Geoscience Reference
In-Depth Information
7.13 Hydrothermal Growth of Phosphates
From the time of its discovery, the hydrothermal technique was known for the syn-
thesis of silicates. In the nineteenth century, the hydrothermal technique was included
under silicate technology. Next to silicates, it was the oxides and hydroxides of vari-
ous metals that dominated the hydrothermal technology during the late nineteenth
and early twentieth centuries. Subsequently, germanates, sulfides, and carbonates,
etc., became popular. However, the growth of phosphates by hydrothermal technique
was not attempted for a long time. It was only during the 1950s that hydrothermal
technique was tested for the synthesis of phosphates
[222]
, and up to the 1980s there
were only scanty reports on the synthesis of phosphates by hydrothermal technique.
Today, the situation has changed dramatically. Hydrothermal growth of phosphates
is much more popular than the hydrothermal growth of any other compound. A large
variety of phosphates starting from simple orthophosphates to the most condensed
or ultraphosphates containing a wide range of elements have been obtained. This is
probably due to the improved instrumentation techniques and the discovery of alu-
minophosphate zeolites. Further, the success in the growth of various phosphates has
prompted the hydrothermal researchers to synthesize other inorganic compounds
hitherto unattempted. Similarly, it is appropriate to state that the success in the
growth of phosphates has led to the understanding of hydrothermal crystallization
mechanism for various inorganic compounds. It ultimately has set a new trend in the
synthesis of inorganic compounds under mild hydrothermal conditions.
There is a growing interest in the study of phosphates in the last two decades
owing to their widespread application, e.g., piezoelectric, luminescent, magnetic,
superionic, ceramic, solid-state laser, ultra-low thermal expansion, nonlinear optic
and moisture sensors as hosts for actinide ions in modern technology. Similarly,
phosphate minerals are the important source materials of almost all the rare earth
elements. Thus, the phase equilibria studies in phosphate systems help to understand
the interaction between various metals and P
2
O
5
, and it might give possible clues to
separate these rare earth elements from phosphate minerals and also to understand
their genesis. It is interesting to note that there are over 300 phosphate minerals in
nature that have been classified into various groups depending upon the cationic ele-
ments present in them
[223]
. The phosphates containing rare earth elements repre-
sent the smallest group having only 15 minerals, out of which 4 are anhydrous:
monazite-(Ce, La, Y, Th)PO
4
; xenotime-YPO
4
; cheralite-(R, Ce, Th)PO
4
, and
vitosite-Na
3
R(PO
4
)
[224]
(a recently reported mineral from Lovozerskii alkaline
mass), where R is rare earth element. It is interesting to note that all the naturally
occurring phosphates are represented by one anionic group—orthophosphate.
Although a wide range of physicochemical conditions exist in nature, we find phos-
phate minerals without poly-, pyro-, meta-, ultra-, etc. types, unlike the silicate
minerals with a very wide variety of condensed silicates like meta-, poly-, soro-,
phyllo-, etc. The reasons for the absence of condensed phosphates in nature are not
clearly known. In order to understand the peculiarities among natural phosphate
minerals, one has to synthesize them in the laboratory by simulating the natural
