl is the largest crystal produced
by nature and is commonly found in pegmatites. It is one of the most important
gemstones. The presence of a little Cr 1 3 gives a green color and it is known as
emerald. The bluish green variety is known as aquamarine. The greenish/yellow to
iron-yellow and honey-yellow are called golden beryl. The rose-colored variety is
called morganite or vorobyevite. The alkali elements like Na 1 ,Li 1 , and Cs 1 are
sometimes present replacing the beryllium from 0.25% to 5%.
Artificial growth of beryl crystals began in the nineteenth century. Hautefeuille
and Perrey [138] were the first ones to synthesize beryl crystals. Espig [139] stud-
ied the crystal structure of beryl. Rogers and Sperisen [140] prepared beryl crystals
by the hydrothermal technique. Wyart and Scavnicar [141] synthesized beryl crys-
tals by the hydrothermal technique using NaHCO 3 solvents. Nassau [142] has
grown large crystals of beryl by the hydrothermal method. Beryl, Be 3 Al 2 Si 6 O 18 ,is
found with a variety of impurity cations leading to its well-known beautiful colors
and its favorable use as a gemstone. Probably the best work on hydrothermal
growth of emerald was by Nacken. He made large numbers of synthetic emeralds,
using a trace of chromium to produce the color. Hexagonal prisms weighing about
0.2 gm were grown in a few days. Nacken [142] and Hosaka [143] reviewed
the hydrothermal growth of emerald. Johann Lechleitner of Innsbruck, Austria,
released the first and not completely satisfactory product into the market during
1960 [144
146] . He used faceted beryl gemstones as the seed and grew a thin
layer of hydrothermal emerald on the surface, which was subsequently given a light
polish (not necessarily on all faces). Such stones were marketed for a short time
under the names Emerita and Symerald. Since the green skin was quite thin, the
color of these emeralds was not intense, and the skin could be readily viewed by
immersion of the stone in a liquid. Lechleitner emerald can be synthesized under
the following conditions:
Growth temperature: 300 400 C
Pressure: 1020 kg/cm 2
Solvent: a weak alkaline solution
Nutrient: silica (SiO 2 ), alumina (Al 2 O 3 ), beryllium oxide (BeO).
The first completely synthetic hydrothermal emerald was put on the market by
the Linde Division, Union Carbide Corp., in 1965. Figure 7.29 shows the emeralds
grown hydrothermally by the Linde Co.; the size of the crystal is 50 3 22 3 6mm 3 .
The first patent on beryl revealed that it could be grown in a neutral to alkali
medium (pH of 7
12.5) using mineralizers such as alkali or ammonium halides
(e.g., NH 4 F
NH 4 OH or KF) and that Fe, Ni, or Nd could be used as dopants
[147] . The second patent dealt with the analogous growth of beryl in an acid
medium (pH of 0.2
1
4.5), using similar halide mineralizers but with an acid reac-
tion (e.g., 8 N NH 4 Cl) or with extra acid added [148] . Typical emerald growth con-
ditions included pressures of 10,000
600 C
20,000 psi at temperatures of 500
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