Geoscience Reference
In-Depth Information
Table 6.4 Coordination of Cation with Oxygen in Silicate Structures
[22]
Radius (
˚
)
Ion
Radius Ratio
Coordination No.
Bond Strength
B
1
3
0.20
0.20
3,4
1 or 3
=
4
Be
1
2
1
=
2
0.31
0.25
4
Li
1
1
=
4
0.60
0.34
4
Si
1
4
0.41
0.37
4,6
1
P
1
5
0.34
0.34
4,6
5
=
4or5
=
6
Al
1
3
0.50
0.41
4,5,6
3
=
4or1
=
2
Ge
1
4
0.44
0.43
4,6
1 or 2
=
3
Ga
1
3
0.62
0.46
4,6
3
=
4or1
=
2
Mg
1
2
0.65
0.47
6
1
=
3
Na
1
0.95
0.54
6,8
1
=
6or1
=
8
Ca
1
2
0.99
0.67
7,8,9
1
=
4
K
1
1.33
0.75
6,7,8,9,10,12
1
=
9
NH
4
1.50
0.85
9,12
1
=
9or1
=
12
a distinct gap between zeolite-type and dense tetrahedral framework structures is
seen. The boundary ranges from just under 20 to over 21 T-atoms per 1000
˚
3
,
depending on the type of smallest rings present.
Figure 6.3b
shows the correlation
between the pore size of molecular sieves and the diameter (
) of various mole-
cules. While, for zeolites, the observed values range from about 11.1, for structures
with the largest pore volume, to around 20.5. To date, framework densities of less
than 12 have only been encountered for interrupted framework, like in cloverite,
and hypothetical networks
[24,25]
. The FD is obviously related to the pore volume,
but does not reflect the size of the pore openings, and to some extent it depends on
the composition for structural and chemical reasons. The list of zeolite structure
types also comprises interrupted frameworks of zeolite-like materials, provided the
framework atoms, other than oxygen, are tetrahedrally coordinated, at least in the
calcined form.
The chemistry of zeolites is based on the tetrahedron (TO
4
), where, T
σ
Si, Al,
or P. However, zeolites are so diverse that secondary structural units based on
small grouping of linked tetrahedra are needed in describing and systematizing
their topologies. A detailed structural classification of natural and synthetic zeolites
has been proposed by several workers like Breck
[21]
, Smith
[24]
, and Barrer
[25]
.
However, Meier
[26]
was the first to propose a classification based on second
building units contained in the structure. The classification based upon SBUs is
given below.
Table 6.5
gives the building units in zeolite structures
[27]
. Some of
the synthetic zeolites are structurally related to minerals and are included in the
above classification; however, for many of the synthetic zeolites structures are not
known, therefore, in many cases the framework may be considered as made up of
larger polyhedral units, such as the truncated cubo-octahedron (a-cage), truncated
octahedron (b-cage), double 8 with 4-ring bridges (g-cage), double 8 ring (d-cage),
and cancrinite
[19]
.
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