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face (1011) is the fastest growing face. Hence, natural calcite crystals cleaved at
(1011) plane are used generally as seed crystals. Yanagisawa et al. (1996) [182]
have obtained the highest growth rate reported so far for calcite, 213
m/day using
ammonium acetate solvent at 230 C and a temperature gradient of 20 C. However,
the optical quality of the calcite crystal was relatively poor. Only under experimen-
tal conditions, such as temperature gradient of 15 C and growth temperature of
235 C, was the growth of transparent calcite crystals achieved at 57
μ
m/day.
Figure 5.52 shows the hydrothermally grown calcite crystals by different authors.
When as-cleaved crystals are used, the calcite crystals obtained are usually opaque
or not transparent, and the surfaces show a greater degree of defects like growth
hillocks, a lot of small secondary grown crystals, and strains. The crystals grown
on the etched seed crystals are usually transparent and the surfaces are very
smooth. Figure 5.53 represents the characteristic photographs of calcite single crys-
tal grown in NH 4 NO 3 solution using as-cleaved and etched seeds. Similarly,
Figure 5.54 represents the surface morphology of the calcite crystals using as-cleaved
μ
Figure 5.51 Schematic
diagram of the autoclave [181] .
Gauge
CO 2
container
Rupture
disk
Valve 1
Valve 2
Furnace
Valve 4
Valve 3
Hand pump
Autoclave
Figure 5.52 Hydrothermally
grown calcite crystals [178] .
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