Geoscience Reference
In-Depth Information
This is an important conclusion, particularly for geoscientists to explain the possible
reasons for the absence of condensed phosphates in nature
[254]
. The experiments car-
ried out under hydrothermal conditions showed that when P
H
2
O
.
crystallization
of polyphosphates or ultraphosphates ceases and is replaced by crystallization of simple
orthophosphates irrespective of the temperature of the system. Similarly, even an
increase in the concentration of M
2
O in the system leads to decondensation, and ultra-
phosphates having a highly condensed state or a complex anionic structure [P
5
O
14
]dis-
appear from the products of crystallization and are replaced by polyphosphates with
radicals [P
2
O
7
], [P
3
O
9
], [P
4
O
12
], and finally [PO
4
]. These experiments demonstrate that
the structures of condensed phosphates are not stable, at least not in the presence of
water. It is well known that the magmatic or the postmagmatic processes of mineral
formation are always accompanied by a certain amount of water and proceed in the
presence of many types of metals. These are the conditions for the formation of ortho-
phosphates and not condensed phosphates
[254]
. From the above studies, it is clear that
only the hydrothermal method of studying alkaline rare earth phosphate systems
revealed the formation of various phosphates depending upon the PT conditions.
The growth of rare earth phosphates in the form of single crystals is very interest-
ing owing to the high reaction susceptibility of phosphorus and the easy formation
of phosphoro-oxygen radicals in a given system. For example, any anionic group of
Nd-phosphate can be readily obtained from phosphoric acid solutions. In the liquid,
from which the Nd-phosphates grow, Nd should be dissociated to form individual
ions. For this purpose, usually concentrated solutions or alkali melts are used.
However, the process of Nd dissociation often carries a reversible character and the
phase develops with Nd ions in the condensed pattern during the crystallization—e.
g., as in NdP
3
O
9
[255]
. In order to achieve this, it is necessary that in the process of
crystallization, the Nd ions are to be held in a dissociated state, i.e., in the form of a
nezoite complex (
Figure 7.59
). A shell of ligands around Nd prevents the ions from
intercondensation and allows them to enter the crystal lattice as isolated ions. These
ligands which often occur as tetrahedral oxy-anions [TO
4
] is the formation of a
nezoite complex of type NdT
8
O
24
, where T
6atm
;
S, P, W, Mo, etc.
[256]
Thus, to obtain
5
Figure 7.59 Nezoite complex
[240]
.
[TO
4
]
[NdO
8
]
