Geoscience Reference
In-Depth Information
Fig. 5.4
The arrangement of
Si tetrahedrons in a plain: A
tetrahedral sheet
binding bridge B. The vertices of an octahedron are again occupied by spheres of O
or OH whenever they are not coincident with those of the tetrahedron below.
Inasmuch as the bond O-B-O has less energy than that of O-Si-O, its presence
provides a favorable, less resistant location for the initiation of weathering. If we
remember that chemical weathering is mainly the braking of chemical bonds, we
can relate the intensity of weathering to the relative number of weak B bonds. Let
us consider minerals having 24 O atoms linked within each of their structural units
that contain a maximum of 12 B bonds. Accordingly, the degree of lowest to highest
weatherability of these minerals is estimated by a scheme of the number of B bonds
in each of their structural units ranging from 0 to 12. It is usually necessary to
improve this scheme by considering one additional characteristic - the individual
type of cation. For example, the chemical bonding energy between O-Al-O is
