Geoscience Reference
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0.02 ˚ ,P
1.51 ˚ , angle O
follows: d(P
a
O)
1.54
a
O (terminal)
a
P
a
O
5
1
5
5
1.64 ˚ . Although
the bond length tends to be close to its mean value, the bond angle, especially angle
P
5
167 .P
109.4
and angle P
a
O
a
P
156
a
O (bridging)
6
5
5
P linkage allows a
variety of related orientation of adjacent tetrahedral [5] . From the crystal chemistry
of phosphates, it is evident that the variation in the bond lengths and bond angles is
quite large and it is difficult to find an analog to [PO 4 ]-tetrahedra among the inor-
ganic compounds [6] . Hence, a thorough study of the crystal chemistry of silicates
and phosphates, in turn, helps to understand the crystal chemistry of analogous
compounds like germanates, vanadates, arsenates, sulfates, and borates.
Germanates and phosphates form the nearest analogs of silicates owing to the
nearer ionic radii and charge values. All three types of tetrahedra [SiO 4 , GeO 4 , and
PO 4 ] form condensed radicals, but the ability to undergo polycondensation is high
for silica tetrahedra when compared with Ge, P, and other tetrahedra (VO 4 , AsO 4 ,
SO 4 ,BO 4 , BeF 4 , etc.) having nearer ionic radii. This is probably connected with a
significantly lower number of different P:O and Ge:O ratios (10 and 9, respec-
tively) when compared with a similar Si:O, which is 28 as observed in phosphates,
germanates, and silicates [7] . This characteristic property of Si-tetrahedra can be
explained based on two factors: (i) A suitable value of the valence charge of silicon
equivalent to one (this separates silicon from phosphorus), (ii) characteristic radius
ratio rSi 4 1 :rO 2 2 for the formation of tetrahedral coordination. In this respect,
Si-tetrahedra differ from Ge-tetrahedra for which the radius ratio rGe 4 1 :rO 2 2
(0.38) is nearer to the upper stability limit of the tetrahedral coordination (0.41).
The possible valence states and the coordination numbers of basic cations capable
of forming oxide complexes are given in Table 7.3 . All these elements represented
in Table 7.3 form the tetrahedral coordination. Besides, the linking of [TO 4 ]-
tetrahedra (T is the tetrahedral cation) is different for different cations. For exam-
ple, out of more than 600 structures refined for natural and synthetic sulfates, only
in four structures, namely, K 2 S 2 O 7 [8] , nK 2 S 2 CyV 2 O 5 [9] , CdS 2 O 7 [10] , and
Te(S 2 O 7 ) 2 [11] , have the pyrogroups [S 2 O 7 ] been established recently. Structurally,
a very important phosphate mineral, canaphite, having pyrophosphate groups
a
O
a
P, is obtuse and the rotation flexibility along the P
a
O
a
Table 7.3 Valency States and Coordination Numbers of Basic Oxide Complexes [1]
Group
Element
Valency States
Coordination
2
3
4
5
6
3
4
5
6
III
B
212221122
IV
Si
221222111
Ge
221222121
V
P
211122122
V
111122111
As
212122122
VI
S
12122
1
1
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