Geoscience Reference
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[P 2 O 7 ] formed by the linking of two tetrahedra at the vertices was reported recently
[12] . Before the discovery of this mineral in 1988, there was a unanimous opinion
with regard to the absence of condensed phosphate minerals, although more than
320 phosphate minerals are known in nature. However, vanadates, which usually
form four-, five,- and sixfold coordinations with oxygen, contain the condensed
V-tetrahedra. For example, chervetite, Pb 2 V 2 O 7 having pyrogroup-[V 2 O 7 ] formed
by the linking of two [VO 4 ]-teterahedra with one of the corners sharing, has been
reported so far [13] . In contrast, such condensed radical groups or pyrogroups, i.e.,
[As 2 O 7 ] complexes, have not been reported among arsenates.
Main concept of silicate structures. In silicate structures (Si,O)-tetrahedral anions
should be adjusted to the cationic polyhedral fragments [14] . In particular, this
principle was confirmed by the structures with big cations (K, Na, Ca, RE, etc.),
which always contain (Si,O)-complexes on the basis of pyrogroups [Si 2 O 7 ] com-
mensurable with the edges of cationic polyhedra.
Several quantitative correlations actually complete this qualitative approach. For
example, there are more than 60 compounds with a general formula M a [T 2 O 7 ] b ,
where T
As, Be, Cr, Ge, P, S, Si. They can be subdivided into two groups:
(i) Tortveitite-like with angle T
5
140 ; (ii) bichromate like with angle
a
O
a
T
.
140 Ref [15] . Figure 7.1 illustrates that the frontier between both groups
corresponds to the equation r/T
a
a
T
O
T
,
1.1, where r is the radii of tetrahedral
(T) and nontetrahedral (M) cations [15] . Thus, the increasing number of refined
structures allows the establishment of a quantitative relationship between the config-
uration of tetrahedral complexes and the properties of the nontetrahedral cations.
Among these complex coordinated compounds, silicates have been studied ex-
tensively, followed by germanates, phosphates, and others. Although silicates and
germanates have a systematic structural classification, their analogous compounds
—like phosphates, vanadates, and arsenates— do not have a systematic classifica-
tion. It is extremely difficult to classify phosphates, unlike silicates, because of
high complexity in the phosphates' internal structures [16] . Similarly, the other
1.5r/M
5
2
0.4
Ge
V
As
Cr
Si
0.3
Tortveitite like
Bichromate like
0.2
P
S
0.1
0.5
0.8 1.0
Radius of M m+
1.5
Figure 7.1 Correlation between the stability of tortveitite-like and bichromate-like structures
Ma[T 2 O 7 ] and radii of M- and T-cations [1] .
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