Geoscience Reference
In-Depth Information
The growth of KTP family of materials on the whole is fascinating and challeng-
ing for both technologists and academicians.
5.7 Calcite
Calcite, CaCO
3
, is an important carbonate mineral and it occurs commonly in
nature as both well-developed crystals and amorphous material. The pure and
optically clear calcite is called Iceland spar. Calcite is polymorphous and exists in
at least five modifications. The two polymorphs commonly found in nature are cal-
cite and aragonite. In addition, there are two synthetic forms known only at high
pressures: calcite II and calcite III. Vaterite (
-CaCO
3
) is a metastable hexagonal
form which crystallizes at ordinary temperatures and pressures
[161]
.
Calcite single crystals form an important optical material owing to its large bire-
fringence and transparency over a wide range of wavelengths. This property makes
it one of the significant materials for polarized devices such as optical isolaters and
Q switches. It also exhibits antiferromagnetic properties. Although there are many
deposits of calcite in the world, the optically clear quality calcite (e.g., Iceland
spar) has been depleted in recent years, leading to its shortage in nature. However,
the demand for optically clear calcite single crystals is increasing greatly with the
development of laser devices such as optical isolators
[162,163]
, and similarly, the
chemical impurities and growth defects depend upon the geological settings and
often lead to variations of refractive index and internal light scattering centers, and
low optical damage resistance. Hence, the artificial means of obtaining optically
pure calcite is becoming very popular. Many laboratories throughout the world
have attempted to obtain calcite, but most of them have suffered from size or qual-
ity problems. In fact, the first publication on the synthesis of calcite appeared in
1883. Lemberg
[164]
heated basalt for 6 months with distilled water. It took up
2.43% H
2
O, which was not removed over H
2
SO
4
in 2 weeks; the solution remain-
ing in the tube had a slightly alkaline reaction. The same basalt heated with K
2
CO
3
solution for 9 months gave a glass rich in H
2
O and CaCO
3
crystals. A glass made
from palagonit from Vidoe heated for 3 months with distilled H
2
O was little
attacked, but the product contained 8.61% H
2
O. Similarly, melted labradorite,
heated for 13 months with K
2
CO
3
solution gave a glass rich in K, and the Ca
replaced had formed CaCO
3
. Further, Lemberg obtained CaCO
3
, while the MgO
remained combined with the SiO
2
after heating augite with K
2
CO
3
solution for
1 year at 100
C. When anorthite, with a little augite, was heated for 189 h at
180
γ
190
with a Na
2
CO
3
solution, the product consisted of calcite crystals, amor-
phous substances, and a columnar mineral similar to cancrinite. Later, Friedel and
Sarasin (1885)
[165]
obtained small calcite crystals by heating CaCl
2
, precipitated
CaCO
3
, and 60
70 ml H
2
O for 10 h at 500
C; with 20 gm CaCl
2
and a few
grams CaCO
3
, the crystals were large enough for goniometric measurements.
Crystals obtained were simple rhombohedra. There was no aragonite in the prod-
uct. However, all these earlier works on the synthesis of calcite were purely of
geological/mineralogical interest, and the size of the crystals obtained was in no
