Geoscience Reference
In-Depth Information
a change in the Si:Al ratio, whereas the structure of felspathoids have a more open
framework so that the ions can be moved, removed, or interchanged through the
channels/cavities without disturbing the framework. The cavities are usually filled
with water molecules.
Finally, the zeolites' structures contain larger cavities/channels and, therefore,
many exhibit, to a greater extent, the properties of ion exchange and molecular
adsorption. Furthermore, these cavities or channels in zeolites are usually filled
with water molecules. These are relatively loosely bound to the framework and
cations and, like the cations, these can also be removed and replaced without dis-
rupting framework bonds. Accordingly, the felspars are anhydrous aluminosilicate
minerals with a density of 2.6
2.7, followed by felspathoids (density 2.3
2.5) and
zeolites (density 2
2.3). The hardness decreases from felspar to the zeolite end of
the minerals owing to the above-stated structural features. Thus, the characteristic
feature of zeolite mineralogy is the presence of OH
2
molecules in the structural
channels, which are loosely bonded to the cation framework. The cations can be
removed and replaced by other metals without disrupting framework bonds.
There are over 46 naturally occurring zeolite minerals, belonging to some major
structure types. The channel systems in various zeolites are formed by different
combinations of linked rings of tetrahedra. The wider the channels at their narrow-
est parts, the larger the cation that can be introduced into the structure. A grading
of the zeolites according to the openness of their structure is given in
Table 6.1
.
When zeolites are heated, water is given off continuously rather than in separate
stages at definite temperatures, and the dehydrated or partially dehydrated mineral
can reabsorb water to its original amount when it is again exposed to water vapor.
Dehydrated zeolites can also absorb other liquids or vapors in place of water, e.g.,
ammonia, alcohol, NO
2
, and H
2
S. The base-exchange properties of zeolites were
first reported by Eichorn
[6]
and have been reviewed by Hey
[7]
who also investi-
gated in detail the base exchange of various members of the group. The colloidal
properties of
the zeolites have been reviewed by Marshall
[8]
. McBain
[9]
Table 6.1 Numbers of Tetrahedra in Rings and Approximate Diameters
of Channels in Various Felspathoids and Zeolites
Minerals
No. of
Tetrahedra
in Rings
Minimum Diameter
of Widest Channel
2.2
˚
Sodalite, Nosean
4 and 6
2.2
˚
Analcite
4 and 6
3.2
˚
Harmotome, Phillipsite
4 and 8
3.2
˚
Levyne
4, 6, and 8
3.6
˚
Erionite
4, 6, and 8
3.9
˚
Chabazite
4, 6, and 8
Heulandite
5, 6, and 8
6.4
˚
Gmelinite
4, 6, 8, and 12
9
˚
Faujasite
4, 6, and 12
